perlfunc
(1)
Name
perlfunc - Perl builtin functions
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an
expression. They fall into two major categories: list
operators and named unary operators. These differ in their
precedence relationship with a following comma. (See the
precedence table in perlop.) List operators take more than
one argument, while unary operators can never take more than
one argument. Thus, a comma terminates the argument of a
unary operator, but merely separates the arguments of a list
operator. A unary operator generally provides a scalar
context to its argument, while a list operator may provide
either scalar or list contexts for its arguments. If it
does both, scalar arguments come first and list argument
follow, and there can only ever be one such list argument.
For instance, splice() has three scalar arguments followed
by a list, whereas gethostbyname() has four scalar
arguments.
In the syntax descriptions that follow, list operators that
expect a list (and provide list context for elements of the
list) are shown with LIST as an argument. Such a list may
consist of any combination of scalar arguments or list
values; the list values will be included in the list as if
each individual element were interpolated at that point in
the list, forming a longer single-dimensional list value.
Commas should separate literal elements of the LIST.
Any function in the list below may be used either with or
without parentheses around its arguments. (The syntax
descriptions omit the parentheses.) If you use parentheses,
the simple but occasionally surprising rule is this: It
looks like a function, therefore it is a function, and
precedence doesn't matter. Otherwise it's a list operator
or unary operator, and precedence does matter. Whitespace
between the function and left parenthesis doesn't count, so
sometimes you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about
this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
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A few functions take no arguments at all, and therefore work
as neither unary nor list operators. These include such
functions as "time" and "endpwent". For example,
"time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list
context, nonabortive failure is generally indicated in a
scalar context by returning the undefined value, and in a
list context by returning the empty list.
Remember the following important rule: There is no rule that
relates the behavior of an expression in list context to its
behavior in scalar context, or vice versa. It might do two
totally different things. Each operator and function
decides which sort of value would be most appropriate to
return in scalar context. Some operators return the length
of the list that would have been returned in list context.
Some operators return the first value in the list. Some
operators return the last value in the list. Some operators
return a count of successful operations. In general, they
do what you want, unless you want consistency.
A named array in scalar context is quite different from what
would at first glance appear to be a list in scalar context.
You can't get a list like "(1,2,3)" into being in scalar
context, because the compiler knows the context at compile
time. It would generate the scalar comma operator there,
not the list construction version of the comma. That means
it was never a list to start with.
In general, functions in Perl that serve as wrappers for
system calls ("syscalls") of the same name (like chown(2),
fork(2), closedir(2), etc.) all return true when they
succeed and "undef" otherwise, as is usually mentioned in
the descriptions below. This is different from the C
interfaces, which return "-1" on failure. Exceptions to
this rule are "wait", "waitpid", and "syscall". System
calls also set the special $! variable on failure. Other
functions do not, except accidentally.
Extension modules can also hook into the Perl parser to
define new kinds of keyword-headed expression. These may
look like functions, but may also look completely different.
The syntax following the keyword is defined entirely by the
extension. If you are an implementor, see
"PL_keyword_plugin" in perlapi for the mechanism. If you
are using such a module, see the module's documentation for
details of the syntax that it defines.
Perl Functions by Category
Here are Perl's functions (including things that look like
functions, like some keywords and named operators) arranged
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by category. Some functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc",
"lcfirst", "length", "oct", "ord", "pack", "q//",
"qq//", "reverse", "rindex", "sprintf", "substr",
"tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study",
"qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log",
"oct", "rand", "sin", "sqrt", "srand"
Functions for real @ARRAYs
"each", "keys", "pop", "push", "shift", "splice",
"unshift", "values"
Functions for list data
"grep", "join", "map", "qw//", "reverse", "sort",
"unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen",
"die", "eof", "fileno", "flock", "format", "getc",
"print", "printf", "read", "readdir", "rewinddir",
"say", "seek", "seekdir", "select", "syscall",
"sysread", "sysseek", "syswrite", "tell", "telldir",
"truncate", "warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite",
"unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl",
"glob", "ioctl", "link", "lstat", "mkdir", "open",
"opendir", "readlink", "rename", "rmdir", "stat",
"symlink", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval",
"exit", "goto", "last", "next", "redo", "return", "sub",
"wantarray"
Keywords related to switch
"break", "continue", "given", "when", "default"
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(These are available only if you enable the "switch"
feature. See feature and "Switch statements" in
perlsyn.)
Keywords related to scoping
"caller", "import", "local", "my", "our", "state",
"package", "use"
("state" is available only if the "state" feature is
enabled. See feature.)
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my",
"our", "reset", "scalar", "state", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid",
"getpriority", "kill", "pipe", "qx//", "setpgrp",
"setpriority", "sleep", "system", "times", "wait",
"waitpid"
Keywords related to Perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientation
"bless", "dbmclose", "dbmopen", "package", "ref", "tie",
"tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername",
"getsockname", "getsockopt", "listen", "recv", "send",
"setsockopt", "shutdown", "socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl",
"semget", "semop", "shmctl", "shmget", "shmread",
"shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent",
"getgrent", "getgrgid", "getgrnam", "getlogin",
"getpwent", "getpwnam", "getpwuid", "setgrent",
"setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr",
"gethostbyname", "gethostent", "getnetbyaddr",
"getnetbyname", "getnetent", "getprotobyname",
"getprotobynumber", "getprotoent", "getservbyname",
"getservbyport", "getservent", "sethostent",
"setnetent", "setprotoent", "setservent"
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Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "break", "chomp", "chr", "continue",
"default", "exists", "formline", "given", "glob",
"import", "lc", "lcfirst", "lock", "map", "my", "no",
"our", "prototype", "qr//", "qw//", "qx//", "readline",
"readpipe", "ref", "sub"*, "sysopen", "tie", "tied",
"uc", "ucfirst", "untie", "use", "when"
* "sub" was a keyword in Perl 4, but in Perl 5 it is an
operator, which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common
Unix system calls. In non-Unix environments, the
functionality of some Unix system calls may not be
available, or details of the available functionality may
differ slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt",
"dbmclose", "dbmopen", "dump", "endgrent", "endhostent",
"endnetent", "endprotoent", "endpwent", "endservent",
"exec", "fcntl", "flock", "fork", "getgrent", "getgrgid",
"gethostbyname", "gethostent", "getlogin", "getnetbyaddr",
"getnetbyname", "getnetent", "getppid", "getpgrp",
"getpriority", "getprotobynumber", "getprotoent",
"getpwent", "getpwnam", "getpwuid", "getservbyport",
"getservent", "getsockopt", "glob", "ioctl", "kill", "link",
"lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open",
"pipe", "readlink", "rename", "select", "semctl", "semget",
"semop", "setgrent", "sethostent", "setnetent", "setpgrp",
"setpriority", "setprotoent", "setpwent", "setservent",
"setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
"socket", "socketpair", "stat", "symlink", "syscall",
"sysopen", "system", "times", "truncate", "umask", "unlink",
"utime", "wait", "waitpid"
For more information about the portability of these
functions, see perlport and other available platform-
specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X DIRHANDLE
-X A file test, where X is one of the letters listed below.
This unary operator takes one argument, either a
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filename, a filehandle, or a dirhandle, and tests the
associated file to see if something is true about it.
If the argument is omitted, tests $_, except for "-t",
which tests STDIN. Unless otherwise documented, it
returns 1 for true and '' for false, or the undefined
value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary
operator. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators
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"-r", "-R", "-w", "-W", "-x", and "-X" is by default
based solely on the mode of the file and the uids and
gids of the user. There may be other reasons you can't
actually read, write, or execute the file: for example
network filesystem access controls, ACLs (access control
lists), read-only filesystems, and unrecognized
executable formats. Note that the use of these six
specific operators to verify if some operation is
possible is usually a mistake, because it may be open to
race conditions.
Also note that, for the superuser on the local
filesystems, the "-r", "-R", "-w", and "-W" tests always
return 1, and "-x" and "-X" return 1 if any execute bit
is set in the mode. Scripts run by the superuser may
thus need to do a stat() to determine the actual mode of
the file, or temporarily set their effective uid to
something else.
If you are using ACLs, there is a pragma called
"filetest" that may produce more accurate results than
the bare stat() mode bits. When under the "use filetest
'access'" the above-mentioned filetests test whether the
permission can (not) be granted using the access(2)
family of system calls. Also note that the "-x" and
"-X" may under this pragma return true even if there are
no execute permission bits set (nor any extra execute
permission ACLs). This strangeness is due to the
underlying system calls' definitions. Note also that,
due to the implementation of "use filetest 'access'",
the "_" special filehandle won't cache the results of
the file tests when this pragma is in effect. Read the
documentation for the "filetest" pragma for more
information.
Note that "-s/a/b/" does not do a negated substitution.
Saying "-exp($foo)" still works as expected, however:
only single letters following a minus are interpreted as
file tests.
The "-T" and "-B" switches work as follows. The first
block or so of the file is examined for odd characters
such as strange control codes or characters with the
high bit set. If too many strange characters (>30%) are
found, it's a "-B" file; otherwise it's a "-T" file.
Also, any file containing a zero byte in the first block
is considered a binary file. If "-T" or "-B" is used on
a filehandle, the current IO buffer is examined rather
than the first block. Both "-T" and "-B" return true on
an empty file, or a file at EOF when testing a
filehandle. Because you have to read a file to do the
"-T" test, on most occasions you want to use a "-f"
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against the file first, as in "next unless -f $file &&
-T $file".
If any of the file tests (or either the "stat" or
"lstat" operators) are given the special filehandle
consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator)
is used, saving a system call. (This doesn't work with
"-t", and you need to remember that lstat() and "-l"
leave values in the stat structure for the symbolic
link, not the real file.) (Also, if the stat buffer was
filled by an "lstat" call, "-T" and "-B" will reset it
with the results of "stat _"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
As of Perl 5.9.1, as a form of purely syntactic sugar,
you can stack file test operators, in a way that "-f -w
-x $file" is equivalent to "-x $file && -w _ && -f _".
(This is only fancy fancy: if you use the return value
of "-f $file" as an argument to another filetest
operator, no special magic will happen.)
abs VALUE
abs Returns the absolute value of its argument. If VALUE is
omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as accept(2)
does. Returns the packed address if it succeeded, false
otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files,
the flag will be set for the newly opened file
descriptor, as determined by the value of $^F. See
"$^F" in perlvar.
alarm SECONDS
alarm
Arranges to have a SIGALRM delivered to this process
after the specified number of wallclock seconds has
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elapsed. If SECONDS is not specified, the value stored
in $_ is used. (On some machines, unfortunately, the
elapsed time may be up to one second less or more than
you specified because of how seconds are counted, and
process scheduling may delay the delivery of the signal
even further.)
Only one timer may be counting at once. Each call
disables the previous timer, and an argument of 0 may be
supplied to cancel the previous timer without starting a
new one. The returned value is the amount of time
remaining on the previous timer.
For delays of finer granularity than one second, the
Time::HiRes module (from CPAN, and starting from Perl
5.8 part of the standard distribution) provides
ualarm(). You may also use Perl's four-argument version
of select() leaving the first three arguments undefined,
or you might be able to use the "syscall" interface to
access setitimer(2) if your system supports it. See
perlfaq8 for details.
It is usually a mistake to intermix "alarm" and "sleep"
calls, because "sleep" may be internally implemented on
your system with "alarm".
If you want to use "alarm" to time out a system call you
need to use an "eval"/"die" pair. You can't rely on the
alarm causing the system call to fail with $! set to
"EINTR" because Perl sets up signal handlers to restart
system calls on some systems. Using "eval"/"die" always
works, modulo the caveats given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
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For the tangent operation, you may use the
"Math::Trig::tan" function, or use the familiar
relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-
defined; consult your atan2(3) manpage for more
information.
bind SOCKET,NAME
Binds a network address to a socket, just as bind(2)
does. Returns true if it succeeded, false otherwise.
NAME should be a packed address of the appropriate type
for the socket. See the examples in "Sockets:
Client/Server Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in
"binary" or "text" mode on systems where the run-time
libraries distinguish between binary and text files. If
FILEHANDLE is an expression, the value is taken as the
name of the filehandle. Returns true on success,
otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based
systems) binmode() is necessary when you're not working
with a text file. For the sake of portability it is a
good idea to always use it when appropriate, and to
never use it when it isn't appropriate. Also, people
can set their I/O to be by default UTF-8 encoded
Unicode, not bytes.
In other words: regardless of platform, use binmode() on
binary data, like for example images.
If LAYER is present it is a single string, but may
contain multiple directives. The directives alter the
behaviour of the filehandle. When LAYER is present
using binmode on a text file makes sense.
If LAYER is omitted or specified as ":raw" the
filehandle is made suitable for passing binary data.
This includes turning off possible CRLF translation and
marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in "Programming
Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is
not simply the inverse of ":crlf". Other layers that
would affect the binary nature of the stream are also
disabled. See PerlIO, perlrun, and the discussion about
the PERLIO environment variable.
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The ":bytes", ":crlf", ":utf8", and any other directives
of the form ":...", are called I/O layers. The "open"
pragma can be used to establish default I/O layers. See
open.
The LAYER parameter of the binmode() function is
described as "DISCIPLINE" in "Programming Perl, 3rd
Edition". However, since the publishing of this book,
by many known as "Camel III", the consensus of the
naming of this functionality has moved from "discipline"
to "layer". All documentation of this version of Perl
therefore refers to "layers" rather than to
"disciplines". Now back to the regularly scheduled
documentation...
To mark FILEHANDLE as UTF-8, use ":utf8" or
":encoding(utf8)". ":utf8" just marks the data as UTF-8
without further checking, while ":encoding(utf8)" checks
the data for actually being valid UTF-8. More details
can be found in PerlIO::encoding.
In general, binmode() should be called after open() but
before any I/O is done on the filehandle. Calling
binmode() normally flushes any pending buffered output
data (and perhaps pending input data) on the handle. An
exception to this is the ":encoding" layer that changes
the default character encoding of the handle, see open.
The ":encoding" layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream. The
":encoding" also implicitly pushes on top of itself the
":utf8" layer because internally Perl operates on
UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and
Perl run-time system all work together to let the
programmer treat a single character ("\n") as the line
terminator, irrespective of the external representation.
On many operating systems, the native text file
representation matches the internal representation, but
on some platforms the external representation of "\n" is
made up of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS
use a single character to end each line in the external
representation of text (even though that single
character is CARRIAGE RETURN on Mac OS and LINE FEED on
Unix and most VMS files). In other systems like OS/2,
DOS and the various flavors of MS-Windows your program
sees a "\n" as a simple "\cJ", but what's stored in text
files are the two characters "\cM\cJ". That means that,
if you don't use binmode() on these systems, "\cM\cJ"
sequences on disk will be converted to "\n" on input,
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and any "\n" in your program will be converted back to
"\cM\cJ" on output. This is what you want for text
files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems)
is that special end-of-file markers will be seen as part
of the data stream. For systems from the Microsoft
family this means that if your binary data contains
"\cZ", the I/O subsystem will regard it as the end of
the file, unless you use binmode().
binmode() is important not only for readline() and
print() operations, but also when using read(), seek(),
sysread(), syswrite() and tell() (see perlport for more
details). See the $/ and "$\" variables in perlvar for
how to manually set your input and output line-
termination sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it
is now an object in the CLASSNAME package. If CLASSNAME
is omitted, the current package is used. Because a
"bless" is often the last thing in a constructor, it
returns the reference for convenience. Always use the
two-argument version if a derived class might inherit
the function doing the blessing. See perltoot and
perlobj for more about the blessing (and blessings) of
objects.
Consider always blessing objects in CLASSNAMEs that are
mixed case. Namespaces with all lowercase names are
considered reserved for Perl pragmata. Builtin types
have all uppercase names. To prevent confusion, you may
wish to avoid such package names as well. Make sure
that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
break
Break out of a "given()" block.
This keyword is enabled by the "switch" feature: see
feature for more information.
caller EXPR
caller
Returns the context of the current subroutine call. In
scalar context, returns the caller's package name if
there is a caller (that is, if we're in a subroutine or
"eval" or "require") and the undefined value otherwise.
In list context, returns
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# 0 1 2
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the
debugger uses to print a stack trace. The value of EXPR
indicates how many call frames to go back before the
current one.
# 0 1 2 3 4
($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);
Here $subroutine may be "(eval)" if the frame is not a
subroutine call, but an "eval". In such a case
additional elements $evaltext and $is_require are set:
$is_require is true if the frame is created by a
"require" or "use" statement, $evaltext contains the
text of the "eval EXPR" statement. In particular, for
an "eval BLOCK" statement, $subroutine is "(eval)", but
$evaltext is undefined. (Note also that each "use"
statement creates a "require" frame inside an "eval
EXPR" frame.) $subroutine may also be "(unknown)" if
this particular subroutine happens to have been deleted
from the symbol table. $hasargs is true if a new
instance of @_ was set up for the frame. $hints and
$bitmask contain pragmatic hints that the caller was
compiled with. The $hints and $bitmask values are
subject to change between versions of Perl, and are not
meant for external use.
$hinthash is a reference to a hash containing the value
of "%^H" when the caller was compiled, or "undef" if
"%^H" was empty. Do not modify the values of this hash,
as they are the actual values stored in the optree.
Furthermore, when called from within the DB package,
caller returns more detailed information: it sets the
list variable @DB::args to be the arguments with which
the subroutine was invoked.
Be aware that the optimizer might have optimized call
frames away before "caller" had a chance to get the
information. That means that caller(N) might not return
information about the call frame you expect it to, for
"N > 1". In particular, @DB::args might have
information from the previous time "caller" was called.
Also be aware that setting @DB::args is best effort,
intended for debugging or generating backtraces, and
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should not be relied upon. In particular, as @_ contains
aliases to the caller's arguments, Perl does not take a
copy of @_, so @DB::args will contain modifications the
subroutine makes to @_ or its contents, not the original
values at call time. @DB::args, like @_, does not hold
explicit references to its elements, so under certain
cases its elements may have become freed and reallocated
for other variables or temporary values. Finally, a side
effect of the current implementation means that the
effects of "shift @_" can normally be undone (but not
"pop @_" or other splicing, and not if a reference to @_
has been taken, and subject to the caveat about
reallocated elements), so @DB::args is actually a hybrid
of the current state and initial state of @_. Buyer
beware.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir
Changes the working directory to EXPR, if possible. If
EXPR is omitted, changes to the directory specified by
$ENV{HOME}, if set; if not, changes to the directory
specified by $ENV{LOGDIR}. (Under VMS, the variable
$ENV{SYS$LOGIN} is also checked, and used if it is set.)
If neither is set, "chdir" does nothing. It returns true
on success, false otherwise. See the example under
"die".
On systems that support fchdir(2), you may pass a
filehandle or directory handle as argument. On systems
that don't support fchdir(2), passing handles raises an
exception.
chmod LIST
Changes the permissions of a list of files. The first
element of the list must be the numerical mode, which
should probably be an octal number, and which definitely
should not be a string of octal digits: 0644 is okay,
but "0644" is not. Returns the number of files
successfully changed. See also "oct", if all you have
is a string.
$cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
$mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
$mode = "0644"; chmod oct($mode), "foo"; # this is better
$mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass
filehandles among the files. On systems that don't
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support fchmod(2), passing filehandles raises an
exception. Filehandles must be passed as globs or glob
references to be recognized; barewords are considered
filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from
the "Fcntl" module:
use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.
chomp VARIABLE
chomp( LIST )
chomp
This safer version of "chop" removes any trailing string
that corresponds to the current value of $/ (also known
as $INPUT_RECORD_SEPARATOR in the "English" module). It
returns the total number of characters removed from all
its arguments. It's often used to remove the newline
from the end of an input record when you're worried that
the final record may be missing its newline. When in
paragraph mode ("$/ = """), it removes all trailing
newlines from the string. When in slurp mode ("$/ =
undef") or fixed-length record mode ($/ is a reference
to an integer or the like, see perlvar) chomp() won't
remove anything. If VARIABLE is omitted, it chomps $_.
Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but
not its keys.
You can actually chomp anything that's an lvalue,
including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the
total number of characters removed is returned.
Note that parentheses are necessary when you're chomping
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anything that is not a simple variable. This is because
"chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) =
`pwd`;", rather than as "chomp( $cwd = `pwd` )" which
you might expect. Similarly, "chomp $a, $b" is
interpreted as "chomp($a), $b" rather than as "chomp($a,
$b)".
chop VARIABLE
chop( LIST )
chop
Chops off the last character of a string and returns the
character chopped. It is much more efficient than
"s/.$//s" because it neither scans nor copies the
string. If VARIABLE is omitted, chops $_. If VARIABLE
is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue,
including an assignment.
If you chop a list, each element is chopped. Only the
value of the last "chop" is returned.
Note that "chop" returns the last character. To return
all but the last character, use "substr($string, 0,
-1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The
first two elements of the list must be the numeric uid
and gid, in that order. A value of -1 in either
position is interpreted by most systems to leave that
value unchanged. Returns the number of files
successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass
filehandles among the files. On systems that don't
support fchown(2), passing filehandles raises an
exception. Filehandles must be passed as globs or glob
references to be recognized; barewords are considered
filenames.
Here's an example that looks up nonnumeric uids in the
passwd file:
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print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the
ownership of the file unless you're the superuser,
although you should be able to change the group to any
of your secondary groups. On insecure systems, these
restrictions may be relaxed, but this is not a portable
assumption. On POSIX systems, you can detect this
condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the
character set. For example, "chr(65)" is "A" in either
ASCII or Unicode, and chr(0x263a) is a Unicode smiley
face.
