perlhack
(1)
Name
perlhack - How to hack at the Perl internals
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLHACK(1)
NAME
perlhack - How to hack at the Perl internals
DESCRIPTION
This document attempts to explain how Perl development takes
place, and ends with some suggestions for people wanting to
become bona fide porters.
The perl5-porters mailing list is where the Perl standard
distribution is maintained and developed. The list can get
anywhere from 10 to 150 messages a day, depending on the
heatedness of the debate. Most days there are two or three
patches, extensions, features, or bugs being discussed at a
time.
A searchable archive of the list is at either:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
or
http://archive.develooper.com/perl5-porters@perl.org/
List subscribers (the porters themselves) come in several
flavours. Some are quiet curious lurkers, who rarely pitch
in and instead watch the ongoing development to ensure
they're forewarned of new changes or features in Perl. Some
are representatives of vendors, who are there to make sure
that Perl continues to compile and work on their platforms.
Some patch any reported bug that they know how to fix, some
are actively patching their pet area (threads, Win32, the
regexp engine), while others seem to do nothing but
complain. In other words, it's your usual mix of technical
people.
Over this group of porters presides Larry Wall. He has the
final word in what does and does not change in the Perl
language. Various releases of Perl are shepherded by a
"pumpking", a porter responsible for gathering patches,
deciding on a patch-by-patch, feature-by-feature basis what
will and will not go into the release. For instance,
Gurusamy Sarathy was the pumpking for the 5.6 release of
Perl, and Jarkko Hietaniemi was the pumpking for the 5.8
release, and Rafael Garcia-Suarez holds the pumpking crown
for the 5.10 release.
In addition, various people are pumpkings for different
things. For instance, Andy Dougherty and Jarkko Hietaniemi
did a grand job as the Configure pumpkin up till the 5.8
release. For the 5.10 release H.Merijn Brand took over.
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Larry sees Perl development along the lines of the US
government: there's the Legislature (the porters), the
Executive branch (the pumpkings), and the Supreme Court
(Larry). The legislature can discuss and submit patches to
the executive branch all they like, but the executive branch
is free to veto them. Rarely, the Supreme Court will side
with the executive branch over the legislature, or the
legislature over the executive branch. Mostly, however, the
legislature and the executive branch are supposed to get
along and work out their differences without impeachment or
court cases.
You might sometimes see reference to Rule 1 and Rule 2.
Larry's power as Supreme Court is expressed in The Rules:
1. Larry is always by definition right about how Perl
should behave. This means he has final veto power on
the core functionality.
2. Larry is allowed to change his mind about any matter at
a later date, regardless of whether he previously
invoked Rule 1.
Got that? Larry is always right, even when he was wrong.
It's rare to see either Rule exercised, but they are often
alluded to.
New features and extensions to the language are contentious,
because the criteria used by the pumpkings, Larry, and other
porters to decide which features should be implemented and
incorporated are not codified in a few small design goals as
with some other languages. Instead, the heuristics are
flexible and often difficult to fathom. Here is one
person's list, roughly in decreasing order of importance, of
heuristics that new features have to be weighed against:
Does concept match the general goals of Perl?
These haven't been written anywhere in stone, but one
approximation is:
1. Keep it fast, simple, and useful.
2. Keep features/concepts as orthogonal as possible.
3. No arbitrary limits (platforms, data sizes, cultures).
4. Keep it open and exciting to use/patch/advocate Perl everywhere.
5. Either assimilate new technologies, or build bridges to them.
Where is the implementation?
All the talk in the world is useless without an
implementation. In almost every case, the person or
people who argue for a new feature will be expected to
be the ones who implement it. Porters capable of coding
new features have their own agendas, and are not
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available to implement your (possibly good) idea.
Backwards compatibility
It's a cardinal sin to break existing Perl programs.
New warnings are contentious--some say that a program
that emits warnings is not broken, while others say it
is. Adding keywords has the potential to break
programs, changing the meaning of existing token
sequences or functions might break programs.
Could it be a module instead?
Perl 5 has extension mechanisms, modules and XS,
specifically to avoid the need to keep changing the Perl
interpreter. You can write modules that export
functions, you can give those functions prototypes so
they can be called like built-in functions, you can even
write XS code to mess with the runtime data structures
of the Perl interpreter if you want to implement really
complicated things. If it can be done in a module
instead of in the core, it's highly unlikely to be
added.
Is the feature generic enough?
Is this something that only the submitter wants added to
the language, or would it be broadly useful? Sometimes,
instead of adding a feature with a tight focus, the
porters might decide to wait until someone implements
the more generalized feature. For instance, instead of
implementing a "delayed evaluation" feature, the porters
are waiting for a macro system that would permit delayed
evaluation and much more.
Does it potentially introduce new bugs?
Radical rewrites of large chunks of the Perl interpreter
have the potential to introduce new bugs. The smaller
and more localized the change, the better.
Does it preclude other desirable features?
A patch is likely to be rejected if it closes off future
avenues of development. For instance, a patch that
placed a true and final interpretation on prototypes is
likely to be rejected because there are still options
for the future of prototypes that haven't been
addressed.