Negative values give the Unicode replacement character
(chr(0xfffd)), except under the bytes pragma, where the
low eight bits of the value (truncated to an integer)
are used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by
default internally not encoded as UTF-8 for backward
compatibility reasons.
See perlunicode for more about Unicode.
chroot FILENAME
chroot
This function works like the system call by the same
name: it makes the named directory the new root
directory for all further pathnames that begin with a
"/" by your process and all its children. (It doesn't
change your current working directory, which is
unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted,
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does a "chroot" to $_.
close FILEHANDLE
close
Closes the file or pipe associated with the filehandle,
flushes the IO buffers, and closes the system file
descriptor. Returns true if those operations have
succeeded and if no error was reported by any PerlIO
layer. Closes the currently selected filehandle if the
argument is omitted.
You don't have to close FILEHANDLE if you are
immediately going to do another "open" on it, because
"open" closes it for you. (See "open".) However, an
explicit "close" on an input file resets the line
counter ($.), while the implicit close done by "open"
does not.
If the filehandle came from a piped open, "close"
returns false if one of the other syscalls involved
fails or if its program exits with non-zero status. If
the only problem was that the program exited non-zero,
$! will be set to 0. Closing a pipe also waits for the
process executing on the pipe to exit--in case you wish
to look at the output of the pipe afterwards--and
implicitly puts the exit status value of that command
into $? and "${^CHILD_ERROR_NATIVE}".
Closing the read end of a pipe before the process
writing to it at the other end is done writing results
in the writer receiving a SIGPIPE. If the other end
can't handle that, be sure to read all the data before
closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used
as an indirect filehandle, usually the real filehandle
name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the
success of that system call.
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connect SOCKET,NAME
Attempts to connect to a remote socket, just like
connect(2). Returns true if it succeeded, false
otherwise. NAME should be a packed address of the
appropriate type for the socket. See the examples in
"Sockets: Client/Server Communication" in perlipc.
continue BLOCK
continue
"continue" is actually a flow control statement rather
than a function. If there is a "continue" BLOCK
attached to a BLOCK (typically in a "while" or
"foreach"), it is always executed just before the
conditional is about to be evaluated again, just like
the third part of a "for" loop in C. Thus it can be
used to increment a loop variable, even when the loop
has been continued via the "next" statement (which is
similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue"
block; "last" and "redo" behave as if they had been
executed within the main block. So will "next", but
since it will execute a "continue" block, it may be more
entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is equivalent to using
an empty one, logically enough, so "next" goes directly
back to check the condition at the top of the loop.
If the "switch" feature is enabled, "continue" is also a
function that exits the current "when" (or "default")
block and falls through to the next one. See feature
and "Switch statements" in perlsyn for more information.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If
EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
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crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3)
function in the C library (assuming that you actually
have a version there that has not been extirpated as a
potential munition).
crypt() is a one-way hash function. The PLAINTEXT and
SALT is turned into a short string, called a digest,
which is returned. The same PLAINTEXT and SALT will
always return the same string, but there is no (known)
way to get the original PLAINTEXT from the hash. Small
changes in the PLAINTEXT or SALT will result in large
changes in the digest.
There is no decrypt function. This function isn't all
that useful for cryptography (for that, look for Crypt
modules on your nearby CPAN mirror) and the name "crypt"
is a bit of a misnomer. Instead it is primarily used to
check if two pieces of text are the same without having
to transmit or store the text itself. An example is
checking if a correct password is given. The digest of
the password is stored, not the password itself. The
user types in a password that is crypt()'d with the same
salt as the stored digest. If the two digests match the
password is correct.
When verifying an existing digest string you should use
the digest as the salt (like "crypt($plain, $digest) eq
$digest"). The SALT used to create the digest is
visible as part of the digest. This ensures crypt()
will hash the new string with the same salt as the
digest. This allows your code to work with the standard
crypt and with more exotic implementations. In other
words, do not assume anything about the returned string
itself, or how many bytes in the digest matter.
Traditionally the result is a string of 13 bytes: two
first bytes of the salt, followed by 11 bytes from the
set "[./0-9A-Za-z]", and only the first eight bytes of
PLAINTEXT mattered. But alternative hashing schemes
(like MD5), higher level security schemes (like C2), and
implementations on non-Unix platforms may produce
different strings.
When choosing a new salt create a random two character
string whose characters come from the set
"[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9,
'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This set of
characters is just a recommendation; the characters
allowed in the salt depend solely on your system's crypt
library, and Perl can't restrict what salts "crypt()"
accepts.
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Here's an example that makes sure that whoever runs this
program knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks
you for it is unwise.
The crypt function is unsuitable for hashing large
quantities of data, not least of all because you can't
get the information back. Look at the Digest module for
more robust algorithms.
If using crypt() on a Unicode string (which potentially
has characters with codepoints above 255), Perl tries to
make sense of the situation by trying to downgrade (a
copy of the string) the string back to an eight-bit byte
string before calling crypt() (on that copy). If that
works, good. If not, crypt() dies with "Wide character
in crypt".
dbmclose HASH
[This function has been largely superseded by the
"untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie"
function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or
Berkeley DB file to a hash. HASH is the name of the
hash. (Unlike normal "open", the first argument is not
a filehandle, even though it looks like one). DBNAME is
the name of the database (without the .dir or .pag
extension if any). If the database does not exist, it
is created with protection specified by MASK (as
modified by the "umask"). If your system supports only
the older DBM functions, you may make only one "dbmopen"
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call in your program. In older versions of Perl, if
your system had neither DBM nor ndbm, calling "dbmopen"
produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can
only read hash variables, not set them. If you want to
test whether you can write, either use file tests or try
setting a dummy hash entry inside an "eval" to trap the
error.
Note that functions such as "keys" and "values" may
return huge lists when used on large DBM files. You may
prefer to use the "each" function to iterate over large
DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of
the pros and cons of the various dbm approaches, as well
as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading
that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value
other than the undefined value "undef". If EXPR is not
present, $_ is checked.
Many operations return "undef" to indicate failure, end
of file, system error, uninitialized variable, and other
exceptional conditions. This function allows you to
distinguish "undef" from other values. (A simple
Boolean test will not distinguish among "undef", zero,
the empty string, and "0", which are all equally false.)
Note that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition:
"pop" returns "undef" when its argument is an empty
array, or when the element to return happens to be
"undef".
You may also use "defined(&func)" to check whether
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subroutine &func has ever been defined. The return
value is unaffected by any forward declarations of
&func. A subroutine that is not defined may still be
callable: its package may have an "AUTOLOAD" method that
makes it spring into existence the first time that it is
called; see perlsub.
Use of "defined" on aggregates (hashes and arrays) is
deprecated. It used to report whether memory for that
aggregate has ever been allocated. This behavior may
disappear in future versions of Perl. You should
instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the
value is defined, not whether the key exists in the
hash. Use "exists" for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then
are surprised to discover that the number 0 and "" (the
zero-length string) are, in fact, defined values. For
example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds and $1 is defined, although
it matched "nothing". It didn't really fail to match
anything. Rather, it matched something that happened to
be zero characters long. This is all very above-board
and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest
answer. So you should use "defined" only when
questioning the integrity of what you're trying to do.
At other times, a simple comparison to 0 or "" is what
you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies an element or slice
of a hash, "delete" deletes the specified elements from
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that hash so that exists() on that element no longer
returns true. Setting a hash element to the undefined
value does not remove its key, but deleting it does; see
"exists".
It returns the value or values deleted in list context,
or the last such element in scalar context. The return
list's length always matches that of the argument list:
deleting non-existent elements returns the undefined
value in their corresponding positions.
delete() may also be used on arrays and array slices,
but its behavior is less straightforward. Although
exists() will return false for deleted entries, deleting
array elements never changes indices of existing values;
use shift() or splice() for that. However, if all
deleted elements fall at the end of an array, the
array's size shrinks to the position of the highest
element that still tests true for exists(), or to 0 if
none do.
Be aware that calling delete on array values is
deprecated and likely to be removed in a future version
of Perl.
Deleting from %ENV modifies the environment. Deleting
from a hash tied to a DBM file deletes the entry from
the DBM file. Deleting from a "tied" hash or array may
not necessarily return anything; it depends on the
implementation of the "tied" package's DELETE method,
which may do whatever it pleases.
The "delete local EXPR" construct localizes the deletion
to the current block at run time. Until the block
exits, elements locally deleted temporarily no longer
exist. See "Localized deletion of elements of composite
types" in perlsub.
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
The following (inefficiently) deletes all the values of
%HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
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}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or
undefining %HASH or @ARRAY, which is the customary way
to empty out an aggregate:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its
final operation is an element or slice of an aggregate:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
"die" raises an exception. Inside an "eval" the error
message is stuffed into $@ and the "eval" is terminated
with the undefined value. If the exception is outside
of all enclosing "eval"s, then the uncaught exception
prints LIST to "STDERR" and exits with a non-zero value.
If you need to exit the process with a specific exit
code, see exit.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline,
the current script line number and input line number (if
any) are also printed, and a newline is supplied. Note
that the "input line number" (also known as "chunk") is
subject to whatever notion of "line" happens to be
currently in effect, and is also available as the
special variable $.. See "$/" in perlvar and "$." in
perlvar.
Hint: sometimes appending ", stopped" to your message
will cause it to make better sense when the string "at
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foo line 123" is appended. Suppose you are running
script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
If the output is empty and $@ already contains a value
(typically from a previous eval) that value is reused
after appending "\t...propagated". This is useful for
propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If the output is empty and $@ contains an object
reference that has a "PROPAGATE" method, that method
will be called with additional file and line number
parameters. The return value replaces the value in $@.
i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string "Died" is used.
If an uncaught exception results in interpreter exit,
the exit code is determined from the values of $! and $?
with this pseudocode:
exit $! if $!; # errno
exit $? >> 8 if $? >> 8; # child exit status
exit 255; # last resort
The intent is to squeeze as much possible information
about the likely cause into the limited space of the
system exit code. However, as $! is the value of C's
"errno", which can be set by any system call, this means
that the value of the exit code used by "die" can be
non-predictable, so should not be relied upon, other
than to be non-zero.
You can also call "die" with a reference argument, and
if this is trapped within an "eval", $@ contains that
reference. This permits more elaborate exception
handling using objects that maintain arbitrary state
about the exception. Such a scheme is sometimes
preferable to matching particular string values of $@
with regular expressions. Because $@ is a global
variable and "eval" may be used within object
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implementations, be careful that analyzing the error
object doesn't replace the reference in the global
variable. It's easiest to make a local copy of the
reference before any manipulations. Here's an example:
use Scalar::Util "blessed";
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because Perl stringifies uncaught exception messages
before display, you'll probably want to overload
stringification operations on exception objects. See
overload for details about that.
You can arrange for a callback to be run just before the
"die" does its deed, by setting the $SIG{__DIE__} hook.
The associated handler is called with the error text and
can change the error message, if it sees fit, by calling
"die" again. See "$SIG{expr}" in perlvar for details on
setting %SIG entries, and "eval BLOCK" for some
examples. Although this feature was to be run only
right before your program was to exit, this is not
currently so: the $SIG{__DIE__} hook is currently called
even inside eval()ed blocks/strings! If one wants the
hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar).
Because this promotes strange action at a distance, this
counterintuitive behavior may be fixed in a future
release.
See also exit(), warn(), and the Carp module.
do BLOCK
Not really a function. Returns the value of the last
command in the sequence of commands indicated by BLOCK.
When modified by the "while" or "until" loop modifier,
executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test
the conditional first.)
"do BLOCK" does not count as a loop, so the loop control
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statements "next", "last", or "redo" cannot be used to
leave or restart the block. See perlsyn for alternative
strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See
perlsub.
do EXPR
Uses the value of EXPR as a filename and executes the
contents of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track
of the current filename for error messages, searches the
@INC directories, and updates %INC if the file is found.
See "Predefined Names" in perlvar for these variables.
It also differs in that code evaluated with "do
FILENAME" cannot see lexicals in the enclosing scope;
"eval STRING" does. It's the same, however, in that it
does reparse the file every time you call it, so you
probably don't want to do this inside a loop.
If "do" cannot read the file, it returns undef and sets
$! to the error. If "do" can read the file but cannot
compile it, it returns undef and sets an error message
in $@. If the file is successfully compiled, "do"
returns the value of the last expression evaluated.
Inclusion of library modules is better done with the
"use" and "require" operators, which also do automatic
error checking and raise an exception if there's a
problem.
You might like to use "do" to read in a program
configuration file. Manual error checking can be done
this way:
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# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump
This function causes an immediate core dump. See also
the -u command-line switch in perlrun, which does the
same thing. Primarily this is so that you can use the
undump program (not supplied) to turn your core dump
into an executable binary after having initialized all
your variables at the beginning of the program. When
the new binary is executed it will begin by executing a
"goto LABEL" (with all the restrictions that "goto"
suffers). Think of it as a goto with an intervening
core dump and reincarnation. If "LABEL" is omitted,
restarts the program from the top.
WARNING: Any files opened at the time of the dump will
not be open any more when the program is reincarnated,
with possible resulting confusion by Perl.
This function is now largely obsolete, mostly because
it's very hard to convert a core file into an
executable. That's why you should now invoke it as
"CORE::dump()", if you don't want to be warned against a
possible typo.
each HASH
each ARRAY
When called in list context, returns a 2-element list
consisting of the key and value for the next element of
a hash, or the index and value for the next element of
an array, so that you can iterate over it. When called
in scalar context, returns only the key (not the value)
in a hash, or the index in an array.
Hash entries are returned in an apparently random order.
The actual random order is subject to change in future
versions of Perl, but it is guaranteed to be in the same
order as either the "keys" or "values" function would
produce on the same (unmodified) hash. Since Perl 5.8.2
the ordering can be different even between different
runs of Perl for security reasons (see "Algorithmic
Complexity Attacks" in perlsec).
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After "each" has returned all entries from the hash or
array, the next call to "each" returns the empty list in
list context and "undef" in scalar context. The next
call following that one restarts iteration. Each hash
or array has its own internal iterator, accessed by
"each", "keys", and "values". The iterator is
implicitly reset when "each" has reached the end as just
described; it can be explicitly reset by calling "keys"
or "values" on the hash or array. If you add or delete
a hash's elements while iterating over it, entries may
be skipped or duplicated--so don't do that. Exception:
It is always safe to delete the item most recently
returned by "each()", so the following code works
properly:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
This prints out your environment like the printenv(1)
program, but in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end
of file, or if FILEHANDLE is not open. FILEHANDLE may
be an expression whose value gives the real filehandle.
(Note that this function actually reads a character and
then "ungetc"s it, so isn't useful in an interactive
context.) Do not read from a terminal file (or call
"eof(FILEHANDLE)" on it) after end-of-file is reached.
File types such as terminals may lose the end-of-file
condition if you do.
An "eof" without an argument uses the last file read.
Using "eof()" with empty parentheses is different. It
refers to the pseudo file formed from the files listed
on the command line and accessed via the "<>" operator.
Since "<>" isn't explicitly opened, as a normal
filehandle is, an "eof()" before "<>" has been used will
cause @ARGV to be examined to determine if input is
available. Similarly, an "eof()" after "<>" has
returned end-of-file will assume you are processing
another @ARGV list, and if you haven't set @ARGV, will
read input from "STDIN"; see "I/O Operators" in perlop.
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In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used
to detect the end of each file, "eof()" will detect the
end of only the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in
Perl, because the input operators typically return
"undef" when they run out of data, or if there was an
error.
eval EXPR
eval BLOCK
eval
In the first form, the return value of EXPR is parsed
and executed as if it were a little Perl program. The
value of the expression (which is itself determined
within scalar context) is first parsed, and if there
weren't any errors, executed in the lexical context of
the current Perl program, so that any variable settings
or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the "eval"
executes. If EXPR is omitted, evaluates $_. This form
is typically used to delay parsing and subsequent
execution of the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed
only once--at the same time the code surrounding the
"eval" itself was parsed--and executed within the
context of the current Perl program. This form is
typically used to trap exceptions more efficiently than
the first (see below), while also providing the benefit
of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the
value of EXPR or within the BLOCK.
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In both forms, the value returned is the value of the
last expression evaluated inside the mini-program; a
return statement may be also used, just as with
subroutines. The expression providing the return value
is evaluated in void, scalar, or list context, depending
on the context of the "eval" itself. See "wantarray"
for more on how the evaluation context can be
determined.
If there is a syntax error or runtime error, or a "die"
statement is executed, "eval" returns an undefined value
in scalar context or an empty list in list context, and
$@ is set to the error message. If there was no error,
$@ is guaranteed to be the empty string. Beware that
using "eval" neither silences Perl from printing
warnings to STDERR, nor does it stuff the text of
warning messages into $@. To do either of those, you
have to use the $SIG{__WARN__} facility, or turn off
warnings inside the BLOCK or EXPR using
"no warnings 'all'". See "warn", perlvar, warnings and
perllexwarn.
Note that, because "eval" traps otherwise-fatal errors,
it is useful for determining whether a particular
feature (such as "socket" or "symlink") is implemented.
It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.
If you want to trap errors when loading an XS module,
some problems with the binary interface (such as Perl
version skew) may be fatal even with "eval" unless
$ENV{PERL_DL_NONLAZY} is set. See perlrun.
If the code to be executed doesn't vary, you may use the
eval-BLOCK form to trap run-time errors without
incurring the penalty of recompiling each time. The
error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in
libraries does have some issues. Due to the current
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arguably broken state of "__DIE__" hooks, you may wish
not to trigger any "__DIE__" hooks that user code may
have installed. You can use the "local $SIG{__DIE__}"
construct for this purpose, as this example shows:
# a private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__"
hooks can call "die" again, which has the effect of
changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this
counterintuitive behavior may be fixed in a future
release.
With an "eval", you should be especially careful to
remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the
code contained in the variable $x. (Although case 2 has
misleading double quotes making the reader wonder what
else might be happening (nothing is).) Cases 3 and 4
likewise behave in the same way: they run the code '$x',
which does nothing but return the value of $x. (Case 4
is preferred for purely visual reasons, but it also has
the advantage of compiling at compile-time instead of at
run-time.) Case 5 is a place where normally you would
like to use double quotes, except that in this
particular situation, you can just use symbolic
references instead, as in case 6.
The assignment to $@ occurs before restoration of
localised variables, which means a temporary is required
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if you want to mask some but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # DOES NOT WORK
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}
"eval BLOCK" does not count as a loop, so the loop
control statements "next", "last", or "redo" cannot be
used to leave or restart the block.
An "eval ''" executed within the "DB" package doesn't
see the usual surrounding lexical scope, but rather the
scope of the first non-DB piece of code that called it.
You don't normally need to worry about this unless you
are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never
returns; use "system" instead of "exec" if you want it
to return. It fails and returns false only if the
command does not exist and it is executed directly
instead of via your system's command shell (see below).
Since it's a common mistake to use "exec" instead of
"system", Perl warns you if there is a following
statement that isn't "die", "warn", or "exit" (if "-w"
is set--but you always do that, right?). If you really
want to follow an "exec" with some other statement, you
can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST
is an array with more than one value, calls execvp(3)
with the arguments in LIST. If there is only one scalar
argument or an array with one element in it, the
argument is checked for shell metacharacters, and if
there are any, the entire argument is passed to the
system's command shell for parsing (this is "/bin/sh -c"
on Unix platforms, but varies on other platforms). If
there are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp", which
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is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument,
but want to lie to the program you are executing about
its own name, you can specify the program you actually
want to run as an "indirect object" (without a comma) in
front of the LIST. (This always forces interpretation
of the LIST as a multivalued list, even if there is only
a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell,
results are subject to its quirks and capabilities. See
"`STRING`" in perlop for details.
Using an indirect object with "exec" or "system" is also
more secure. This usage (which also works fine with
system()) forces interpretation of the arguments as a
multivalued list, even if the list had just one
argument. That way you're safe from the shell expanding
wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object,
ran the echo program, passing it "surprise" an argument.
The second version didn't; it tried to run a program
named "echo surprise", didn't find it, and set $? to a
non-zero value indicating failure.
Beginning with v5.6.0, Perl attempts to flush all files
opened for output before the exec, but this may not be
supported on some platforms (see perlport). To be safe,
you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open
handles to avoid lost output.
Note that "exec" will not call your "END" blocks, nor
will it invoke "DESTROY" methods on your objects.
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exists EXPR
Given an expression that specifies an element of a hash,
returns true if the specified element in the hash has
ever been initialized, even if the corresponding value
is undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
exists may also be called on array elements, but its
behavior is much less obvious, and is strongly tied to
the use of "delete" on arrays. Be aware that calling
exists on array values is deprecated and likely to be
removed in a future version of Perl.
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's
defined, and defined if it exists, but the reverse
doesn't necessarily hold true.
Given an expression that specifies the name of a
subroutine, returns true if the specified subroutine has
ever been declared, even if it is undefined. Mentioning
a subroutine name for exists or defined does not count
as declaring it. Note that a subroutine that does not
exist may still be callable: its package may have an
"AUTOLOAD" method that makes it spring into existence
the first time that it is called; see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash or array key
lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the mostly deeply nested array or hash will not
spring into existence just because its existence was
tested, any intervening ones will. Thus "$ref->{"A"}"
and "$ref->{"A"}->{"B"}" will spring into existence due
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to the existence test for the $key element above. This
happens anywhere the arrow operator is used, including
even here:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at
first--or even second--glance appear to be an lvalue
context may be fixed in a future release.
Use of a subroutine call, rather than a subroutine name,
as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit
Evaluates EXPR and exits immediately with that value.
Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0
status. The only universally recognized values for EXPR
are 0 for success and 1 for error; other values are
subject to interpretation depending on the environment
in which the Perl program is running. For example,
exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-
mail filter will cause the mailer to return the item
undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any
chance that someone might want to trap whatever error
happened. Use "die" instead, which can be trapped by an
"eval".
The exit() function does not always exit immediately.
It calls any defined "END" routines first, but these
"END" routines may not themselves abort the exit.
Likewise any object destructors that need to be called
are called before the real exit. If this is a problem,
you can call "POSIX:_exit($status)" to avoid END and
destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of
EXPR. If EXPR is omitted, gives "exp($_)".
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fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have
to say
use Fcntl;
first to get the correct constant definitions. Argument
processing and value returned work just like "ioctl"
below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from
"fcntl". Like "ioctl", it maps a 0 return from the
system call into "0 but true" in Perl. This string is
true in boolean context and 0 in numeric context. It is
also exempt from the normal -w warnings on improper
numeric conversions.
Note that "fcntl" raises an exception if used on a
machine that doesn't implement fcntl(2). See the Fcntl
module or your fcntl(2) manpage to learn what functions
are available on your system.
Here's an example of setting a filehandle named "REMOTE"
to be non-blocking at the system level. You'll have to
negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or
undefined if the filehandle is not open. This is mainly
useful for constructing bitmaps for "select" and low-
level POSIX tty-handling operations. If FILEHANDLE is
an expression, the value is taken as an indirect
filehandle, generally its name.
You can use this to find out whether two handles refer
to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
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(Filehandles connected to memory objects via new
features of "open" may return undefined even though they
are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE.
Returns true for success, false on failure. Produces a
fatal error if used on a machine that doesn't implement
flock(2), fcntl(2) locking, or lockf(3). "flock" is
Perl's portable file locking interface, although it
locks entire files only, not records.
Two potentially non-obvious but traditional "flock"
semantics are that it waits indefinitely until the lock
is granted, and that its locks merely advisory. Such
discretionary locks are more flexible, but offer fewer
guarantees. This means that programs that do not also
use "flock" may modify files locked with "flock". See
perlport, your port's specific documentation, or your
system-specific local manpages for details. It's best
to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as
always feel perfectly free to write for your own
system's idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN,
possibly combined with LOCK_NB. These constants are
traditionally valued 1, 2, 8 and 4, but you can use the
symbolic names if you import them from the Fcntl module,
either individually, or as a group using the ':flock'
tag. LOCK_SH requests a shared lock, LOCK_EX requests
an exclusive lock, and LOCK_UN releases a previously
requested lock. If LOCK_NB is bitwise-or'ed with
LOCK_SH or LOCK_EX then "flock" returns immediately
rather than blocking waiting for the lock; check the
return status to see if you got it.
To avoid the possibility of miscoordination, Perl now
flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't
provide shared locks, and it requires that FILEHANDLE be
open with write intent. These are the semantics that
lockf(3) implements. Most if not all systems implement
lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires
that FILEHANDLE be open with read intent to use LOCK_SH
and requires that it be open with write intent to use
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LOCK_EX.
Note also that some versions of "flock" cannot lock
things over the network; you would need to use the more
system-specific "fcntl" for that. If you like you can
force Perl to ignore your system's flock(2) function,
and so provide its own fcntl(2)-based emulation, by
passing the switch "-Ud_flock" to the Configure program
when you configure Perl.
Here's a mailbox appender for BSD systems.
use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
# and, in case someone appended while we were waiting...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);
On systems that support a real flock(2), locks are
inherited across fork() calls, whereas those that must
resort to the more capricious fcntl(2) function lose
their locks, making it seriously harder to write
servers.
See also DB_File for other flock() examples.
fork
Does a fork(2) system call to create a new process
running the same program at the same point. It returns
the child pid to the parent process, 0 to the child
process, or "undef" if the fork is unsuccessful. File
descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied. On most
systems supporting fork(), great care has gone into
making it extremely efficient (for example, using copy-
on-write technology on data pages), making it the
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dominant paradigm for multitasking over the last few
decades.
Beginning with v5.6.0, Perl attempts to flush all files
opened for output before forking the child process, but
this may not be supported on some platforms (see
perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()" method
of "IO::Handle" on any open handles to avoid duplicate
output.
If you "fork" without ever waiting on your children, you
will accumulate zombies. On some systems, you can avoid
this by setting $SIG{CHLD} to "IGNORE". See also
perlipc for more examples of forking and reaping
moribund children.
Note that if your forked child inherits system file
descriptors like STDIN and STDOUT that are actually
connected by a pipe or socket, even if you exit, then
the remote server (such as, say, a CGI script or a
backgrounded job launched from a remote shell) won't
think you're done. You should reopen those to /dev/null
if it's any issue.
format
Declare a picture format for use by the "write"
function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though
you may call it, too. It formats (see perlform) a list
of values according to the contents of PICTURE, placing
the output into the format output accumulator, $^A (or
$ACCUMULATOR in English). Eventually, when a "write" is
done, the contents of $^A are written to some
filehandle. You could also read $^A and then set $^A
back to "". Note that a format typically does one
"formline" per line of form, but the "formline" function
itself doesn't care how many newlines are embedded in
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the PICTURE. This means that the "~" and "~~" tokens
treat the entire PICTURE as a single line. You may
therefore need to use multiple formlines to implement a
single record format, just like the "format" compiler.
Be careful if you put double quotes around the picture,
because an "@" character may be taken to mean the
beginning of an array name. "formline" always returns
true. See perlform for other examples.
getc FILEHANDLE
getc
Returns the next character from the input file attached
to FILEHANDLE, or the undefined value at end of file or
if there was an error (in the latter case $! is set).
If FILEHANDLE is omitted, reads from STDIN. This is not
particularly efficient. However, it cannot be used by
itself to fetch single characters without waiting for
the user to hit enter. For that, try something more
like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";
Determination of whether $BSD_STYLE should be set is
left as an exercise to the reader.
The "POSIX::getattr" function can do this more portably
on systems purporting POSIX compliance. See also the
"Term::ReadKey" module from your nearest CPAN site;
details on CPAN can be found on "CPAN" in perlmodlib.
getlogin
This implements the C library function of the same name,
which on most systems returns the current login from
/etc/utmp, if any. If it returns the empty string, use
"getpwuid".
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$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not
as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the
SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID.
Use a PID of 0 to get the current process group for the
current process. Will raise an exception if used on a
machine that doesn't implement getpgrp(2). If PID is
omitted, returns process group of current process. Note
that the POSIX version of "getpgrp" does not accept a
PID argument, so only "PID==0" is truly portable.
getppid
Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions
"getpid()" and "getppid()" return different values from
different threads. In order to be portable, this
behavior is not reflected by the Perl-level function
"getppid()", that returns a consistent value across
threads. If you want to call the underlying "getppid()",
you may use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process
group, or a user. (See getpriority(2).) Will raise a
fatal exception if used on a machine that doesn't
implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
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getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines are the same as their counterparts in the
system C library. In list context, the return values
from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get an empty list.)
The exact meaning of the $gcos field varies but it
usually contains the real name of the user (as opposed
to the login name) and other information pertaining to
the user. Beware, however, that in many system users
are able to change this information and therefore it
cannot be trusted and therefore the $gcos is tainted
(see perlsec). The $passwd and $shell, user's encrypted
password and login shell, are also tainted, because of
the same reason.
In scalar context, you get the name, unless the function
was a lookup by name, in which case you get the other
thing, whatever it is. (If the entry doesn't exist you
get the undefined value.) For example:
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$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are
special in that they are unsupported on many systems.
If the $quota is unsupported, it is an empty scalar. If
it is supported, it usually encodes the disk quota. If
the $comment field is unsupported, it is an empty
scalar. If it is supported it usually encodes some
administrative comment about the user. In some systems
the $quota field may be $change or $age, fields that
have to do with password aging. In some systems the
$comment field may be $class. The $expire field, if
present, encodes the expiration period of the account or
the password. For the availability and the exact
meaning of these fields in your system, please consult
your getpwnam(3) documentation and your pwd.h file. You
can also find out from within Perl what your $quota and
$comment fields mean and whether you have the $expire
field by using the "Config" module and the values
"d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and
"d_pwexpire". Shadow password files are supported only
if your vendor has implemented them in the intuitive
fashion that calling the regular C library routines gets
the shadow versions if you're running under privilege or
if there exists the shadow(3) functions as found in
System V (this includes Solaris and Linux.) Those
systems that implement a proprietary shadow password
facility are unlikely to be supported.
The $members value returned by getgr*() is a space
separated list of the login names of the members of the
group.
For the gethost*() functions, if the "h_errno" variable
is supported in C, it will be returned to you via $? if
the function call fails. The @addrs value returned by a
successful call is a list of raw addresses returned by
the corresponding library call. In the Internet domain,
each address is four bytes long; you can unpack it by
saying something like:
($a,$b,$c,$d) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
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use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP
address you can write this:
use Socket;
$packed_ip = gethostbyname("www.perl.org");
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}
Make sure <gethostbyname()> is called in SCALAR context
and that its return value is checked for definedness.
If you get tired of remembering which element of the
return list contains which return value, by-name
interfaces are provided in standard modules:
"File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These override
the normal built-ins, supplying versions that return
objects with the appropriate names for each field. For
example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls
(uid), they aren't, because a "File::stat" object is
different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the
SOCKET connection, in case you don't know the address
because you have several different IPs that the
connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET
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at a given LEVEL. Options may exist at multiple
protocol levels depending on the socket type, but at
least the uppermost socket level SOL_SOCKET (defined in
the "Socket" module) will exist. To query options at
another level the protocol number of the appropriate
protocol controlling the option should be supplied. For
example, to indicate that an option is to be interpreted
by the TCP protocol, LEVEL should be set to the protocol
number of TCP, which you can get using "getprotobyname".
The function returns a packed string representing the
requested socket option, or "undef" on error, with the
reason for the error placed in $!). Just what is in the
packed string depends on LEVEL and OPTNAME; consult
getsockopt(2) for details. A common case is that the
option is an integer, in which case the result is a
packed integer, which you can decode using "unpack" with
the "i" (or "I") format.
An example to test whether Nagle's algorithm is turned
on on a socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "getsockopt TCP_NODELAY: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob
In list context, returns a (possibly empty) list of
filename expansions on the value of EXPR such as the
standard Unix shell /bin/csh would do. In scalar
context, glob iterates through such filename expansions,
returning undef when the list is exhausted. This is the
internal function implementing the "<*.c>" operator, but
you can use it directly. If EXPR is omitted, $_ is used.
The "<*.c>" operator is discussed in more detail in "I/O
Operators" in perlop.
Note that "glob" splits its arguments on whitespace and
treats each segment as separate pattern. As such,
"glob("*.c *.h")" matches all files with a .c or .h
extension. The expression "glob(".* *")" matchs all
files in the current working directory.
If non-empty braces are the only wildcard characters
used in the "glob", no filenames are matched, but
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potentially many strings are returned. For example,
this produces nine strings, one for each pairing of
fruits and colors:
@many = glob "{apple,tomato,cherry}={green,yellow,red}";
Beginning with v5.6.0, this operator is implemented
using the standard "File::Glob" extension. See
File::Glob for details, including "bsd_glob" which does
not treat whitespace as a pattern separator.
gmtime EXPR
gmtime
Works just like localtime but the returned values are
localized for the standard Greenwich time zone.
Note: when called in list context, $isdst, the last
value returned by gmtime is always 0. There is no
Daylight Saving Time in GMT.
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with
LABEL and resumes execution there. It can't be used to
get out of a block or subroutine given to "sort". It
can be used to go almost anywhere else within the
dynamic scope, including out of subroutines, but it's
usually better to use some other construct such as
"last" or "die". The author of Perl has never felt the
need to use this form of "goto" (in Perl, that is; C is
another matter). (The difference is that C does not
offer named loops combined with loop control. Perl
does, and this replaces most structured uses of "goto"
in other languages.)
The "goto-EXPR" form expects a label name, whose scope
will be resolved dynamically. This allows for computed
"goto"s per FORTRAN, but isn't necessarily recommended
if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
Use of "goto-LABEL" or "goto-EXPR" to jump into a
construct is deprecated and will issue a warning. Even
then, it may not be used to go into any construct that
requires initialization, such as a subroutine or a
"foreach" loop. It also can't be used to go into a
construct that is optimized away.
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The "goto-&NAME" form is quite different from the other
forms of "goto". In fact, it isn't a goto in the normal
sense at all, and doesn't have the stigma associated
with other gotos. Instead, it exits the current
subroutine (losing any changes set by local()) and
immediately calls in its place the named subroutine
using the current value of @_. This is used by
"AUTOLOAD" subroutines that wish to load another
subroutine and then pretend that the other subroutine
had been called in the first place (except that any
modifications to @_ in the current subroutine are
propagated to the other subroutine.) After the "goto",
not even "caller" will be able to tell that this routine
was called first.
NAME needn't be the name of a subroutine; it can be a
scalar variable containing a code reference, or a block
that evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as,
grep(1) and its relatives. In particular, it is not
limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value consisting of those elements for which the
expression evaluated to true. In scalar context,
returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be
used to modify the elements of the LIST. While this is
useful and supported, it can cause bizarre results if
the elements of LIST are not variables. Similarly, grep
returns aliases into the original list, much as a for
loop's index variable aliases the list elements. That
is, modifying an element of a list returned by grep (for
example, in a "foreach", "map" or another "grep")
actually modifies the element in the original list.
This is usually something to be avoided when writing
clear code.
If $_ is lexical in the scope where the "grep" appears
(because it has been declared with "my $_") then, in
addition to being locally aliased to the list elements,
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$_ keeps being lexical inside the block; i.e., it can't
be seen from the outside, avoiding any potential side-
effects.
See also "map" for a list composed of the results of the
BLOCK or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the
corresponding value. (To convert strings that might
start with either 0, "0x", or "0b", see "oct".) If EXPR
is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that
would cause integer overflow trigger a warning. Leading
whitespace is not stripped, unlike oct(). To present
something as hex, look into "printf", "sprintf", or
"unpack".
import LIST
There is no builtin "import" function. It is just an
ordinary method (subroutine) defined (or inherited) by
modules that wish to export names to another module.
The "use" function calls the "import" method for the
package used. See also "use", perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within
another, but without the wildcard-like behavior of a
full regular-expression pattern match. It returns the
position of the first occurrence of SUBSTR in STR at or
after POSITION. If POSITION is omitted, starts
searching from the beginning of the string. POSITION
before the beginning of the string or after its end is
treated as if it were the beginning or the end,
respectively. POSITION and the return value are based
at 0 (or whatever you've set the $[ variable to--but
don't do that). If the substring is not found, "index"
returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is
omitted, uses $_. You should not use this function for
rounding: one because it truncates towards 0, and two
because machine representations of floating-point
numbers can sometimes produce counterintuitive results.
For example, "int(-6.725/0.025)" produces -268 rather
than the correct -269; that's because it's really more
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like -268.99999999999994315658 instead. Usually, the
"sprintf", "printf", or the "POSIX::floor" and
"POSIX::ceil" functions will serve you better than will
int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first
have to say
require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If
sys/ioctl.ph doesn't exist or doesn't have the correct
definitions you'll have to roll your own, based on your
C header files such as <sys/ioctl.h>. (There is a Perl
script called h2ph that comes with the Perl kit that may
help you in this, but it's nontrivial.) SCALAR will be
read and/or written depending on the FUNCTION; a C
pointer to the string value of SCALAR will be passed as
the third argument of the actual "ioctl" call. (If
SCALAR has no string value but does have a numeric
value, that value will be passed rather than a pointer
to the string value. To guarantee this to be true, add
a 0 to the scalar before using it.) The "pack" and
"unpack" functions may be needed to manipulate the
values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure,
yet you can still easily determine the actual value
returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w
complaints about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string
with fields separated by the value of EXPR, and returns
that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a
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pattern as its first argument. Compare "split".
keys HASH
keys ARRAY
Returns a list consisting of all the keys of the named
hash, or the indices of an array. (In scalar context,
returns the number of keys or indices.)
The keys of a hash are returned in an apparently random
order. The actual random order is subject to change in
future versions of Perl, but it is guaranteed to be the
same order as either the "values" or "each" function
produces (given that the hash has not been modified).
Since Perl 5.8.1 the ordering is different even between
different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling keys() resets the HASH or
ARRAY's internal iterator (see "each"). In particular,
calling keys() in void context resets the iterator with
no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in
the hash, so modifying them will not affect the original
hash. Compare "values".
To sort a hash by value, you'll need to use a "sort"
function. Here's a descending numeric sort of a hash by
its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
Used as an lvalue, "keys" allows you to increase the
number of hash buckets allocated for the given hash.
This can gain you a measure of efficiency if you know
the hash is going to get big. (This is similar to pre-
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extending an array by assigning a larger number to
$#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for
it--256 of them, in fact, since it rounds up to the next
power of two. These buckets will be retained even if
you do "%hash = ()", use "undef %hash" if you want to
free the storage while %hash is still in scope. You
can't shrink the number of buckets allocated for the
hash using "keys" in this way (but you needn't worry
about doing this by accident, as trying has no effect).
"keys @array" in an lvalue context is a syntax error.
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the
number of processes successfully signaled (which is not
necessarily the same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process, but
"kill" checks whether it's possible to send a signal to
it (that means, to be brief, that the process is owned
by the same user, or we are the super-user). This is
useful to check that a child process is still alive
(even if only as a zombie) and hasn't changed its UID.
See perlport for notes on the portability of this
construct.
Unlike in the shell, if SIGNAL is negative, it kills
process groups instead of processes. That means you
usually want to use positive not negative signals. You
may also use a signal name in quotes.
The behavior of kill when a PROCESS number is zero or
negative depends on the operating system. For example,
on POSIX-conforming systems, zero will signal the
current process group and -1 will signal all processes.
See "Signals" in perlipc for more details.
last LABEL
last
The "last" command is like the "break" statement in C
(as used in loops); it immediately exits the loop in
question. If the LABEL is omitted, the command refers
to the innermost enclosing loop. The "continue" block,
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if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block that returns a
value such as "eval {}", "sub {}" or "do {}", and should
not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to
a loop that executes once. Thus "last" can be used to
effect an early exit out of such a block.
See also "continue" for an illustration of how "last",
"next", and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the
internal function implementing the "\L" escape in
double-quoted strings.
If EXPR is omitted, uses $_.
What gets returned depends on several factors:
If "use bytes" is in effect:
On EBCDIC platforms
The results are what the C language system call
"tolower()" returns.
On ASCII platforms
The results follow ASCII semantics. Only
characters "A-Z" change, to "a-z" respectively.
Otherwise, If EXPR has the UTF8 flag set
If the current package has a subroutine named
"ToLower", it will be used to change the case (See
"User-Defined Case Mappings" in perlunicode.)
Otherwise Unicode semantics are used for the case
change.
Otherwise, if "use locale" is in effect
Respects current LC_CTYPE locale. See perllocale.
Otherwise, if "use feature 'unicode_strings'" is in
effect:
Unicode semantics are used for the case change. Any
subroutine named "ToLower" will not be used.
Otherwise:
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On EBCDIC platforms
The results are what the C language system call
"tolower()" returns.
On ASCII platforms
ASCII semantics are used for the case change.
The lowercase of any character outside the ASCII
range is the character itself.
lcfirst EXPR
lcfirst
Returns the value of EXPR with the first character
lowercased. This is the internal function implementing
the "\l" escape in double-quoted strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma,
such as in a locale, as "lc" does.
length EXPR
length
Returns the length in characters of the value of EXPR.
If EXPR is omitted, returns length of $_. If EXPR is
undefined, returns "undef".
This function cannot be used on an entire array or hash
to find out how many elements these have. For that, use
"scalar @array" and "scalar keys %hash", respectively.
Like all Perl character operations, length() normally
deals in logical characters, not physical bytes. For
how many bytes a string encoded as UTF-8 would take up,
use "length(Encode::encode_utf8(EXPR))" (you'll have to
"use Encode" first). See Encode and perlunicode.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename.
Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen(2) system call does.
Returns true if it succeeded, false otherwise. See the
example in "Sockets: Client/Server Communication" in
perlipc.
local EXPR
You really probably want to be using "my" instead,
because "local" isn't what most people think of as
"local". See "Private Variables via my()" in perlsub
for details.
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A local modifies the listed variables to be local to the
enclosing block, file, or eval. If more than one value
is listed, the list must be placed in parentheses. See
"Temporary Values via local()" in perlsub for details,
including issues with tied arrays and hashes.
The "delete local EXPR" construct can also be used to
localize the deletion of array/hash elements to the
current block. See "Localized deletion of elements of
composite types" in perlsub.
localtime EXPR
localtime
Converts a time as returned by the time function to a
9-element list with the time analyzed for the local time
zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out of
the C `struct tm'. $sec, $min, and $hour are the
seconds, minutes, and hours of the specified time.
$mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January and
11 indicating December. This makes it easy to get a
month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just the
last two digits of the year. That is, $year is 123 in
year 2023. The proper way to get a 4-digit year is
simply:
$year += 1900;
Otherwise you create non-Y2K-compliant programs--and you
wouldn't want to do that, would you?
To get the last two digits of the year (e.g., '01' in
2001) do:
$year = sprintf("%02d", $year % 100);
$wday is the day of the week, with 0 indicating Sunday
and 3 indicating Wednesday. $yday is the day of the
year, in the range 0..364 (or 0..365 in leap years.)
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$isdst is true if the specified time occurs during
Daylight Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current time
(as returned by time(3)).
In scalar context, "localtime()" returns the ctime(3)
value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a Perl
builtin. For GMT instead of local time use the "gmtime"
builtin. See also the "Time::Local" module (to convert
the second, minutes, hours, ... back to the integer
value returned by time()), and the POSIX module's
strftime(3) and mktime(3) functions.
To get somewhat similar but locale dependent date
strings, set up your locale environment variables
appropriately (please see perllocale) and try for
example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day of
the week and the month of the year, may not necessarily
be three characters wide.
See "localtime" in perlport for portability concerns.
The Time::gmtime and Time::localtime modules provides a
convenient, by-name access mechanism to the gmtime() and
localtime() functions, respectively.
For a comprehensive date and time representation look at
the DateTime module on CPAN.
lock THING
This function places an advisory lock on a shared
variable, or referenced object contained in THING until
the lock goes out of scope.
lock() is a "weak keyword" : this means that if you've
defined a function by this name (before any calls to
it), that function will be called instead. If you are
not under "use threads::shared" this does nothing. See
threads::shared.
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log EXPR
log Returns the natural logarithm (base e) of EXPR. If EXPR
is omitted, returns log of $_. To get the log of
another base, use basic algebra: The base-N log of a
number is equal to the natural log of that number
divided by the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat EXPR
lstat
Does the same thing as the "stat" function (including
setting the special "_" filehandle) but stats a symbolic
link instead of the file the symbolic link points to.
If symbolic links are unimplemented on your system, a
normal "stat" is done. For much more detailed
information, please see the documentation for "stat".
If EXPR is omitted, stats $_.
m// The match operator. See "Regexp Quote-Like Operators"
in perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value composed of the results of each such
evaluation. In scalar context, returns the total number
of elements so generated. Evaluates BLOCK or EXPR in
list context, so each element of LIST may produce zero,
one, or more elements in the returned value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding
characters. And
%hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach (@array) {
$hash{get_a_key_for($_)} = $_;
}
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Note that $_ is an alias to the list value, so it can be
used to modify the elements of the LIST. While this is
useful and supported, it can cause bizarre results if
the elements of LIST are not variables. Using a regular
"foreach" loop for this purpose would be clearer in most
cases. See also "grep" for an array composed of those
items of the original list for which the BLOCK or EXPR
evaluates to true.
If $_ is lexical in the scope where the "map" appears
(because it has been declared with "my $_"), then, in
addition to being locally aliased to the list elements,
$_ keeps being lexical inside the block; that is, it
can't be seen from the outside, avoiding any potential
side-effects.
"{" starts both hash references and blocks, so "map {
..." could be either the start of map BLOCK LIST or map
EXPR, LIST. Because Perl doesn't look ahead for the
closing "}" it has to take a guess at which it's dealing
with based on what it finds just after the "{". Usually
it gets it right, but if it doesn't it won't realize
something is wrong until it gets to the "}" and
encounters the missing (or unexpected) comma. The syntax
error will be reported close to the "}", but you'll need
to change something near the "{" such as using a unary
"+" to give Perl some help:
%hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_" => 1) } @array # this also works
%hash = map { lc($_) => 1 } @array # as does this.
%hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
@hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
to get a list of anonymous hashes each with only one
entry apiece.
mkdir FILENAME,MASK
mkdir FILENAME
mkdir
Creates the directory specified by FILENAME, with
permissions specified by MASK (as modified by "umask").
If it succeeds it returns true, otherwise it returns
false and sets $! (errno). If omitted, MASK defaults to
0777. If omitted, FILENAME defaults to $_.
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In general, it is better to create directories with
permissive MASK, and let the user modify that with their
"umask", than it is to supply a restrictive MASK and
give the user no way to be more permissive. The
exceptions to this rule are when the file or directory
should be kept private (mail files, for instance). The
perlfunc(1) entry on "umask" discusses the choice of
MASK in more detail.
Note that according to the POSIX 1003.1-1996 the
FILENAME may have any number of trailing slashes. Some
operating and filesystems do not get this right, so Perl
automatically removes all trailing slashes to keep
everyone happy.
To recursively create a directory structure, look at the
"mkpath" function of the File::Path module.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD
is "IPC_STAT", then ARG must be a variable that will
hold the returned "msqid_ds" structure. Returns like
"ioctl": the undefined value for error, "0 but true" for
zero, or the actual return value otherwise. See also
"SysV IPC" in perlipc, "IPC::SysV", and "IPC::Semaphore"
documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the
message queue id, or the undefined value if there is an
error. See also "SysV IPC" in perlipc and "IPC::SysV"
and "IPC::Msg" documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a
message from message queue ID into variable VAR with a
maximum message size of SIZE. Note that when a message
is received, the message type as a native long integer
will be the first thing in VAR, followed by the actual
message. This packing may be opened with "unpack("l!
a*")". Taints the variable. Returns true if
successful, or false if there is an error. See also
"SysV IPC" in perlipc, "IPC::SysV", and "IPC::SysV::Msg"
documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the
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message MSG to the message queue ID. MSG must begin
with the native long integer message type, and be
followed by the length of the actual message, and
finally the message itself. This kind of packing can be
achieved with "pack("l! a*", $type, $message)". Returns
true if successful, or false if there is an error. See
also "IPC::SysV" and "IPC::SysV::Msg" documentation.
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
A "my" declares the listed variables to be local
(lexically) to the enclosing block, file, or "eval". If
more than one value is listed, the list must be placed
in parentheses.
The exact semantics and interface of TYPE and ATTRS are
still evolving. TYPE is currently bound to the use of
"fields" pragma, and attributes are handled using the
"attributes" pragma, or starting from Perl 5.8.0 also
via the "Attribute::Handlers" module. See "Private
Variables via my()" in perlsub for details, and fields,
attributes, and Attribute::Handlers.
next LABEL
next
The "next" command is like the "continue" statement in
C; it starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the above,
it would get executed even on discarded lines. If LABEL
is omitted, the command refers to the innermost
enclosing loop.
"next" cannot be used to exit a block which returns a
value such as "eval {}", "sub {}" or "do {}", and should
not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to
a loop that executes once. Thus "next" will exit such a
block early.
See also "continue" for an illustration of how "last",
"next", and "redo" work.
no MODULE VERSION LIST
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no MODULE VERSION
no MODULE LIST
no MODULE
no VERSION
See the "use" function, of which "no" is the opposite.
oct EXPR
oct Interprets EXPR as an octal string and returns the
corresponding value. (If EXPR happens to start off with
"0x", interprets it as a hex string. If EXPR starts off
with "0b", it is interpreted as a binary string.
Leading whitespace is ignored in all three cases.) The
following will handle decimal, binary, octal, and hex in
standard Perl notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way
(produce a number in octal), use sprintf() or printf():
$dec_perms = (stat("filename"))[2] & 07777;
$oct_perm_str = sprintf "%o", $perms;
The oct() function is commonly used when a string such
as 644 needs to be converted into a file mode, for
example. Although Perl automatically converts strings
into numbers as needed, this automatic conversion
assumes base 10.
Leading white space is ignored without warning, as too
are any trailing non-digits, such as a decimal point
("oct" only handles non-negative integers, not negative
integers or floating point).
open FILEHANDLE,EXPR
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE.
Simple examples to open a file for reading:
open(my $fh, '<', "input.txt") or die $!;
and for writing:
open(my $fh, '>', "output.txt") or die $!;
(The following is a comprehensive reference to open():
for a gentler introduction you may consider
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perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array
or hash element) the variable is assigned a reference to
a new anonymous filehandle, otherwise if FILEHANDLE is
an expression, its value is used as the name of the real
filehandle wanted. (This is considered a symbolic
reference, so "use strict 'refs'" should not be in
effect.)
If EXPR is omitted, the scalar variable of the same name
as the FILEHANDLE contains the filename. (Note that
lexical variables--those declared with "my"--will not
work for this purpose; so if you're using "my", specify
EXPR in your call to open.)
If three or more arguments are specified then the mode
of opening and the filename are separate. If MODE is '<'
or nothing, the file is opened for input. If MODE is
'>', the file is truncated and opened for output, being
created if necessary. If MODE is '>>', the file is
opened for appending, again being created if necessary.
You can put a '+' in front of the '>' or '<' to indicate
that you want both read and write access to the file;
thus '+<' is almost always preferred for read/write
updates--the '+>' mode would clobber the file first.
You can't usually use either read-write mode for
updating textfiles, since they have variable length
records. See the -i switch in perlrun for a better
approach. The file is created with permissions of 0666
modified by the process's "umask" value.
These various prefixes correspond to the fopen(3) modes
of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
In the two-argument (and one-argument) form of the call,
the mode and filename should be concatenated (in that
order), possibly separated by spaces. You may omit the
mode in these forms when that mode is '<'.
If the filename begins with '|', the filename is
interpreted as a command to which output is to be piped,
and if the filename ends with a '|', the filename is
interpreted as a command that pipes output to us. See
"Using open() for IPC" in perlipc for more examples of
this. (You are not allowed to "open" to a command that
pipes both in and out, but see IPC::Open2, IPC::Open3,
and "Bidirectional Communication with Another Process"
in perlipc for alternatives.)
For three or more arguments if MODE is '|-', the
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filename is interpreted as a command to which output is
to be piped, and if MODE is '-|', the filename is
interpreted as a command that pipes output to us. In
the two-argument (and one-argument) form, one should
replace dash ('-') with the command. See "Using open()
for IPC" in perlipc for more examples of this. (You are
not allowed to "open" to a command that pipes both in
and out, but see IPC::Open2, IPC::Open3, and
"Bidirectional Communication" in perlipc for
alternatives.)
In the form of pipe opens taking three or more
arguments, if LIST is specified (extra arguments after
the command name) then LIST becomes arguments to the
command invoked if the platform supports it. The
meaning of "open" with more than three arguments for
non-pipe modes is not yet defined, but experimental
"layers" may give extra LIST arguments meaning.
In the two-argument (and one-argument) form, opening
'<-' or '-' opens STDIN and opening '>-' opens STDOUT.
You may use the three-argument form of open to specify
I/O layers (sometimes referred to as "disciplines") to
apply to the handle that affect how the input and output
are processed (see open and PerlIO for more details).
For example:
open(my $fh, "<:encoding(UTF-8)", "filename")
|| die "can't open UTF-8 encoded filename: $!";
opens the UTF-8 encoded file containing Unicode
characters; see perluniintro. Note that if layers are
specified in the three-argument form, then default
layers stored in ${^OPEN} (see perlvar; usually set by
the open pragma or the switch -CioD) are ignored.
Open returns nonzero on success, the undefined value
otherwise. If the "open" involved a pipe, the return
value happens to be the pid of the subprocess.
If you're running Perl on a system that distinguishes
between text files and binary files, then you should
check out "binmode" for tips for dealing with this. The
key distinction between systems that need "binmode" and
those that don't is their text file formats. Systems
like Unix, Mac OS, and Plan 9, that end lines with a
single character and encode that character in C as "\n"
do not need "binmode". The rest need it.
When opening a file, it's seldom a good idea to continue
if the request failed, so "open" is frequently used with
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"die". Even if "die" won't do what you want (say, in a
CGI script, where you want to format a suitable error
message (but there are modules that can help with that
problem)) always check the return value from opening a
file.
As a special case the 3-arg form with a read/write mode
and the third argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file. Also
using "+<" works for symmetry, but you really should
consider writing something to the temporary file first.
You will need to seek() to do the reading.
Since v5.8.0, Perl has built using PerlIO by default.
Unless you've changed this (i.e., Configure
-Uuseperlio), you can open filehandles directly to Perl
scalars via:
open($fh, '>', \$variable) || ..
To (re)open "STDOUT" or "STDERR" as an in-memory file,
close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
General examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(my $dbase, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(my $dbase, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
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# in-memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will appear in $var
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
See perliol for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify an
EXPR beginning with '>&', in which case the rest of the
string is interpreted as the name of a filehandle (or
file descriptor, if numeric) to be duped (as dup(2)) and
opened. You may use "&" after ">", ">>", "<", "+>",
"+>>", and "+<". The mode you specify should match the
mode of the original filehandle. (Duping a filehandle
does not take into account any existing contents of IO
buffers.) If you use the 3-arg form then you can pass
either a number, the name of a filehandle or the normal
"reference to a glob".
Here is a script that saves, redirects, and restores
"STDOUT" and "STDERR" using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
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open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor
number or a filehandle, then Perl will do an equivalent
of C's "fdopen" of that file descriptor (and not call
dup(2)); this is more parsimonious of file descriptors.
For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful
(besides being parsimonious) for example when something
is dependent on file descriptors, like for example
locking using flock(). If you do just "open(A,
'>>&B')", the filehandle A will not have the same file
descriptor as B, and therefore flock(A) will not
flock(B), and vice versa. But with "open(A, '>>&=B')"
the filehandles will share the same file descriptor.
Note that if you are using Perls older than 5.8.0, Perl
will be using the standard C libraries' fdopen() to
implement the "=" functionality. On many Unix systems
fdopen() fails when file descriptors exceed a certain
value, typically 255. For Perls 5.8.0 and later, PerlIO
is most often the default.
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You can see whether Perl has been compiled with PerlIO
or not by running "perl -V" and looking for "useperlio="
line. If "useperlio" is "define", you have PerlIO,
otherwise you don't.
If you open a pipe on the command '-', i.e., either '|-'
or '-|' with 2-arguments (or 1-argument) form of open(),
then there is an implicit fork done, and the return
value of open is the pid of the child within the parent
process, and 0 within the child process. (Use
"defined($pid)" to determine whether the open was
successful.) The filehandle behaves normally for the
parent, but I/O to that filehandle is piped from/to the
STDOUT/STDIN of the child process. In the child
process, the filehandle isn't opened--I/O happens
from/to the new STDOUT/STDIN. Typically this is used
like the normal piped open when you want to exercise
more control over just how the pipe command gets
executed, such as when running setuid and you don't want
to have to scan shell commands for metacharacters.
The following triples are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as "list
form", which is not yet supported on all platforms. A
good rule of thumb is that if your platform has true
"fork()" (in other words, if your platform is Unix) you
can use the list form.
See "Safe Pipe Opens" in perlipc for more examples of
this.
Beginning with v5.6.0, Perl will attempt to flush all
files opened for output before any operation that may do
a fork, but this may not be supported on some platforms
(see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()" method
of "IO::Handle" on any open handles.
On systems that support a close-on-exec flag on files,
the flag will be set for the newly opened file
descriptor as determined by the value of $^F. See "$^F"
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in perlvar.
Closing any piped filehandle causes the parent process
to wait for the child to finish, and returns the status
value in $? and "${^CHILD_ERROR_NATIVE}".
The filename passed to 2-argument (or 1-argument) form
of open() will have leading and trailing whitespace
deleted, and the normal redirection characters honored.
This property, known as "magic open", can often be used
to good effect. A user could specify a filename of "rsh
cat file |", or you could change certain filenames as
needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary weird
characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and
trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems). One
should conscientiously choose between the magic and
3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the form
"rsh cat file |", but will not work on a filename that
happens to have a trailing space, while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your
system), then you should use the "sysopen" function,
which involves no such magic (but may use subtly
different filemodes than Perl open(), which is mapped to
C fopen()). This is another way to protect your
filenames from interpretation. For example:
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use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package (or
one of its subclasses, such as "IO::File" or
"IO::Socket"), you can generate anonymous filehandles
that have the scope of whatever variables hold
references to them, and automatically close whenever and
however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = IO::File->new;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See "seek" for some details about mixing reading and
writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
"readdir", "telldir", "seekdir", "rewinddir", and
"closedir". Returns true if successful. DIRHANDLE may
be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name. If
DIRHANDLE is an undefined scalar variable (or array or
hash element), the variable is assigned a reference to a
new anonymous dirhandle. DIRHANDLEs have their own
namespace separate from FILEHANDLEs.
See example at "readdir".
ord EXPR
ord Returns the numeric (the native 8-bit encoding, like
ASCII or EBCDIC, or Unicode) value of the first
character of EXPR. If EXPR is omitted, uses $_.
For the reverse, see "chr". See perlunicode for more
about Unicode.
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our EXPR
our TYPE EXPR
our EXPR : ATTRS
our TYPE EXPR : ATTRS
"our" associates a simple name with a package variable
in the current package for use within the current scope.
When "use strict 'vars'" is in effect, "our" lets you
use declared global variables without qualifying them
with package names, within the lexical scope of the
"our" declaration. In this way "our" differs from "use
vars", which is package scoped.
Unlike "my", which both allocates storage for a variable
and associates a simple name with that storage for use
within the current scope, "our" associates a simple name
with a package variable in the current package, for use
within the current scope. In other words, "our" has the
same scoping rules as "my", but does not necessarily
create a variable.
If more than one value is listed, the list must be
placed in parentheses.
our $foo;
our($bar, $baz);
An "our" declaration declares a global variable that
will be visible across its entire lexical scope, even
across package boundaries. The package in which the
variable is entered is determined at the point of the
declaration, not at the point of use. This means the
following behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the
same lexical scope are allowed if they are in different
packages. If they happen to be in the same package,
Perl will emit warnings if you have asked for them, just
like multiple "my" declarations. Unlike a second "my"
declaration, which will bind the name to a fresh
variable, a second "our" declaration in the same
package, in the same scope, is merely redundant.
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use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30
An "our" declaration may also have a list of attributes
associated with it.
The exact semantics and interface of TYPE and ATTRS are
still evolving. TYPE is currently bound to the use of
"fields" pragma, and attributes are handled using the
"attributes" pragma, or starting from Perl 5.8.0 also
via the "Attribute::Handlers" module. See "Private
Variables via my()" in perlsub for details, and fields,
attributes, and Attribute::Handlers.
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string
using the rules given by the TEMPLATE. The resulting
string is the concatenation of the converted values.
Typically, each converted value looks like its machine-
level representation. For example, on 32-bit machines
an integer may be represented by a sequence of 4 bytes,
which will in Perl be presented as a string that's 4
characters long.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the
order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
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S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Raises an exception otherwise.)
i A signed integer value.
I A unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in native format.
d A double-precision float in native format.
F A Perl internal floating-point value (NV) in native format
D A float of long-double precision in native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Raises an exception otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in character mode
and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
details). Its bytes represent an unsigned integer in base 128,
most significant digit first, with as few digits as possible. Bit
eight (the high bit) is set on each byte except the last.
x A null byte (a.k.a ASCII NUL, "\000", chr(0))
X Back up a byte.
@ Null-fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null-fill or truncate to absolute position specified by the value.
( Start of a ()-group.
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One or more modifiers below may optionally follow
certain letters in the TEMPLATE (the second column lists
letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
xX Make x and X act as alignment commands.
nNvV Treat integers as signed instead of unsigned.
@. Specify position as byte offset in the internal
representation of the packed string. Efficient but
dangerous.
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()"
groups to force a particular byte-order on all
components in that group, including all its subgroups.
The following rules apply:
o Each letter may optionally be followed by a number
indicating the repeat count. A numeric repeat count
may optionally be enclosed in brackets, as in
"pack("C[80]", @arr)". The repeat count gobbles
that many values from the LIST when used with all
format types other than "a", "A", "Z", "b", "B",
"h", "H", "@", ".", "x", "X", and "P", where it
means something else, dscribed below. Supplying a
"*" for the repeat count instead of a number means
to use however many items are left, except for:
o "@", "x", and "X", where it is equivalent to 0.
o <.>, where it means relative to the start of the
string.
o "u", where it is equivalent to 1 (or 45, which
here is equivalent).
One can replace a numeric repeat count with a
template letter enclosed in brackets to use the
packed byte length of the bracketed template for the
repeat count.
For example, the template "x[L]" skips as many bytes
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as in a packed long, and the template "$t X[$t] $t"
unpacks twice whatever $t (when variable-expanded)
unpacks. If the template in brackets contains
alignment commands (such as "x![d]"), its packed
length is calculated as if the start of the template
had the maximal possible alignment.
When used with "Z", a "*" as the repeat count is
guaranteed to add a trailing null byte, so the
resulting string is always one byte longer than the
byte length of the item itself.
When used with "@", the repeat count represents an
offset from the start of the innermost "()" group.
When used with ".", the repeat count determines the
starting position to calculate the value offset as
follows:
o If the repeat count is 0, it's relative to the
current position.
o If the repeat count is "*", the offset is
relative to the start of the packed string.
o And if it's an integer n, the offset is relative
to the start of the nth innermost "()" group, or
to the start of the string if n is bigger then
the group level.
The repeat count for "u" is interpreted as the
maximal number of bytes to encode per line of
output, with 0, 1 and 2 replaced by 45. The repeat
count should not be more than 65.
o The "a", "A", and "Z" types gobble just one value,
but pack it as a string of length count, padding
with nulls or spaces as needed. When unpacking, "A"
strips trailing whitespace and nulls, "Z" strips
everything after the first null, and "a" returns
data without any sort of trimming.
If the value to pack is too long, the result is
truncated. If it's too long and an explicit count
is provided, "Z" packs only "$count-1" bytes,
followed by a null byte. Thus "Z" always packs a
trailing null, except for when the count is 0.
o Likewise, the "b" and "B" formats pack a string
that's that many bits long. Each such format
generates 1 bit of the result.
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Each result bit is based on the least-significant
bit of the corresponding input character, i.e., on
"ord($char)%2". In particular, characters "0" and
"1" generate bits 0 and 1, as do characters "\000"
and "\001".
Starting from the beginning of the input string,
each 8-tuple of characters is converted to 1
character of output. With format "b", the first
character of the 8-tuple determines the least-
significant bit of a character; with format "B", it
determines the most-significant bit of a character.
If the length of the input string is not evenly
divisible by 8, the remainder is packed as if the
input string were padded by null characters at the
end. Similarly during unpacking, "extra" bits are
ignored.
If the input string is longer than needed, remaining
characters are ignored.
A "*" for the repeat count uses all characters of
the input field. On unpacking, bits are converted
to a string of "0"s and "1"s.
o The "h" and "H" formats pack a string that many
nybbles (4-bit groups, representable as hexadecimal
digits, "0".."9" "a".."f") long.
For each such format, pack() generates 4 bits of the
result. With non-alphabetical characters, the
result is based on the 4 least-significant bits of
the input character, i.e., on "ord($char)%16". In
particular, characters "0" and "1" generate nybbles
0 and 1, as do bytes "\0" and "\1". For characters
"a".."f" and "A".."F", the result is compatible with
the usual hexadecimal digits, so that "a" and "A"
both generate the nybble "0xa==10". Do not use any
characters but these with this format.
Starting from the beginning of the template to
pack(), each pair of characters is converted to 1
character of output. With format "h", the first
character of the pair determines the least-
significant nybble of the output character; with
format "H", it determines the most-significant
nybble.
If the length of the input string is not even, it
behaves as if padded by a null character at the end.
Similarly, "extra" nybbles are ignored during
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unpacking.
If the input string is longer than needed, extra
characters are ignored.
A "*" for the repeat count uses all characters of
the input field. For unpack(), nybbles are
converted to a string of hexadecimal digits.
o The "p" format packs a pointer to a null-terminated
string. You are responsible for ensuring that the
string is not a temporary value, as that could
potentially get deallocated before you got around to
using the packed result. The "P" format packs a
pointer to a structure of the size indicated by the
length. A null pointer is created if the
corresponding value for "p" or "P" is "undef";
similarly with unpack(), where a null pointer
unpacks into "undef".
If your system has a strange pointer size--meaning a
pointer is neither as big as an int nor as big as a
long--it may not be possible to pack or unpack
pointers in big- or little-endian byte order.
Attempting to do so raises an exception.
o The "/" template character allows packing and
unpacking of a sequence of items where the packed
structure contains a packed item count followed by
the packed items themselves. This is useful when
the structure you're unpacking has encoded the sizes
or repeat counts for some of its fields within the
structure itself as separate fields.
For "pack", you write length-item"/"sequence-item,
and the length-item describes how the length value
is packed. Formats likely to be of most use are
integer-packing ones like "n" for Java strings, "w"
for ASN.1 or SNMP, and "N" for Sun XDR.
For "pack", sequence-item may have a repeat count,
in which case the minimum of that and the number of
available items is used as the argument for length-
item. If it has no repeat count or uses a '*', the
number of available items is used.
For "unpack", an internal stack of integer arguments
unpacked so far is used. You write "/"sequence-item
and the repeat count is obtained by popping off the
last element from the stack. The sequence-item must
not have a repeat count.
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If sequence-item refers to a string type ("A", "a",
or "Z"), the length-item is the string length, not
the number of strings. With an explicit repeat
count for pack, the packed string is adjusted to
that length. For example:
unpack("W/a", "\04Gurusamy") gives ("Guru")
unpack("a3/A A*", "007 Bond J ") gives (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") gives ("Bond, J", ".")
pack("n/a* w/a","hello,","world") gives "\000\006hello,\005world"
pack("a/W2", ord("a") .. ord("z")) gives "2ab"
The length-item is not returned explicitly from
"unpack".
Supplying a count to the length-item format letter
is only useful with "A", "a", or "Z". Packing with
a length-item of "a" or "Z" may introduce "\000"
characters, which Perl does not regard as legal in
numeric strings.
o The integer types "s", "S", "l", and "L" may be
followed by a "!" modifier to specify native shorts
or longs. As shown in the example above, a bare "l"
means exactly 32 bits, although the native "long" as
seen by the local C compiler may be larger. This is
mainly an issue on 64-bit platforms. You can see
whether using "!" makes any difference this way:
printf "format s is %d, s! is %d\n",
length pack("s"), length pack("s!");
printf "format l is %d, l! is %d\n",
length pack("l"), length pack("l!");
"i!" and "I!" are also allowed, but only for
completeness' sake: they are identical to "i" and
"I".
The actual sizes (in bytes) of native shorts, ints,
longs, and long longs on the platform where Perl was
built are also available from the command line:
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';
or programmatically via the "Config" module:
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use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
$Config{longlongsize} is undefined on systems
without long long support.
o The integer formats "s", "S", "i", "I", "l", "L",
"j", and "J" are inherently non-portable between
processors and operating systems because they obey
native byteorder and endianness. For example, a
4-byte integer 0x12345678 (305419896 decimal) would
be ordered natively (arranged in and handled by the
CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, Intel and VAX CPUs are little-endian,
while everybody else, including Motorola m68k/88k,
PPC, Sparc, HP PA, Power, and Cray, are big-endian.
Alpha and MIPS can be either: Digital/Compaq
used/uses them in little-endian mode, but SGI/Cray
uses them in big-endian mode.
The names big-endian and little-endian are comic
references to the egg-eating habits of the little-
endian Lilliputians and the big-endian Blefuscudians
from the classic Jonathan Swift satire, Gulliver's
Travels. This entered computer lingo via the paper
"On Holy Wars and a Plea for Peace" by Danny Cohen,
USC/ISI IEN 137, April 1, 1980.
Some systems may have even weirder byte orders such
as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can determine your system endianness with this
incantation:
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built
is also available via Config:
use Config;
print "$Config{byteorder}\n";
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or from the command line:
$ perl -V:byteorder
Byteorders "1234" and "12345678" are little-endian;
"4321" and "87654321" are big-endian.
For portably packed integers, either use the formats
"n", "N", "v", and "V" or else use the ">" and "<"
modifiers described immediately below. See also
perlport.
o Starting with Perl 5.9.2, integer and floating-point
formats, along with the "p" and "P" formats and "()"
groups, may all be followed by the ">" or "<"
endianness modifiers to respectively enforce big- or
little-endian byte-order. These modifiers are
especially useful given how "n", "N", "v" and "V"
don't cover signed integers, 64-bit integers, or
floating-point values.
Here are some concerns to keep in mind when using
endianness modifier:
o Exchanging signed integers between different
platforms works only when all platforms store
them in the same format. Most platforms store
signed integers in two's-complement notation, so
usually this is not an issue.
o The ">" or "<" modifiers can only be used on
floating-point formats on big- or little-endian
machines. Otherwise, attempting to use them
raises an exception.
o Forcing big- or little-endian byte-order on
floating-point values for data exchange can work
only if all platforms use the same binary
representation such as IEEE floating-point.
Even if all platforms are using IEEE, there may
still be subtle differences. Being able to use
">" or "<" on floating-point values can be
useful, but also dangerous if you don't know
exactly what you're doing. It is not a general
way to portably store floating-point values.
o When using ">" or "<" on a "()" group, this
affects all types inside the group that accept
byte-order modifiers, including all subgroups.
It is silently ignored for all other types. You
are not allowed to override the byte-order
within a group that already has a byte-order
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modifier suffix.
o Real numbers (floats and doubles) are in native
machine format only. Due to the multiplicity of
floating-point formats and the lack of a standard
"network" representation for them, no facility for
interchange has been made. This means that packed
floating-point data written on one machine may not
be readable on another, even if both use IEEE
floating-point arithmetic (because the endianness of
the memory representation is not part of the IEEE
spec). See also perlport.
If you know exactly what you're doing, you can use
the ">" or "<" modifiers to force big- or little-
endian byte-order on floating-point values.
Because Perl uses doubles (or long doubles, if
configured) internally for all numeric calculation,
converting from double into float and thence to
double again loses precision, so "unpack("f",
pack("f", $foo)") will not in general equal $foo.
o Pack and unpack can operate in two modes: character
mode ("C0" mode) where the packed string is
processed per character, and UTF-8 mode ("U0" mode)
where the packed string is processed in its
UTF-8-encoded Unicode form on a byte-by-byte basis.
Character mode is the default unless the format
string starts with "U". You can always switch mode
mid-format with an explicit "C0" or "U0" in the
format. This mode remains in effect until the next
mode change, or until the end of the "()" group it
(directly) applies to.
o You must yourself do any alignment or padding by
inserting, for example, enough "x"es while packing.
There is no way for pack() and unpack() to know
where characters are going to or coming from, so
they handle their output and input as flat sequences
of characters.
o A "()" group is a sub-TEMPLATE enclosed in
parentheses. A group may take a repeat count either
as postfix, or for unpack(), also via the "/"
template character. Within each repetition of a
group, positioning with "@" starts over at 0.
Therefore, the result of
pack("@1A((@2A)@3A)", qw[X Y Z])
is the string "\0X\0\0YZ".
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o "x" and "X" accept the "!" modifier to act as
alignment commands: they jump forward or back to the
closest position aligned at a multiple of "count"
characters. For example, to pack() or unpack() a C
structure like
struct {
char c; /* one signed, 8-bit character */
double d;
char cc[2];
}
one may need to use the template "c x![d] d c[2]".
This assumes that doubles must be aligned to the
size of double.
For alignment commands, a "count" of 0 is equivalent
to a "count" of 1; both are no-ops.
o "n", "N", "v" and "V" accept the "!" modifier to
represent signed 16-/32-bit integers in
big-/little-endian order. This is portable only
when all platforms sharing packed data use the same
binary representation for signed integers; for
example, when all platforms use two's-complement
representation.
o Comments can be embedded in a TEMPLATE using "#"
through the end of line. White space can separate
pack codes from each other, but modifiers and repeat
counts must follow immediately. Breaking complex
templates into individual line-by-line components,
suitably annotated, can do as much to improve
legibility and maintainability of pack/unpack
formats as "/x" can for complicated pattern matches.
o If TEMPLATE requires more arguments that pack() is
given, pack() assumes additional "" arguments. If
TEMPLATE requires fewer arguments than given, extra
arguments are ignored.
Examples:
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$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the UTF-8
# bytes because the U at the start of the format caused a switch to
# U0-mode, so the UTF-8 bytes get joined into characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. On EBCDIC systems, the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
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# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same
The same template may generally also be used in
unpack().
package NAMESPACE VERSION
package NAMESPACE
Declares the compilation unit as being in the given
namespace. The scope of the package declaration is from
the declaration itself through the end of the enclosing
block, file, or eval (the same as the "my" operator).
All further unqualified dynamic identifiers will be in
this namespace. A package statement affects dynamic
variables only, including those you've used "local" on,
but not lexical variables, which are created with "my"
(or "our" (or "state")). Typically it would be the
first declaration in a file included by "require" or
"use". You can switch into a package in more than one
place, since this only determines which default symbol
table the compiler uses for the rest of that block. You
can refer to identifiers in other packages than the
current one by prefixing the identifier with the package
name and a double colon, as in $SomePack::var or
"ThatPack::INPUT_HANDLE". If package name is omitted,
the "main" package as assumed. That is, $::sail is
equivalent to $main::sail (as well as to "$main'sail",
still seen in ancient code, mostly from Perl 4).
If VERSION is provided, "package" sets the $VERSION
variable in the given namespace to a version object with
the VERSION provided. VERSION must be a "strict" style
version number as defined by the version module: a
positive decimal number (integer or decimal-fraction)
without exponentiation or else a dotted-decimal v-string
with a leading 'v' character and at least three
components. You should set $VERSION only once per
package.
See "Packages" in perlmod for more information about
packages, modules, and classes. See perlsub for other
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scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding
system call. Note that if you set up a loop of piped
processes, deadlock can occur unless you are very
careful. In addition, note that Perl's pipes use IO
buffering, so you may need to set $| to flush your
WRITEHANDLE after each command, depending on the
application.
See IPC::Open2, IPC::Open3, and "Bidirectional
Communication" in perlipc for examples of such things.
On systems that support a close-on-exec flag on files,
that flag is set on all newly opened file descriptors
whose "fileno"s are higher than the current value of $^F
(by default 2 for "STDERR"). See "$^F" in perlvar.
pop ARRAY
pop Pops and returns the last value of the array, shortening
the array by one element.
Returns the undefined value if the array is empty,
although this may also happen at other times. If ARRAY
is omitted, pops the @ARGV array in the main program,
but the @_ array in subroutines, just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search left
off for the variable in question ($_ is used when the
variable is not specified). Note that 0 is a valid
match offset. "undef" indicates that the search
position is reset (usually due to match failure, but can
also be because no match has yet been run on the
scalar). "pos" directly accesses the location used by
the regexp engine to store the offset, so assigning to
"pos" will change that offset, and so will also
influence the "\G" zero-width assertion in regular
expressions. Because a failed "m//gc" match doesn't
reset the offset, the return from "pos" won't change
either in this case. See perlre and perlop.
print FILEHANDLE LIST
print LIST
print
Prints a string or a list of strings. Returns true if
successful. FILEHANDLE may be a scalar variable
containing the name of or a reference to the filehandle,
thus introducing one level of indirection. (NOTE: If
FILEHANDLE is a variable and the next token is a term,
it may be misinterpreted as an operator unless you
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interpose a "+" or put parentheses around the
arguments.) If FILEHANDLE is omitted, prints to
standard output by default, or to the last selected
output channel; see "select". If LIST is also omitted,
prints $_ to the currently selected output handle. To
set the default output handle to something other than
STDOUT use the select operation. The current value of
$, (if any) is printed between each LIST item. The
current value of "$\" (if any) is printed after the
entire LIST has been printed. Because print takes a
LIST, anything in the LIST is evaluated in list context,
and any subroutine that you call will have one or more
of its expressions evaluated in list context. Also be
careful not to follow the print keyword with a left
parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print; put
parentheses around all the arguments (or interpose a
"+", but that doesn't look as good).
Note that if you're storing FILEHANDLEs in an array, or
if you're using any other expression more complex than a
scalar variable to retrieve it, you will have to use a
block returning the filehandle value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
Printing to a closed pipe or socket will generate a
SIGPIPE signal. See perlipc for more on signal
handling.
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)",
except that "$\" (the output record separator) is not
appended. The first argument of the list will be
interpreted as the "printf" format. See "sprintf" for an
explanation of the format argument. If "use locale" is
in effect, and POSIX::setlocale() has been called, the
character used for the decimal separator in formatted
floating-point numbers is affected by the LC_NUMERIC
locale. See perllocale and POSIX.
Don't fall into the trap of using a "printf" when a
simple "print" would do. The "print" is more efficient
and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string (or
"undef" if the function has no prototype). FUNCTION is
a reference to, or the name of, the function whose
prototype you want to retrieve.
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If FUNCTION is a string starting with "CORE::", the rest
is taken as a name for a Perl builtin. If the builtin
is not overridable (such as "qw//") or if its arguments
cannot be adequately expressed by a prototype (such as
"system"), prototype() returns "undef", because the
builtin does not really behave like a Perl function.
Otherwise, the string describing the equivalent
prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY. The length of ARRAY increases by
the length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the number of elements
in the array following the completed "push".
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See "Quote-Like Operators" in
perlop.
qr/STRING/
Regexp-like quote. See "Regexp Quote-Like Operators" in
perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word" characters
backslashed. (That is, all characters not matching
"/[A-Za-z_0-9]/" will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing the "\Q"
escape in double-quoted strings.
If EXPR is omitted, uses $_.
quotemeta (and "\Q" ... "\E") are useful when
interpolating strings into regular expressions, because
by default an interpolated variable will be considered a
mini-regular expression. For example:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};
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Will cause $sentence to become 'The big bad wolf jumped
over...'.
On the other hand:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};
Or:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};
Will both leave the sentence as is. Normally, when
accepting string input from the user, quotemeta() or
"\Q" must be used.
rand EXPR
rand
Returns a random fractional number greater than or equal
to 0 and less than the value of EXPR. (EXPR should be
positive.) If EXPR is omitted, the value 1 is used.
Currently EXPR with the value 0 is also special-cased as
1 (this was undocumented before Perl 5.8.0 and is
subject to change in future versions of Perl).
Automatically calls "srand" unless "srand" has already
been called. See also "srand".
Apply "int()" to the value returned by "rand()" if you
want random integers instead of random fractional
numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns
numbers that are too large or too small, then your
version of Perl was probably compiled with the wrong
number of RANDBITS.)
"rand()" is not cryptographically secure. You should
not rely on it in security-sensitive situations. As of
this writing, a number of third-party CPAN modules offer
random number generators intended by their authors to be
cryptographically secure, including:
Math::Random::Secure, Math::Random::MT::Perl, and
Math::TrulyRandom.
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read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable
SCALAR from the specified FILEHANDLE. Returns the
number of characters actually read, 0 at end of file, or
undef if there was an error (in the latter case $! is
also set). SCALAR will be grown or shrunk so that the
last character actually read is the last character of
the scalar after the read.
An OFFSET may be specified to place the read data at
some place in the string other than the beginning. A
negative OFFSET specifies placement at that many
characters counting backwards from the end of the
string. A positive OFFSET greater than the length of
SCALAR results in the string being padded to the
required size with "\0" bytes before the result of the
read is appended.
The call is implemented in terms of either Perl's or
your system's native fread(3) library function. To get
a true read(2) system call, see "sysread".
Note the characters: depending on the status of the
filehandle, either (8-bit) bytes or characters are read.
By default all filehandles operate on bytes, but for
example if the filehandle has been opened with the
":utf8" I/O layer (see "open", and the "open" pragma,
open), the I/O will operate on UTF-8 encoded Unicode
characters, not bytes. Similarly for the ":encoding"
pragma: in that case pretty much any characters can be
read.
readdir DIRHANDLE
Returns the next directory entry for a directory opened
by "opendir". If used in list context, returns all the
rest of the entries in the directory. If there are no
more entries, returns the undefined value in scalar
context and the empty list in list context.
If you're planning to filetest the return values out of
a "readdir", you'd better prepend the directory in
question. Otherwise, because we didn't "chdir" there,
it would have been testing the wrong file.
opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;
As of Perl 5.11.2 you can use a bare "readdir" in a
"while" loop, which will set $_ on every iteration.
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opendir(my $dh, $some_dir) || die;
while(readdir $dh) {
print "$some_dir/$_\n";
}
closedir $dh;
readline EXPR
readline
Reads from the filehandle whose typeglob is contained in
EXPR (or from *ARGV if EXPR is not provided). In scalar
context, each call reads and returns the next line until
end-of-file is reached, whereupon the subsequent call
returns "undef". In list context, reads until end-of-
file is reached and returns a list of lines. Note that
the notion of "line" used here is whatever you may have
defined with $/ or $INPUT_RECORD_SEPARATOR). See "$/"
in perlvar.
When $/ is set to "undef", when "readline" is in scalar
context (i.e., file slurp mode), and when an empty file
is read, it returns '' the first time, followed by
"undef" subsequently.
This is the internal function implementing the "<EXPR>"
operator, but you can use it directly. The "<EXPR>"
operator is discussed in more detail in "I/O Operators"
in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If "readline" encounters an operating system error, $!
will be set with the corresponding error message. It
can be helpful to check $! when you are reading from
filehandles you don't trust, such as a tty or a socket.
The following example uses the operator form of
"readline" and dies if the result is not defined.
while ( ! eof($fh) ) {
defined( $_ = <$fh> ) or die "readline failed: $!";
...
}
Note that you have can't handle "readline" errors that
way with the "ARGV" filehandle. In that case, you have
to open each element of @ARGV yourself since "eof"
handles "ARGV" differently.
foreach my $arg (@ARGV) {
open(my $fh, $arg) or warn "Can't open $arg: $!";
while ( ! eof($fh) ) {
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defined( $_ = <$fh> )
or die "readline failed for $arg: $!";
...
}
}
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links
are implemented. If not, raises an exception. If there
is a system error, returns the undefined value and sets
$! (errno). If EXPR is omitted, uses $_.
readpipe EXPR
readpipe
EXPR is executed as a system command. The collected
standard output of the command is returned. In scalar
context, it comes back as a single (potentially multi-
line) string. In list context, returns a list of lines
(however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR). This is the internal function
implementing the "qx/EXPR/" operator, but you can use it
directly. The "qx/EXPR/" operator is discussed in more
detail in "I/O Operators" in perlop. If EXPR is
omitted, uses $_.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive
LENGTH characters of data into variable SCALAR from the
specified SOCKET filehandle. SCALAR will be grown or
shrunk to the length actually read. Takes the same
flags as the system call of the same name. Returns the
address of the sender if SOCKET's protocol supports
this; returns an empty string otherwise. If there's an
error, returns the undefined value. This call is
actually implemented in terms of recvfrom(2) system
call. See "UDP: Message Passing" in perlipc for
examples.
Note the characters: depending on the status of the
socket, either (8-bit) bytes or characters are received.
By default all sockets operate on bytes, but for example
if the socket has been changed using binmode() to
operate with the ":encoding(utf8)" I/O layer (see the
"open" pragma, open), the I/O will operate on UTF-8
encoded Unicode characters, not bytes. Similarly for
the ":encoding" pragma: in that case pretty much any
characters can be read.
redo LABEL
redo
The "redo" command restarts the loop block without
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evaluating the conditional again. The "continue" block,
if any, is not executed. If the LABEL is omitted, the
command refers to the innermost enclosing loop.
Programs that want to lie to themselves about what was
just input normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block that returns a
value such as "eval {}", "sub {}" or "do {}", and should
not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to
a loop that executes once. Thus "redo" inside such a
block will effectively turn it into a looping construct.
See also "continue" for an illustration of how "last",
"next", and "redo" work.
ref EXPR
ref Returns a non-empty string if EXPR is a reference, the
empty string otherwise. If EXPR is not specified, $_
will be used. The value returned depends on the type of
thing the reference is a reference to. Builtin types
include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp
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If the referenced object has been blessed into a
package, then that package name is returned instead.
You can think of "ref" as a "typeof" operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
The return value "LVALUE" indicates a reference to an
lvalue that is not a variable. You get this from taking
the reference of function calls like "pos()" or
"substr()". "VSTRING" is returned if the reference
points to a version string.
The result "Regexp" indicates that the argument is a
regular expression resulting from "qr//".
See also perlref.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME
will be clobbered. Returns true for success, false
otherwise.
Behavior of this function varies wildly depending on
your system implementation. For example, it will
usually not work across file system boundaries, even
though the system mv command sometimes compensates for
this. Other restrictions include whether it works on
directories, open files, or pre-existing files. Check
perlport and either the rename(2) manpage or equivalent
system documentation for details.
For a platform independent "move" function look at the
File::Copy module.
require VERSION
require EXPR
require
Demands a version of Perl specified by VERSION, or
demands some semantics specified by EXPR or by $_ if
EXPR is not supplied.
VERSION may be either a numeric argument such as 5.006,
which will be compared to $], or a literal of the form
v5.6.1, which will be compared to $^V (aka
$PERL_VERSION). An exception is raised if VERSION is
greater than the version of the current Perl
interpreter. Compare with "use", which can do a similar
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check at compile time.
Specifying VERSION as a literal of the form v5.6.1
should generally be avoided, because it leads to
misleading error messages under earlier versions of Perl
that do not support this syntax. The equivalent numeric
version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, "require" demands that a library file be
included if it hasn't already been included. The file
is included via the do-FILE mechanism, which is
essentially just a variety of "eval" with the caveat
that lexical variables in the invoking script will be
invisible to the included code. Has semantics similar
to the following subroutine:
sub require {
my ($filename) = @_;
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
die "Compilation failed in require";
}
my ($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
if ($@) {
$INC{$filename} = undef;
die $@;
} elsif (!$result) {
delete $INC{$filename};
die "$filename did not return true value";
} else {
return $result;
}
}
Note that the file will not be included twice under the
same specified name.
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The file must return true as the last statement to
indicate successful execution of any initialization
code, so it's customary to end such a file with "1;"
unless you're sure it'll return true otherwise. But
it's better just to put the "1;", in case you add more
statements.
If EXPR is a bareword, the require assumes a ".pm"
extension and replaces "::" with "/" in the filename for
you, to make it easy to load standard modules. This
form of loading of modules does not risk altering your
namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the
"Foo/Bar.pm" file in the directories specified in the
@INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file
in the @INC array and will complain about not finding
"Foo::Bar" there. In this case you can do:
eval "require $class";
Now that you understand how "require" looks for files
with a bareword argument, there is a little extra
functionality going on behind the scenes. Before
"require" looks for a ".pm" extension, it will first
look for a similar filename with a ".pmc" extension. If
this file is found, it will be loaded in place of any
file ending in a ".pm" extension.
You can also insert hooks into the import facility, by
putting Perl code directly into the @INC array. There
are three forms of hooks: subroutine references, array
references and blessed objects.
Subroutine references are the simplest case. When the
inclusion system walks through @INC and encounters a
subroutine, this subroutine gets called with two
parameters, the first a reference to itself, and the
second the name of the file to be included (e.g.,
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"Foo/Bar.pm"). The subroutine should return either
nothing or else a list of up to three values in the
following order:
1. A filehandle, from which the file will be read.
2. A reference to a subroutine. If there is no
filehandle (previous item), then this subroutine is
expected to generate one line of source code per
call, writing the line into $_ and returning 1, then
returning 0 at end of file. If there is a
filehandle, then the subroutine will be called to
act as a simple source filter, with the line as read
in $_. Again, return 1 for each valid line, and 0
after all lines have been returned.
3. Optional state for the subroutine. The state is
passed in as $_[1]. A reference to the subroutine
itself is passed in as $_[0].
If an empty list, "undef", or nothing that matches the
first 3 values above is returned, then "require" looks
at the remaining elements of @INC. Note that this
filehandle must be a real filehandle (strictly a
typeglob or reference to a typeglob, blessed or
unblessed); tied filehandles will be ignored and return
value processing will stop there.
If the hook is an array reference, its first element
must be a subroutine reference. This subroutine is
called as above, but the first parameter is the array
reference. This lets you indirectly pass arguments to
the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
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If the hook is an object, it must provide an INC method
that will be called as above, the first parameter being
the object itself. (Note that you must fully qualify
the sub's name, as unqualified "INC" is always forced
into package "main".) Here is a typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, Foo->new(...);
These hooks are also permitted to set the %INC entry
corresponding to the files they have loaded. See "%INC"
in perlvar.
For a yet-more-powerful import facility, see "use" and
perlmod.
reset EXPR
reset
Generally used in a "continue" block at the end of a
loop to clear variables and reset "??" searches so that
they work again. The expression is interpreted as a
list of single characters (hyphens allowed for ranges).
All variables and arrays beginning with one of those
letters are reset to their pristine state. If the
expression is omitted, one-match searches ("?pattern?")
are reset to match again. Only resets variables or
searches in the current package. Always returns 1.
Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you'll wipe
out your @ARGV and @INC arrays and your %ENV hash.
Resets only package variables; lexical variables are
unaffected, but they clean themselves up on scope exit
anyway, so you'll probably want to use them instead.
See "my".
return EXPR
return
Returns from a subroutine, "eval", or "do FILE" with the
value given in EXPR. Evaluation of EXPR may be in list,
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scalar, or void context, depending on how the return
value will be used, and the context may vary from one
execution to the next (see "wantarray"). If no EXPR is
given, returns an empty list in list context, the
undefined value in scalar context, and (of course)
nothing at all in void context.
(In the absence of an explicit "return", a subroutine,
eval, or do FILE automatically returns the value of the
last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of the
elements of LIST in the opposite order. In scalar
context, concatenates the elements of LIST and returns a
string value with all characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world
print scalar reverse "dlrow ,", "olleH"; # Hello, world
Used without arguments in scalar context, reverse()
reverses $_.
$_ = "dlrow ,olleH";
print reverse; # No output, list context
print scalar reverse; # Hello, world
Note that reversing an array to itself (as in "@a =
reverse @a") will preserve non-existent elements
whenever possible, i.e., for non magical arrays or tied
arrays with "EXISTS" and "DELETE" methods.
This operator is also handy for inverting a hash,
although there are some caveats. If a value is
duplicated in the original hash, only one of those can
be represented as a key in the inverted hash. Also,
this has to unwind one hash and build a whole new one,
which may take some time on a large hash, such as from a
DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the
directory for the "readdir" routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the
position of the last occurrence of SUBSTR in STR. If
POSITION is specified, returns the last occurrence
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beginning at or before that position.
rmdir FILENAME
rmdir
Deletes the directory specified by FILENAME if that
directory is empty. If it succeeds it returns true,
otherwise it returns false and sets $! (errno). If
FILENAME is omitted, uses $_.
To remove a directory tree recursively ("rm -rf" on
Unix) look at the "rmtree" function of the File::Path
module.
s///
The substitution operator. See "Regexp Quote-Like
Operators" in perlop.
say FILEHANDLE LIST
say LIST
say Just like "print", but implicitly appends a newline.
"say LIST" is simply an abbreviation for "{ local $\ =
"\n"; print LIST }".
This keyword is available only when the "say" feature is
enabled: see feature.
scalar EXPR
Forces EXPR to be interpreted in scalar context and
returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression
to be interpolated in list context because in practice,
this is never needed. If you really wanted to do so,
however, you could use the construction "@{[ (some
expression) ]}", but usually a simple "(some
expression)" suffices.
Because "scalar" is a unary operator, if you
accidentally use for EXPR a parenthesized list, this
behaves as a scalar comma expression, evaluating all but
the last element in void context and returning the final
element evaluated in scalar context. This is seldom
what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
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&foo;
print(uc($bar),$baz);
See perlop for more details on unary operators and the
comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the "fseek" call
of "stdio". FILEHANDLE may be an expression whose value
gives the name of the filehandle. The values for WHENCE
are 0 to set the new position in bytes to POSITION, 1 to
set it to the current position plus POSITION, and 2 to
set it to EOF plus POSITION (typically negative). For
WHENCE you may use the constants "SEEK_SET", "SEEK_CUR",
and "SEEK_END" (start of the file, current position, end
of the file) from the Fcntl module. Returns 1 on
success, 0 otherwise.
Note the in bytes: even if the filehandle has been set
to operate on characters (for example by using the
":encoding(utf8)" open layer), tell() will return byte
offsets, not character offsets (because implementing
that would render seek() and tell() rather slow).
If you want to position the file for "sysread" or
"syswrite", don't use "seek", because buffering makes
its effect on the file's read-write position
unpredictable and non-portable. Use "sysseek" instead.
Due to the rules and rigors of ANSI C, on some systems
you have to do a seek whenever you switch between
reading and writing. Amongst other things, this may
have the effect of calling stdio's clearerr(3). A
WHENCE of 1 ("SEEK_CUR") is useful for not moving the
file position:
seek(TEST,0,1);
This is also useful for applications emulating "tail
-f". Once you hit EOF on your read and then sleep for a
while, you (probably) have to stick in a dummy seek() to
reset things. The "seek" doesn't change the position,
but it does clear the end-of-file condition on the
handle, so that the next "<FILE>" makes Perl try again
to read something. (We hope.)
If that doesn't work (some I/O implementations are
particularly cantankerous), you might need something
like this:
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for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine on
DIRHANDLE. POS must be a value returned by "telldir".
"seekdir" also has the same caveats about possible
directory compaction as the corresponding system library
routine.
select FILEHANDLE
select
Returns the currently selected filehandle. If
FILEHANDLE is supplied, sets the new current default
filehandle for output. This has two effects: first, a
"write" or a "print" without a filehandle will default
to this FILEHANDLE. Second, references to variables
related to output will refer to this output channel.
For example, if you have to set the top of form format
for more than one output channel, you might do the
following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the
name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as
objects with methods, preferring to write the last
example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) syscall with the bit masks
specified, which can be constructed using "fileno" and
"vec", along these lines:
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$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles, you may wish
to write a subroutine like this:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in
$timeleft, so calling select() in scalar context just
returns $nfound.
Any of the bit masks can also be undef. The timeout, if
specified, is in seconds, which may be fractional.
Note: not all implementations are capable of returning
the $timeleft. If not, they always return $timeleft
equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after signals
(say, SIGALRM) is implementation-dependent. See also
perlport for notes on the portability of "select".
On error, "select" behaves like select(2): it returns -1
and sets $!.
On some Unixes, select(2) may report a socket file
descriptor as "ready for reading" when no data is
available, and thus a subsequent read blocks. This can
be avoided if you always use O_NONBLOCK on the socket.
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See select(2) and fcntl(2) for further details.
WARNING: One should not attempt to mix buffered I/O
(like "read" or <FH>) with "select", except as permitted
by POSIX, and even then only on POSIX systems. You have
to use "sysread" instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl(2). You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD
is IPC_STAT or GETALL, then ARG must be a variable that
will hold the returned semid_ds structure or semaphore
value array. Returns like "ioctl": the undefined value
for error, ""0 but true"" for zero, or the actual return
value otherwise. The ARG must consist of a vector of
native short integers, which may be created with
"pack("s!",(0)x$nsem)". See also "SysV IPC" in perlipc,
"IPC::SysV", "IPC::Semaphore" documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget(2). Returns the
semaphore id, or the undefined value if there is an
error. See also "SysV IPC" in perlipc, "IPC::SysV",
"IPC::SysV::Semaphore" documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop(2) for semaphore
operations such as signalling and waiting. OPSTRING
must be a packed array of semop structures. Each semop
structure can be generated with "pack("s!3", $semnum,
$semop, $semflag)". The length of OPSTRING implies the
number of semaphore operations. Returns true if
successful, or false if there is an error. As an
example, the following code waits on semaphore $semnum
of semaphore id $semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See also
"SysV IPC" in perlipc, "IPC::SysV", and
"IPC::SysV::Semaphore" documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the
scalar MSG to the SOCKET filehandle. Takes the same
flags as the system call of the same name. On
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unconnected sockets, you must specify a destination to
send to, in which case it does a sendto(2) syscall.
Returns the number of characters sent, or the undefined
value on error. The sendmsg(2) syscall is currently
unimplemented. See "UDP: Message Passing" in perlipc
for examples.
Note the characters: depending on the status of the
socket, either (8-bit) bytes or characters are sent. By
default all sockets operate on bytes, but for example if
the socket has been changed using binmode() to operate
with the ":encoding(utf8)" I/O layer (see "open", or the
"open" pragma, open), the I/O will operate on UTF-8
encoded Unicode characters, not bytes. Similarly for
the ":encoding" pragma: in that case pretty much any
characters can be sent.
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0
for the current process. Raises an exception when used
on a machine that doesn't implement POSIX setpgid(2) or
BSD setpgrp(2). If the arguments are omitted, it
defaults to "0,0". Note that the BSD 4.2 version of
"setpgrp" does not accept any arguments, so only
"setpgrp(0,0)" is portable. See also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process
group, or a user. (See setpriority(2).) Raises an
exception when used on a machine that doesn't implement
setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if
there is an error. Use integer constants provided by
the "Socket" module for LEVEL and OPNAME. Values for
LEVEL can also be obtained from getprotobyname. OPTVAL
might either be a packed string or an integer. An
integer OPTVAL is shorthand for pack("i", OPTVAL).
An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
shift ARRAY
shift
Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down.
If there are no elements in the array, returns the
undefined value. If ARRAY is omitted, shifts the @_
array within the lexical scope of subroutines and
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formats, and the @ARGV array outside a subroutine and
also within the lexical scopes established by the "eval
STRING", "BEGIN {}", "INIT {}", "CHECK {}", "UNITCHECK
{}" and "END {}" constructs.
See also "unshift", "push", and "pop". "shift" and
"unshift" do the same thing to the left end of an array
that "pop" and "push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably
have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD
is "IPC_STAT", then ARG must be a variable that will
hold the returned "shmid_ds" structure. Returns like
ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise. See also
"SysV IPC" in perlipc and "IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the
shared memory segment id, or the undefined value if
there is an error. See also "SysV IPC" in perlipc and
"IPC::SysV" documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID
starting at position POS for size SIZE by attaching to
it, copying in/out, and detaching from it. When
reading, VAR must be a variable that will hold the data
read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are
written to fill out SIZE bytes. Return true if
successful, or false if there is an error. shmread()
taints the variable. See also "SysV IPC" in perlipc,
"IPC::SysV" documentation, and the "IPC::Shareable"
module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated
by HOW, which has the same interpretation as in the
syscall of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the
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other side you're done writing but not done reading, or
vice versa. It's also a more insistent form of close
because it also disables the file descriptor in any
forked copies in other processes.
Returns 1 for success; on error, returns "undef" if the
first argument is not a valid filehandle, or returns 0
and sets $! for any other failure.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If
EXPR is omitted, returns sine of $_.
For the inverse sine operation, you may use the
"Math::Trig::asin" function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep
Causes the script to sleep for (integer) EXPR seconds,
or forever if no argument is given. Returns the integer
number of seconds actually slept.
May be interrupted if the process receives a signal such
as "SIGALRM".
eval {
local $SIG{ALARM} = sub { die "Alarm!\n" };
sleep;
};
die $@ unless $@ eq "Alarm!\n";
You probably cannot mix "alarm" and "sleep" calls,
because "sleep" is often implemented using "alarm".
On some older systems, it may sleep up to a full second
less than what you requested, depending on how it counts
seconds. Most modern systems always sleep the full
amount. They may appear to sleep longer than that,
however, because your process might not be scheduled
right away in a busy multitasking system.
For delays of finer granularity than one second, the
Time::HiRes module (from CPAN, and starting from Perl
5.8 part of the standard distribution) provides
usleep(). You may also use Perl's four-argument version
of select() leaving the first three arguments undefined,
or you might be able to use the "syscall" interface to
access setitimer(2) if your system supports it. See
perlfaq8 for details.
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See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are
specified the same as for the syscall of the same name.
You should "use Socket" first to get the proper
definitions imported. See the examples in "Sockets:
Client/Server Communication" in perlipc.
On systems that support a close-on-exec flag on files,
the flag will be set for the newly opened file
descriptor, as determined by the value of $^F. See
"$^F" in perlvar.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified
domain, of the specified type. DOMAIN, TYPE, and
PROTOCOL are specified the same as for the syscall of
the same name. If unimplemented, raises an exception.
Returns true if successful.
On systems that support a close-on-exec flag on files,
the flag will be set for the newly opened file
descriptors, as determined by the value of $^F. See
"$^F" in perlvar.
Some systems defined "pipe" in terms of "socketpair", in
which a call to "pipe(Rdr, Wtr)" is essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8
and later will emulate socketpair using IP sockets to
localhost if your system implements sockets but not
socketpair.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the
sorted list value. In scalar context, the behaviour of
"sort()" is undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard
string comparison order. If SUBNAME is specified, it
gives the name of a subroutine that returns an integer
less than, equal to, or greater than 0, depending on how
the elements of the list are to be ordered. (The "<=>"
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and "cmp" operators are extremely useful in such
routines.) SUBNAME may be a scalar variable name
(unsubscripted), in which case the value provides the
name of (or a reference to) the actual subroutine to
use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.
If the subroutine's prototype is "($$)", the elements to
be compared are passed by reference in @_, as for a
normal subroutine. This is slower than unprototyped
subroutines, where the elements to be compared are
passed into the subroutine as the package global
variables $a and $b (see example below). Note that in
the latter case, it is usually counter-productive to
declare $a and $b as lexicals.
The values to be compared are always passed by reference
and should not be modified.
You also cannot exit out of the sort block or subroutine
using any of the loop control operators described in
perlsyn or with "goto".
When "use locale" is in effect, "sort LIST" sorts LIST
according to the current collation locale. See
perllocale.
sort() returns aliases into the original list, much as a
for loop's index variable aliases the list elements.
That is, modifying an element of a list returned by
sort() (for example, in a "foreach", "map" or "grep")
actually modifies the element in the original list.
This is usually something to be avoided when writing
clear code.
Perl 5.6 and earlier used a quicksort algorithm to
implement sort. That algorithm was not stable, and
could go quadratic. (A stable sort preserves the input
order of elements that compare equal. Although
quicksort's run time is O(NlogN) when averaged over all
arrays of length N, the time can be O(N**2), quadratic
behavior, for some inputs.) In 5.7, the quicksort
implementation was replaced with a stable mergesort
algorithm whose worst-case behavior is O(NlogN). But
benchmarks indicated that for some inputs, on some
platforms, the original quicksort was faster. 5.8 has a
sort pragma for limited control of the sort. Its rather
blunt control of the underlying algorithm may not
persist into future Perls, but the ability to
characterize the input or output in implementation
independent ways quite probably will. See the sort
pragma.
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Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
my @new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
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# for speed
my @nums = @caps = ();
for (@old) {
push @nums, ( /=(\d+)/ ? $1 : undef );
push @caps, uc($_);
}
my @new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
Warning: syntactical care is required when sorting the
list returned from a function. If you want to sort the
list returned by the function call "find_records(@key)",
you can use:
@contact = sort { $a cmp $b } find_records @key;
@contact = sort +find_records(@key);
@contact = sort &find_records(@key);
@contact = sort(find_records(@key));
If instead you want to sort the array @key with the
comparison routine "find_records()" then you can use:
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@contact = sort { find_records() } @key;
@contact = sort find_records(@key);
@contact = sort(find_records @key);
@contact = sort(find_records (@key));
If you're using strict, you must not declare $a and $b
as lexicals. They are package globals. That means that
if you're in the "main" package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a and
$::b), but if you're in the "FooPack" package, it's the
same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it
returns inconsistent results (sometimes saying $x[1] is
less than $x[2] and sometimes saying the opposite, for
example) the results are not well-defined.
Because "<=>" returns "undef" when either operand is
"NaN" (not-a-number), and because "sort" raises an
exception unless the result of a comparison is defined,
when sorting with a comparison function like "$a <=>
$b", be careful about lists that might contain a "NaN".
The following example takes advantage that "NaN != NaN"
to eliminate any "NaN"s from the input list.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH
from an array, and replaces them with the elements of
LIST, if any. In list context, returns the elements
removed from the array. In scalar context, returns the
last element removed, or "undef" if no elements are
removed. The array grows or shrinks as necessary. If
OFFSET is negative then it starts that far from the end
of the array. If LENGTH is omitted, removes everything
from OFFSET onward. If LENGTH is negative, removes the
elements from OFFSET onward except for -LENGTH elements
at the end of the array. If both OFFSET and LENGTH are
omitted, removes everything. If OFFSET is past the end
of the array, Perl issues a warning, and splices at the
end of the array.
The following equivalences hold (assuming "$[ == 0 and
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$#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before
arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split
Splits the string EXPR into a list of strings and
returns that list. By default, empty leading fields are
preserved, and empty trailing ones are deleted. (If all
fields are empty, they are considered to be trailing.)
In scalar context, returns the number of fields found.
If EXPR is omitted, splits the $_ string. If PATTERN is
also omitted, splits on whitespace (after skipping any
leading whitespace). Anything matching PATTERN is taken
to be a delimiter separating the fields. (Note that the
delimiter may be longer than one character.)
If LIMIT is specified and positive, it represents the
maximum number of fields the EXPR will be split into,
though the actual number of fields returned depends on
the number of times PATTERN matches within EXPR. If
LIMIT is unspecified or zero, trailing null fields are
stripped (which potential users of "pop" would do well
to remember). If LIMIT is negative, it is treated as if
an arbitrarily large LIMIT had been specified. Note
that splitting an EXPR that evaluates to the empty
string always returns the empty list, regardless of the
LIMIT specified.
A pattern matching the empty string (not to be confused
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with an empty pattern "//", which is just one member of
the set of patterns matching the epmty string), splits
EXPR into individual characters. For example:
print join(':', split(/ */, 'hi there')), "\n";
produces the output 'h:i:t:h:e:r:e'.
As a special case for "split", the empty pattern "//"
specifically matches the empty string; this is not be
confused with the normal use of an empty pattern to mean
the last successful match. So to split a string into
individual characters, the following:
print join(':', split(//, 'hi there')), "\n";
produces the output 'h:i: :t:h:e:r:e'.
Empty leading fields are produced when there are
positive-width matches at the beginning of the string; a
zero-width match at the beginning of the string does not
produce an empty field. For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'. Empty trailing
fields, on the other hand, are produced when there is a
match at the end of the string (and when LIMIT is given
and is not 0), regardless of the length of the match.
For example:
print join(':', split(//, 'hi there!', -1)), "\n";
print join(':', split(/\W/, 'hi there!', -1)), "\n";
produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:',
respectively, both with an empty trailing field.
The LIMIT parameter can be used to split a line
partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or zero,
Perl supplies a LIMIT one larger than the number of
variables in the list, to avoid unnecessary work. For
the list above LIMIT would have been 4 by default. In
time critical applications it behooves you not to split
into more fields than you really need.
If the PATTERN contains parentheses, additional list
elements are created from each matching substring in the
delimiter.
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split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email
message in $header, you could split it up into fields
and their values this way:
$header =~ s/\n(?=\s)//g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an
expression to specify patterns that vary at runtime.
(To do runtime compilation only once, use
"/$variable/o".)
As a special case, specifying a PATTERN of space (' ')
will split on white space just as "split" with no
arguments does. Thus, "split(' ')" can be used to
emulate awk's default behavior, whereas "split(/ /)"
will give you as many initial null fields (empty string)
as there are leading spaces. A "split" on "/\s+/" is
like a "split(' ')" except that any leading whitespace
produces a null first field. A "split" with no
arguments really does a "split(' ', $_)" internally.
A PATTERN of "/^/" is treated as if it were "/^/m",
since it isn't much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing
parentheses that are not matched in a "split()" will be
set to "undef" when returned:
@fields = split /(A)|B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf"
conventions of the C library function "sprintf". See
below for more details and see sprintf(3) or printf(3)
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on your system for an explanation of the general
principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting: it emulates the
C function sprintf(3), but doesn't use it except for
floating-point numbers, and even then only standard
modifiers are allowed. Non-standard extensions in your
local sprintf(3) are therefore unavailable from Perl.
Unlike "printf", "sprintf" does not do what you probably
mean when you pass it an array as your first argument.
The array is given scalar context, and instead of using
the 0th element of the array as the format, Perl will
use the count of elements in the array as the format,
which is almost never useful.
Perl's "sprintf" permits the following universally-known
conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported
conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward")
compatibility, Perl permits these unnecessary but
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widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the
scientific notation produced by %e, %E, %g and %G for
numbers with the modulus of the exponent less than 100
is system-dependent: it may be three or less (zero-
padded as necessary). In other words, 1.23 times ten to
the 99th may be either "1.23e99" or "1.23e099".
Between the "%" and the format letter, you may specify
several additional attributes controlling the
interpretation of the format. In order, these are:
format parameter index
An explicit format parameter index, such as "2$". By
default sprintf will format the next unused argument
in the list, but this allows you to take the
arguments out of order:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix non-negative number with a space
+ prefix non-negative number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# ensure the leading "0" for any octal,
prefix non-zero hexadecimal with "0x" or "0X",
prefix non-zero binary with "0b" or "0B"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#o>', 12; # prints "<014>"
printf '<%#x>', 12; # prints "<0xc>"
printf '<%#X>', 12; # prints "<0XC>"
printf '<%#b>', 12; # prints "<0b1100>"
printf '<%#B>', 12; # prints "<0B1100>"
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When a space and a plus sign are given as the flags
at once, a plus sign is used to prefix a positive
number.
printf '<%+ d>', 12; # prints "<+12>"
printf '<% +d>', 12; # prints "<+12>"
When the # flag and a precision are given in the %o
conversion, the precision is incremented if it's
necessary for the leading "0".
printf '<%#.5o>', 012; # prints "<00012>"
printf '<%#.5o>', 012345; # prints "<012345>"
printf '<%#.0o>', 0; # prints "<0>"
vector flag
This flag tells Perl to interpret the supplied
string as a vector of integers, one for each
character in the string. Perl applies the format to
each integer in turn, then joins the resulting
strings with a separator (a dot "." by default).
This can be useful for displaying ordinal values of
characters in arbitrary strings:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override the
string to use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number
to use for the join string using something like
"*2$v"; for example:
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
(minimum) width
Arguments are usually formatted to be only as wide
as required to display the given value. You can
override the width by putting a number here, or get
the width from the next argument (with "*") or from
a specified argument (e.g., with "*2$"):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
printf '<%*s>', 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf '<%2s>', "long"; # prints "<long>" (does not truncate)
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If a field width obtained through "*" is negative,
it has the same effect as the "-" flag: left-
justification.
precision, or maximum width
You can specify a precision (for numeric
conversions) or a maximum width (for string
conversions) by specifying a "." followed by a
number. For floating-point formats except 'g' and
'G', this specifies how many places right of the
decimal point to show (the default being 6). For
example:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For "g" and "G", this specifies the maximum number
of digits to show, including thoe prior to the
decimal point and those after it; for example:
# These examples are subject to system-specific variation.
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision
implies that the output of the number itself should
be zero-padded to this width, where the 0 flag is
ignored:
printf '<%.6d>', 1; # prints "<000001>"
printf '<%+.6d>', 1; # prints "<+000001>"
printf '<%-10.6d>', 1; # prints "<000001 >"
printf '<%10.6d>', 1; # prints "< 000001>"
printf '<%010.6d>', 1; # prints "< 000001>"
printf '<%+10.6d>', 1; # prints "< +000001>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
printf '<%10.6x>', 1; # prints "< 000001>"
printf '<%010.6x>', 1; # prints "< 000001>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"
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For string conversions, specifying a precision
truncates the string to fit the specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next
argument using ".*":
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
If a precision obtained through "*" is negative, it
counts as having no precision at all.
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*s>', 0, "string"; # prints "<>"
printf '<%.*s>', -1, "string"; # prints "<string>"
printf '<%.*d>', 1, 0; # prints "<0>"
printf '<%.*d>', 0, 0; # prints "<>"
printf '<%.*d>', -1, 0; # prints "<0>"
You cannot currently get the precision from a
specified number, but it is intended that this will
be possible in the future, for example using ".*2$":
printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
size
For numeric conversions, you can specify the size to
interpret the number as using "l", "h", "V", "q",
"L", or "ll". For integer conversions ("d u o x X b
i D U O"), numbers are usually assumed to be
whatever the default integer size is on your
platform (usually 32 or 64 bits), but you can
override this to use instead one of the standard C
types, as supported by the compiler used to build
Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will raise an exception if Perl does not
understand "quads" in your installation. (This
requires either that the platform natively support
quads, or that Perl were specifically compiled to
support quads.) You can find out whether your Perl
supports quads via Config:
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use Config;
if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
print "Nice quads!\n";
}
For floating-point conversions ("e f g E F G"),
numbers are usually assumed to be the default
floating-point size on your platform (double or long
double), but you can force "long double" with "q",
"L", or "ll" if your platform supports them. You can
find out whether your Perl supports long doubles via
Config:
use Config;
print "long doubles\n" if $Config{d_longdbl} eq "define";
You can find out whether Perl considers "long
double" to be the default floating-point size to use
on your platform via Config:
use Config;
if ($Config{uselongdouble} eq "define") {
print "long doubles by default\n";
}
It can also be that long doubles and doubles are the
same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl code,
but is supported for compatibility with XS code. It
means "use the standard size for a Perl integer or
floating-point number", which is the default.
order of arguments
Normally, sprintf() takes the next unused argument
as the value to format for each format
specification. If the format specification uses "*"
to require additional arguments, these are consumed
from the argument list in the order they appear in
the format specification before the value to format.
Where an argument is specified by an explicit index,
this does not affect the normal order for the
arguments, even when the explicitly specified index
would have been the next argument.
So:
printf "<%*.*s>", $a, $b, $c;
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uses $a for the width, $b for the precision, and $c
as the value to format; while:
printf "<%*1$.*s>", $a, $b;
would use $a for the width and precision, and $b as
the value to format.
Here are some more examples; be aware that when
using an explicit index, the "$" may need escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If "use locale" is in effect and POSIX::setlocale() has
been called, the character used for the decimal
separator in formatted floating-point numbers is
affected by the LC_NUMERIC locale. See perllocale and
POSIX.
sqrt EXPR
sqrt
Return the positive square root of EXPR. If EXPR is
omitted, uses $_. Works only for non-negative operands
unless you've loaded the "Math::Complex" module.
use Math::Complex;
print sqrt(-4); # prints 2i
srand EXPR
srand
Sets the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand"
function so that "rand" can produce a different sequence
each time you run your program.
If srand() is not called explicitly, it is called
implicitly at the first use of the "rand" operator.
However, this was not true of versions of Perl before
5.004, so if your script will run under older Perl
versions, it should call "srand".
Most programs won't even call srand() at all, except
those that need a cryptographically-strong starting
point rather than the generally acceptable default,
which is based on time of day, process ID, and memory
allocation, or the /dev/urandom device if available. You
may also want to call srand() after a fork() to avoid
child processes sharing the same seed value as the
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parent (and consequently each other).
You can call srand($seed) with the same $seed to
reproduce the same sequence from rand(), but this is
usually reserved for generating predictable results for
testing or debugging. Otherwise, don't call srand()
more than once in your program.
Do not call srand() (i.e., without an argument) more
than once per process. The internal state of the random
number generator should contain more entropy than can be
provided by any seed, so calling srand() again actually
loses randomness.
Most implementations of "srand" take an integer and will
silently truncate decimal numbers. This means
"srand(42)" will usually produce the same results as
"srand(42.1)". To be safe, always pass "srand" an
integer.
In versions of Perl prior to 5.004 the default seed was
just the current "time". This isn't a particularly good
seed, so many old programs supply their own seed value
(often "time ^ $$" or "time ^ ($$ + ($$ << 15))"), but
that isn't necessary any more.
For cryptographic purposes, however, you need something
much more random than the default seed. Checksumming
the compressed output of one or more rapidly changing
operating system status programs is the usual method.
For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);
If you're particularly concerned with this, search the
CPAN for random number generator modules instead of
rolling out your own.
Frequently called programs (like CGI scripts) that
simply use
time ^ $$
for a seed can fall prey to the mathematical property
that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
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stat DIRHANDLE
stat
Returns a 13-element list giving the status info for a
file, either the file opened via FILEHANDLE or
DIRHANDLE, or named by EXPR. If EXPR is omitted, it
stats $_. Returns the empty list if "stat" fails.
Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types.
Here are the meanings of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem
types. Notably, the ctime field is non-portable. In
particular, you cannot expect it to be a "creation
time", see "Files and Filesystems" in perlport for
details.
If "stat" is passed the special filehandle consisting of
an underline, no stat is done, but the current contents
of the stat structure from the last "stat", "lstat", or
filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number
is negative under NFS.)
Because the mode contains both the file type and its
permissions, you should mask off the file type portion
and (s)printf using a "%o" if you want to see the real
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permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value
indicating success or failure, and, if successful, sets
the information associated with the special filehandle
"_".
The File::stat module provides a convenient, by-name
access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and
functions ("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_directory = S_ISDIR($mode);
You could write the last two using the "-u" and "-d"
operators. Commonly available "S_IF*" constants are:
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
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# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the "S_IF*" functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with (for example) S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for
more details about the "S_*" constants. To get status
info for a symbolic link instead of the target file
behind the link, use the "lstat" function.
state EXPR
state TYPE EXPR
state EXPR : ATTRS
state TYPE EXPR : ATTRS
"state" declares a lexically scoped variable, just like
"my" does. However, those variables will never be
reinitialized, contrary to lexical variables that are
reinitialized each time their enclosing block is
entered.
"state" variables are enabled only when the "use feature
"state"" pragma is in effect. See feature.
study SCALAR
study
Takes extra time to study SCALAR ($_ if unspecified) in
anticipation of doing many pattern matches on the string
before it is next modified. This may or may not save
time, depending on the nature and number of patterns you
are searching on, and on the distribution of character
frequencies in the string to be searched; you probably
want to compare run times with and without it to see
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which runs faster. Those loops that scan for many short
constant strings (including the constant parts of more
complex patterns) will benefit most. You may have only
one "study" active at a time: if you study a different
scalar the first is "unstudied". (The way "study" works
is this: a linked list of every character in the string
to be searched is made, so we know, for example, where
all the 'k' characters are. From each search string,
the rarest character is selected, based on some static
frequency tables constructed from some C programs and
English text. Only those places that contain this
"rarest" character are examined.)
For example, here is a loop that inserts index producing
entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only locations in $_ that
contain "f" will be looked at, because "f" is rarer than
"o". In general, this is a big win except in
pathological cases. The only question is whether it
saves you more time than it took to build the linked
list in the first place.
Note that if you have to look for strings that you don't
know till runtime, you can build an entire loop as a
string and "eval" that to avoid recompiling all your
patterns all the time. Together with undefining $/ to
input entire files as one record, this can be quite
fast, often faster than specialized programs like
fgrep(1). The following scans a list of files (@files)
for a list of words (@words), and prints out the names
of those files that contain a match:
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$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per
se. Without a BLOCK it's just a forward declaration.
Without a NAME, it's an anonymous function declaration,
and does actually return a value: the CODE ref of the
closure you just created.
See perlsub and perlref for details about subroutines
and references, and attributes and Attribute::Handlers
for more information about attributes.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First
character is at offset 0, or whatever you've set $[ to
(but don't do that). If OFFSET is negative (or more
precisely, less than $[), starts that far from the end
of the string. If LENGTH is omitted, returns everything
to the end of the string. If LENGTH is negative, leaves
that many characters off the end of the string.
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr
You can use the substr() function as an lvalue, in which
case EXPR must itself be an lvalue. If you assign
something shorter than LENGTH, the string will shrink,
and if you assign something longer than LENGTH, the
string will grow to accommodate it. To keep the string
the same length, you may need to pad or chop your value
using "sprintf".
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If OFFSET and LENGTH specify a substring that is partly
outside the string, only the part within the string is
returned. If the substring is beyond either end of the
string, substr() returns the undefined value and
produces a warning. When used as an lvalue, specifying
a substring that is entirely outside the string raises
an exception. Here's an example showing the behavior
for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exception
An alternative to using substr() as an lvalue is to
specify the replacement string as the 4th argument.
This allows you to replace parts of the EXPR and return
what was there before in one operation, just as you can
with splice().
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the 3-arg version of
substr() acts as a 'magic bullet'; each time it is
assigned to, it remembers which part of the original
string is being modified; for example:
$x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
Prior to Perl version 5.9.1, the result of using an
lvalue multiple times was unspecified.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old
filename. Returns 1 for success, 0 otherwise. On
systems that don't support symbolic links, raises an
exception. To check for that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall NUMBER, LIST
Calls the system call specified as the first element of
the list, passing the remaining elements as arguments to
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the system call. If unimplemented, raises an exception.
The arguments are interpreted as follows: if a given
argument is numeric, the argument is passed as an int.
If not, the pointer to the string value is passed. You
are responsible to make sure a string is pre-extended
long enough to receive any result that might be written
into a string. You can't use a string literal (or other
read-only string) as an argument to "syscall" because
Perl has to assume that any string pointer might be
written through. If your integer arguments are not
literals and have never been interpreted in a numeric
context, you may need to add 0 to them to force them to
look like numbers. This emulates the "syswrite"
function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14
arguments to your syscall, which in practice should
(usually) suffice.
Syscall returns whatever value returned by the system
call it calls. If the system call fails, "syscall"
returns "-1" and sets $! (errno). Note that some system
calls can legitimately return "-1". The proper way to
handle such calls is to assign "$!=0;" before the call
and check the value of $! if syscall returns "-1".
There's a problem with "syscall(&SYS_pipe)": it returns
the file number of the read end of the pipe it creates.
There is no way to retrieve the file number of the other
end. You can avoid this problem by using "pipe"
instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the real
filehandle wanted. This function calls the underlying
operating system's "open" function with the parameters
FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter
are system-dependent; they are available via the
standard module "Fcntl". See the documentation of your
operating system's "open" to see which values and flag
bits are available. You may combine several flags using
the "|"-operator.
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Some of the most common values are "O_RDONLY" for
opening the file in read-only mode, "O_WRONLY" for
opening the file in write-only mode, and "O_RDWR" for
opening the file in read-write mode.
For historical reasons, some values work on almost every
system supported by Perl: 0 means read-only, 1 means
write-only, and 2 means read/write. We know that these
values do not work under OS/390 & VM/ESA Unix and on the
Macintosh; you probably don't want to use them in new
code.
If the file named by FILENAME does not exist and the
"open" call creates it (typically because MODE includes
the "O_CREAT" flag), then the value of PERMS specifies
the permissions of the newly created file. If you omit
the PERMS argument to "sysopen", Perl uses the octal
value 0666. These permission values need to be in
octal, and are modified by your process's current
"umask".
In many systems the "O_EXCL" flag is available for
opening files in exclusive mode. This is not locking:
exclusiveness means here that if the file already
exists, sysopen() fails. "O_EXCL" may not work on
network filesystems, and has no effect unless the
"O_CREAT" flag is set as well. Setting "O_CREAT|O_EXCL"
prevents the file from being opened if it is a symbolic
link. It does not protect against symbolic links in the
file's path.
Sometimes you may want to truncate an already-existing
file. This can be done using the "O_TRUNC" flag. The
behavior of "O_TRUNC" with "O_RDONLY" is undefined.
You should seldom if ever use 0644 as argument to
"sysopen", because that takes away the user's option to
have a more permissive umask. Better to omit it. See
the perlfunc(1) entry on "umask" for more on this.
Note that "sysopen" depends on the fdopen() C library
function. On many Unix systems, fdopen() is known to
fail when file descriptors exceed a certain value,
typically 255. If you need more file descriptors than
that, consider rebuilding Perl to use the "sfio"
library, or perhaps using the POSIX::open() function.
See perlopentut for a kinder, gentler explanation of
opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
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Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE, using the read(2).
It bypasses buffered IO, so mixing this with other kinds
of reads, "print", "write", "seek", "tell", or "eof" can
cause confusion because the perlio or stdio layers
usually buffers data. Returns the number of bytes
actually read, 0 at end of file, or undef if there was
an error (in the latter case $! is also set). SCALAR
will be grown or shrunk so that the last byte actually
read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at
some place in the string other than the beginning. A
negative OFFSET specifies placement at that many
characters counting backwards from the end of the
string. A positive OFFSET greater than the length of
SCALAR results in the string being padded to the
required size with "\0" bytes before the result of the
read is appended.
There is no syseof() function, which is ok, since eof()
doesn't work well on device files (like ttys) anyway.
Use sysread() and check for a return value for 0 to
decide whether you're done.
Note that if the filehandle has been marked as ":utf8"
Unicode characters are read instead of bytes (the
LENGTH, OFFSET, and the return value of sysread() are in
Unicode characters). The ":encoding(...)" layer
implicitly introduces the ":utf8" layer. See "binmode",
"open", and the "open" pragma, open.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using
lseek(2). FILEHANDLE may be an expression whose value
gives the name of the filehandle. The values for WHENCE
are 0 to set the new position to POSITION, 1 to set the
it to the current position plus POSITION, and 2 to set
it to EOF plus POSITION (typically negative).
Note the in bytes: even if the filehandle has been set
to operate on characters (for example by using the
":encoding(utf8)" I/O layer), tell() will return byte
offsets, not character offsets (because implementing
that would render sysseek() unacceptably slow).
sysseek() bypasses normal buffered IO, so mixing this
with reads (other than "sysread", for example "<>" or
read()) "print", "write", "seek", "tell", or "eof" may
cause confusion.
For WHENCE, you may also use the constants "SEEK_SET",
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"SEEK_CUR", and "SEEK_END" (start of the file, current
position, end of the file) from the Fcntl module. Use
of the constants is also more portable than relying on
0, 1, and 2. For example to define a "systell"
function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on
failure. A position of zero is returned as the string
"0 but true"; thus "sysseek" returns true on success and
false on failure, yet you can still easily determine the
new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except that
a fork is done first, and the parent process waits for
the child process to exit. Note that argument
processing varies depending on the number of arguments.
If there is more than one argument in LIST, or if LIST
is an array with more than one value, starts the program
given by the first element of the list with arguments
given by the rest of the list. If there is only one
scalar argument, the argument is checked for shell
metacharacters, and if there are any, the entire
argument is passed to the system's command shell for
parsing (this is "/bin/sh -c" on Unix platforms, but
varies on other platforms). If there are no shell
metacharacters in the argument, it is split into words
and passed directly to "execvp", which is more
efficient.
Beginning with v5.6.0, Perl will attempt to flush all
files opened for output before any operation that may do
a fork, but this may not be supported on some platforms
(see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()" method
of "IO::Handle" on any open handles.
The return value is the exit status of the program as
returned by the "wait" call. To get the actual exit
value, shift right by eight (see below). See also
"exec". This is not what you want to use to capture the
output from a command, for that you should use merely
backticks or "qx//", as described in "`STRING`" in
perlop. Return value of -1 indicates a failure to start
the program or an error of the wait(2) system call
(inspect $! for the reason).
If you'd like to make "system" (and many other bits of
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Perl) die on error, have a look at the autodie pragma.
Like "exec", "system" allows you to lie to a program
about its name if you use the "system PROGRAM LIST"
syntax. Again, see "exec".
Since "SIGINT" and "SIGQUIT" are ignored during the
execution of "system", if you expect your program to
terminate on receipt of these signals you will need to
arrange to do so yourself based on the return value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
If you'd like to manually inspect "system"'s failure,
you can check all possible failure modes by inspecting
$? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
Alternatively, you may inspect the value of
"${^CHILD_ERROR_NATIVE}" with the "W*()" calls from the
POSIX module.
When "system"'s arguments are executed indirectly by the
shell, results and return codes are subject to its
quirks. See "`STRING`" in perlop and "exec" for
details.
Since "system" does a "fork" and "wait" it may affect a
"SIGCHLD" handler. See perlipc for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable
SCALAR to the specified FILEHANDLE, using write(2). If
LENGTH is not specified, writes whole SCALAR. It
bypasses buffered IO, so mixing this with reads (other
than sysread()), "print", "write", "seek", "tell", or
"eof" may cause confusion because the perlio and stdio
layers usually buffers data. Returns the number of
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bytes actually written, or "undef" if there was an error
(in this case the errno variable $! is also set). If
the LENGTH is greater than the data available in the
SCALAR after the OFFSET, only as much data as is
available will be written.
An OFFSET may be specified to write the data from some
part of the string other than the beginning. A negative
OFFSET specifies writing that many characters counting
backwards from the end of the string. If SCALAR is of
length zero, you can only use an OFFSET of 0.
Warning: If the filehandle is marked ":utf8", Unicode
characters encoded in UTF-8 are written instead of
bytes, and the LENGTH, OFFSET, and return value of
syswrite() are in (UTF-8 encoded Unicode) characters.
The ":encoding(...)" layer implicitly introduces the
":utf8" layer. See "binmode", "open", and the "open"
pragma, open.
tell FILEHANDLE
tell
Returns the current position in bytes for FILEHANDLE, or
-1 on error. FILEHANDLE may be an expression whose
value gives the name of the actual filehandle. If
FILEHANDLE is omitted, assumes the file last read.
Note the in bytes: even if the filehandle has been set
to operate on characters (for example by using the
":encoding(utf8)" open layer), tell() will return byte
offsets, not character offsets (because that would
render seek() and tell() rather slow).
The return value of tell() for the standard streams like
the STDIN depends on the operating system: it may return
-1 or something else. tell() on pipes, fifos, and
sockets usually returns -1.
There is no "systell" function. Use "sysseek(FH, 0, 1)"
for that.
Do not use tell() (or other buffered I/O operations) on
a filehandle that has been manipulated by sysread(),
syswrite() or sysseek(). Those functions ignore the
buffering, while tell() does not.
telldir DIRHANDLE
Returns the current position of the "readdir" routines
on DIRHANDLE. Value may be given to "seekdir" to access
a particular location in a directory. "telldir" has the
same caveats about possible directory compaction as the
corresponding system library routine.
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tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that
will provide the implementation for the variable.
VARIABLE is the name of the variable to be enchanted.
CLASSNAME is the name of a class implementing objects of
correct type. Any additional arguments are passed to
the "new" method of the class (meaning "TIESCALAR",
"TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically these
are arguments such as might be passed to the
"dbm_open()" function of C. The object returned by the
"new" method is also returned by the "tie" function,
which would be useful if you want to access other
methods in CLASSNAME.
Note that functions such as "keys" and "values" may
return huge lists when used on large objects, like DBM
files. You may prefer to use the "each" function to
iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following
methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
A class implementing an ordinary array should have the
following methods:
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TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a filehandle should have the
following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following
methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented.
See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and
Tie::Handle.
Unlike "dbmopen", the "tie" function will not "use" or
"require" a module for you; you need to do that
explicitly yourself. See DB_File or the Config module
for interesting "tie" implementations.
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For further details see perltie, "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARIABLE
(the same value that was originally returned by the
"tie" call that bound the variable to a package.)
Returns the undefined value if VARIABLE isn't tied to a
package.
time
Returns the number of non-leap seconds since whatever
time the system considers to be the epoch, suitable for
feeding to "gmtime" and "localtime". On most systems the
epoch is 00:00:00 UTC, January 1, 1970; a prominent
exception being Mac OS Classic which uses 00:00:00,
January 1, 1904 in the current local time zone for its
epoch.
For measuring time in better granularity than one
second, you may use either the Time::HiRes module (from
CPAN, and starting from Perl 5.8 part of the standard
distribution), or if you have gettimeofday(2), you may
be able to use the "syscall" interface of Perl. See
perlfaq8 for details.
For date and time processing look at the many related
modules on CPAN. For a comprehensive date and time
representation look at the DateTime module.
times
Returns a four-element list giving the user and system
times, in seconds, for this process and the children of
this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
Children's times are only included for terminated
children.
tr///
The transliteration operator. Same as "y///". See
"Quote and Quote-like Operators" in perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by
EXPR, to the specified length. Raises an exception if
truncate isn't implemented on your system. Returns true
if successful, the undefined value otherwise.
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The behavior is undefined if LENGTH is greater than the
length of the file.
The position in the file of FILEHANDLE is left
unchanged. You may want to call seek before writing to
the file.
uc EXPR
uc Returns an uppercased version of EXPR. This is the
internal function implementing the "\U" escape in
double-quoted strings. It does not attempt to do
titlecase mapping on initial letters. See "ucfirst" for
that.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma,
such as in a locale, as "lc" does.
ucfirst EXPR
ucfirst
Returns the value of EXPR with the first character in
uppercase (titlecase in Unicode). This is the internal
function implementing the "\u" escape in double-quoted
strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma,
such as in a locale, as "lc" does.
umask EXPR
umask
Sets the umask for the process to EXPR and returns the
previous value. If EXPR is omitted, merely returns the
current umask.
The Unix permission "rwxr-x---" is represented as three
sets of three bits, or three octal digits: 0750 (the
leading 0 indicates octal and isn't one of the digits).
The "umask" value is such a number representing disabled
permissions bits. The permission (or "mode") values you
pass "mkdir" or "sysopen" are modified by your umask, so
even if you tell "sysopen" to create a file with
permissions 0777, if your umask is 0022 then the file
will actually be created with permissions 0755. If your
"umask" were 0027 (group can't write; others can't read,
write, or execute), then passing "sysopen" 0666 would
create a file with mode 0640 ("0666 &~ 027" is 0640).
Here's some advice: supply a creation mode of 0666 for
regular files (in "sysopen") and one of 0777 for
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directories (in "mkdir") and executable files. This
gives users the freedom of choice: if they want
protected files, they might choose process umasks of
022, 027, or even the particularly antisocial mask of
077. Programs should rarely if ever make policy
decisions better left to the user. The exception to
this is when writing files that should be kept private:
mail files, web browser cookies, .rhosts files, and so
on.
If umask(2) is not implemented on your system and you
are trying to restrict access for yourself (i.e., "(EXPR
& 0700) > 0"), raises an exception. If umask(2) is not
implemented and you are not trying to restrict access
for yourself, returns "undef".
Remember that a umask is a number, usually given in
octal; it is not a string of octal digits. See also
"oct", if all you have is a string.
undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue.
Use only on a scalar value, an array (using "@"), a hash
(using "%"), a subroutine (using "&"), or a typeglob
(using "*"). Saying "undef $hash{$key}" will probably
not do what you expect on most predefined variables or
DBM list values, so don't do that; see delete. Always
returns the undefined value. You can omit the EXPR, in
which case nothing is undefined, but you still get an
undefined value that you could, for instance, return
from a subroutine, assign to a variable, or pass as a
parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink
Deletes a list of files. On success, it returns the
number of files it successfully deleted. On failure, it
returns false and sets $! (errno):
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my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";
On error, "unlink" will not tell you which files it
could not remove. If you want to know which files you
could not remove, try them one at a time:
foreach my $file ( @goners ) {
unlink $file or warn "Could not unlink $file: $!";
}
Note: "unlink" will not attempt to delete directories
unless you are superuser and the -U flag is supplied to
Perl. Even if these conditions are met, be warned that
unlinking a directory can inflict damage on your
filesystem. Finally, using "unlink" on directories is
not supported on many operating systems. Use "rmdir"
instead.
If LIST is omitted, "unlink" uses $_.
unpack TEMPLATE,EXPR
unpack TEMPLATE
"unpack" does the reverse of "pack": it takes a string
and expands it out into a list of values. (In scalar
context, it returns merely the first value produced.)
If EXPR is omitted, unpacks the $_ string. See
perlpacktut for an introduction to this function.
The string is broken into chunks described by the
TEMPLATE. Each chunk is converted separately to a
value. Typically, either the string is a result of
"pack", or the characters of the string represent a C
structure of some kind.
The TEMPLATE has the same format as in the "pack"
function. Here's a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix
a field with a %<number> to indicate that you want a
<number>-bit checksum of the items instead of the items
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themselves. Default is a 16-bit checksum. Checksum is
calculated by summing numeric values of expanded values
(for string fields the sum of "ord($char)" is taken, for
bit fields the sum of zeroes and ones).
For example, the following computes the same number as
the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32W*",<>) % 65535;
};
The following efficiently counts the number of set bits
in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since
Perl has no way of checking whether the value passed to
"unpack()" corresponds to a valid memory location,
passing a pointer value that's not known to be valid is
likely to have disastrous consequences.
If there are more pack codes or if the repeat count of a
field or a group is larger than what the remainder of
the input string allows, the result is not well defined:
the repeat count may be decreased, or "unpack()" may
produce empty strings or zeros, or it may raise an
exception. If the input string is longer than one
described by the TEMPLATE, the remainder of that input
string is ignored.
See "pack" for more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a package.
(See "tie".) Has no effect if the variable is not tied.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of a
"push", depending on how you look at it. Prepends list
to the front of the array, and returns the new number of
elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a
time, so the prepended elements stay in the same order.
Use "reverse" to do the reverse.
use Module VERSION LIST
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use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from the
named module, generally by aliasing certain subroutine
or variable names into your package. It is exactly
equivalent to
BEGIN { require Module; Module->import( LIST ); }
except that Module must be a bareword.
In the peculiar "use VERSION" form, VERSION may be
either a positive decimal fraction such as 5.006, which
will be compared to $], or a v-string of the form
v5.6.1, which will be compared to $^V (aka
$PERL_VERSION). An exception is raised if VERSION is
greater than the version of the current Perl
interpreter; Perl will not attempt to parse the rest of
the file. Compare with "require", which can do a
similar check at run time. Symmetrically, "no VERSION"
allows you to specify that you want a version of Perl
older than the specified one.
Specifying VERSION as a literal of the form v5.6.1
should generally be avoided, because it leads to
misleading error messages under earlier versions of Perl
(that is, prior to 5.6.0) that do not support this
syntax. The equivalent numeric version should be used
instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the current
Perl version before "use"ing library modules that won't
work with older versions of Perl. (We try not to do
this more than we have to.)
Also, if the specified Perl version is greater than or
equal to 5.9.5, "use VERSION" will also load the
"feature" pragma and enable all features available in
the requested version. See feature. Similarly, if the
specified Perl version is greater than or equal to
5.11.0, strictures are enabled lexically as with "use
strict" (except that the strict.pm file is not actually
loaded).
The "BEGIN" forces the "require" and "import" to happen
at compile time. The "require" makes sure the module is
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loaded into memory if it hasn't been yet. The "import"
is not a builtin; it's just an ordinary static method
call into the "Module" package to tell the module to
import the list of features back into the current
package. The module can implement its "import" method
any way it likes, though most modules just choose to
derive their "import" method via inheritance from the
"Exporter" class that is defined in the "Exporter"
module. See Exporter. If no "import" method can be
found then the call is skipped, even if there is an
AUTOLOAD method.
If you do not want to call the package's "import" method
(for instance, to stop your namespace from being
altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and
LIST, then the "use" will call the VERSION method in
class Module with the given version as an argument. The
default VERSION method, inherited from the UNIVERSAL
class, croaks if the given version is larger than the
value of the variable $Module::VERSION.
Again, there is a distinction between omitting LIST
("import" called with no arguments) and an explicit
empty LIST "()" ("import" not called). Note that there
is no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler
directives) are also implemented this way. Currently
implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into the
current block scope (like "strict" or "integer", unlike
ordinary modules, which import symbols into the current
package (which are effective through the end of the
file).
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Because "use" takes effect at compile time, it doesn't
respect the ordinary flow control of the code being
compiled. In particular, putting a "use" inside the
false branch of a conditional doesn't prevent it from
being processed. If a module or pragma only needs to be
loaded conditionally, this can be done using the if
pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);
There's a corresponding "no" command that unimports
meanings imported by "use", i.e., it calls "unimport
Module LIST" instead of "import". It behaves just as
"import" does with VERSION, an omitted or empty LIST, or
no unimport method being found.
no integer;
no strict 'refs';
no warnings;
Care should be taken when using the "no VERSION" form of
"no". It is only meant to be used to assert that the
running perl is of a earlier version than its argument
and not to undo the feature-enabling side effects of
"use VERSION".
See perlmodlib for a list of standard modules and
pragmas. See perlrun for the "-M" and "-m" command-line
options to Perl that give "use" functionality from the
command-line.
utime LIST
Changes the access and modification times on each file
of a list of files. The first two elements of the list
must be the NUMERICAL access and modification times, in
that order. Returns the number of files successfully
changed. The inode change time of each file is set to
the current time. For example, this code has the same
effect as the Unix touch(1) command when the files
already exist and belong to the user running the
program:
#!/usr/bin/perl
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
Since Perl 5.7.2, if the first two elements of the list
are "undef", the utime(2) syscall from your C library is
called with a null second argument. On most systems,
this will set the file's access and modification times
to the current time (i.e., equivalent to the example
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above) and will work even on files you don't own
provided you have write permission:
for $file (@ARGV) {
utime(undef, undef, $file)
|| warn "couldn't touch $file: $!";
}
Under NFS this will use the time of the NFS server, not
the time of the local machine. If there is a time
synchronization problem, the NFS server and local
machine will have different times. The Unix touch(1)
command will in fact normally use this form instead of
the one shown in the first example.
Passing only one of the first two elements as "undef" is
equivalent to passing a 0 and will not have the effect
described when both are "undef". This also triggers an
uninitialized warning.
On systems that support futimes(2), you may pass
filehandles among the files. On systems that don't
support futimes(2), passing filehandles raises an
exception. Filehandles must be passed as globs or glob
references to be recognized; barewords are considered
filenames.
values HASH
values ARRAY
Returns a list consisting of all the values of the named
hash, or the values of an array. (In a scalar context,
returns the number of values.)
The values are returned in an apparently random order.
The actual random order is subject to change in future
versions of Perl, but it is guaranteed to be the same
order as either the "keys" or "each" function would
produce on the same (unmodified) hash. Since Perl 5.8.1
the ordering is different even between different runs of
Perl for security reasons (see "Algorithmic Complexity
Attacks" in perlsec).
As a side effect, calling values() resets the HASH or
ARRAY's internal iterator, see "each". (In particular,
calling values() in void context resets the iterator
with no other overhead. Apart from resetting the
iterator, "values @array" in list context is the same as
plain @array. We recommend that you use void context
"keys @array" for this, but reasoned that it taking
"values @array" out would require more documentation
than leaving it in.)
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Note that the values are not copied, which means
modifying them will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
See also "keys", "each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of
elements of width BITS, and returns the value of the
element specified by OFFSET as an unsigned integer.
BITS therefore specifies the number of bits that are
reserved for each element in the bit vector. This must
be a power of two from 1 to 32 (or 64, if your platform
supports that).
If BITS is 8, "elements" coincide with bytes of the
input string.
If BITS is 16 or more, bytes of the input string are
grouped into chunks of size BITS/8, and each group is
converted to a number as with pack()/unpack() with big-
endian formats "n"/"N" (and analogously for BITS==64).
See "pack" for details.
If bits is 4 or less, the string is broken into bytes,
then the bits of each byte are broken into 8/BITS
groups. Bits of a byte are numbered in a little-endian-
ish way, as in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40,
0x80. For example, breaking the single input byte
"chr(0x36)" into two groups gives a list "(0x6, 0x3)";
breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)".
"vec" may also be assigned to, in which case parentheses
are needed to give the expression the correct precedence
as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value
0 is returned. If an element off the end of the string
is written to, Perl will first extend the string with
sufficiently many zero bytes. It is an error to try to
write off the beginning of the string (i.e., negative
OFFSET).
If the string happens to be encoded as UTF-8 internally
(and thus has the UTF8 flag set), this is ignored by
"vec", and it operates on the internal byte string, not
the conceptual character string, even if you only have
characters with values less than 256.
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Strings created with "vec" can also be manipulated with
the logical operators "|", "&", "^", and "~". These
operators will assume a bit vector operation is desired
when both operands are strings. See "Bitwise String
Operators" in perlop.
The following code will build up an ASCII string saying
'PerlPerlPerl'. The comments show the string after each
step. Note that this code works in the same way on big-
endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0's
and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in
place of the "*".
Here is an example to illustrate how the bits actually
fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
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for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it runs,
the example above should print the following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
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vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
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vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait
Behaves like wait(2) on your system: it waits for a
child process to terminate and returns the pid of the
deceased process, or "-1" if there are no child
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processes. The status is returned in $? and
"${^CHILD_ERROR_NATIVE}". Note that a return value of
"-1" could mean that child processes are being
automatically reaped, as described in perlipc.
If you use wait in your handler for $SIG{CHLD} it may
accidently wait for the child created by qx() or
system(). See perlipc for details.
waitpid PID,FLAGS
Waits for a particular child process to terminate and
returns the pid of the deceased process, or "-1" if
there is no such child process. On some systems, a
value of 0 indicates that there are processes still
running. The status is returned in $? and
"${^CHILD_ERROR_NATIVE}". If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} while $kid > 0;
then you can do a non-blocking wait for all pending
zombie processes. Non-blocking wait is available on
machines supporting either the waitpid(2) or wait4(2)
syscalls. However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere. (Perl emulates
the system call by remembering the status values of
processes that have exited but have not been harvested
by the Perl script yet.)
Note that on some systems, a return value of "-1" could
mean that child processes are being automatically
reaped. See perlipc for details, and for other
examples.
wantarray
Returns true if the context of the currently executing
subroutine or "eval" is looking for a list value.
Returns false if the context is looking for a scalar.
Returns the undefined value if the context is looking
for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
"wantarray()"'s result is unspecified in the top level
of a file, in a "BEGIN", "UNITCHECK", "CHECK", "INIT" or
"END" block, or in a "DESTROY" method.
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This function should have been named wantlist() instead.
warn LIST
Prints the value of LIST to STDERR. If the last element
of LIST does not end in a newline, it appends the same
file/line number text as "die" does.
If the output is empty and $@ already contains a value
(typically from a previous eval) that value is used
after appending "\t...caught" to $@. This is useful for
staying almost, but not entirely similar to "die".
If $@ is empty then the string "Warning: Something's
wrong" is used.
No message is printed if there is a $SIG{__WARN__}
handler installed. It is the handler's responsibility
to deal with the message as it sees fit (like, for
instance, converting it into a "die"). Most handlers
must therefore arrange to actually display the warnings
that they are not prepared to deal with, by calling
"warn" again in the handler. Note that this is quite
safe and will not produce an endless loop, since
"__WARN__" hooks are not called from inside one.
You will find this behavior is slightly different from
that of $SIG{__DIE__} handlers (which don't suppress the
error text, but can instead call "die" again to change
it).
Using a "__WARN__" handler provides a powerful way to
silence all warnings (even the so-called mandatory
ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries, and for
more examples. See the Carp module for other kinds of
warnings using its carp() and cluck() functions.
write FILEHANDLE
write EXPR
write
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Writes a formatted record (possibly multi-line) to the
specified FILEHANDLE, using the format associated with
that file. By default the format for a file is the one
having the same name as the filehandle, but the format
for the current output channel (see the "select"
function) may be set explicitly by assigning the name of
the format to the $~ variable.
Top of form processing is handled automatically: if
there is insufficient room on the current page for the
formatted record, the page is advanced by writing a form
feed, a special top-of-page format is used to format the
new page header, and then the record is written. By
default the top-of-page format is the name of the
filehandle with "_TOP" appended, but it may be
dynamically set to the format of your choice by
assigning the name to the $^ variable while the
filehandle is selected. The number of lines remaining
on the current page is in variable "$-", which can be
set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current
default output channel, which starts out as STDOUT but
may be changed by the "select" operator. If the
FILEHANDLE is an EXPR, then the expression is evaluated
and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see
perlform.
Note that write is not the opposite of "read".
Unfortunately.
y///
The transliteration operator. Same as "tr///". See
"Quote and Quote-like Operators" in perlop.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
perl v5.12.5 Last change: 2012-11-03 153
Perl Programmers Reference Guide PERLFUNC(1)
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 154