Is the implementation robust?
Good patches (tight code, complete, correct) stand more
chance of going in. Sloppy or incorrect patches might
be placed on the back burner until the pumpking has time
to fix, or might be discarded altogether without further
notice.
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Is the implementation generic enough to be portable?
The worst patches make use of a system-specific
features. It's highly unlikely that non-portable
additions to the Perl language will be accepted.
Is the implementation tested?
Patches which change behaviour (fixing bugs or
introducing new features) must include regression tests
to verify that everything works as expected. Without
tests provided by the original author, how can anyone
else changing perl in the future be sure that they
haven't unwittingly broken the behaviour the patch
implements? And without tests, how can the patch's
author be confident that his/her hard work put into the
patch won't be accidentally thrown away by someone in
the future?
Is there enough documentation?
Patches without documentation are probably ill-thought
out or incomplete. Nothing can be added without
documentation, so submitting a patch for the appropriate
manpages as well as the source code is always a good
idea.
Is there another way to do it?
Larry said "Although the Perl Slogan is There's More
Than One Way to Do It, I hesitate to make 10 ways to do
something". This is a tricky heuristic to navigate,
though--one man's essential addition is another man's
pointless cruft.
Does it create too much work?
Work for the pumpking, work for Perl programmers, work
for module authors, ... Perl is supposed to be easy.
Patches speak louder than words
Working code is always preferred to pie-in-the-sky
ideas. A patch to add a feature stands a much higher
chance of making it to the language than does a random
feature request, no matter how fervently argued the
request might be. This ties into "Will it be useful?",
as the fact that someone took the time to make the patch
demonstrates a strong desire for the feature.
If you're on the list, you might hear the word "core"
bandied around. It refers to the standard distribution.
"Hacking on the core" means you're changing the C source
code to the Perl interpreter. "A core module" is one that
ships with Perl.
Keeping in sync
The source code to the Perl interpreter, in its different
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versions, is kept in a repository managed by the git
revision control system. The pumpkings and a few others have
write access to the repository to check in changes.
How to clone and use the git perl repository is described in
perlrepository.
You can also choose to use rsync to get a copy of the
current source tree for the bleadperl branch and all
maintenance branches:
$ rsync -avz rsync://perl5.git.perl.org/perl-current .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.12.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.10.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.8.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.6.x .
$ rsync -avz rsync://perl5.git.perl.org/perl-5.005xx .
(Add the "--delete" option to remove leftover files)
To get a full list of the available sync points:
$ rsync perl5.git.perl.org::
You may also want to subscribe to the perl5-changes mailing
list to receive a copy of each patch that gets submitted to
the maintenance and development "branches" of the perl
repository. See http://lists.perl.org/ for subscription
information.
If you are a member of the perl5-porters mailing list, it is
a good thing to keep in touch with the most recent changes.
If not only to verify if what you would have posted as a bug
report isn't already solved in the most recent available
perl development branch, also known as perl-current,
bleading edge perl, bleedperl or bleadperl.
Needless to say, the source code in perl-current is usually
in a perpetual state of evolution. You should expect it to
be very buggy. Do not use it for any purpose other than
testing and development.
Perlbug administration
There is a single remote administrative interface for
modifying bug status, category, open issues etc. using the
RT bugtracker system, maintained by Robert Spier. Become an
administrator, and close any bugs you can get your sticky
mitts on:
http://bugs.perl.org/
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To email the bug system administrators:
"perlbug-admin" <perlbug-admin@perl.org>
Submitting patches
Always submit patches to perl5-porters@perl.org. If you're
patching a core module and there's an author listed, send
the author a copy (see "Patching a core module"). This lets
other porters review your patch, which catches a surprising
number of errors in patches. Please patch against the
latest development version. (e.g., even if you're fixing a
bug in the 5.8 track, patch against the "blead" branch in
the git repository.)
If changes are accepted, they are applied to the development
branch. Then the maintenance pumpking decides which of those
patches is to be backported to the maint branch. Only
patches that survive the heat of the development branch get
applied to maintenance versions.
Your patch should update the documentation and test suite.
See "Writing a test". If you have added or removed files in
the distribution, edit the MANIFEST file accordingly, sort
the MANIFEST file using "make manisort", and include those
changes as part of your patch.
Patching documentation also follows the same order: if
accepted, a patch is first applied to development, and if
relevant then it's backported to maintenance. (With an
exception for some patches that document behaviour that only
appears in the maintenance branch, but which has changed in
the development version.)
To report a bug in Perl, use the program perlbug which comes
with Perl (if you can't get Perl to work, send mail to the
address perlbug@perl.org or perlbug@perl.com). Reporting
bugs through perlbug feeds into the automated bug-tracking
system, access to which is provided through the web at
http://rt.perl.org/rt3/ . It often pays to check the
archives of the perl5-porters mailing list to see whether
the bug you're reporting has been reported before, and if so
whether it was considered a bug. See above for the location
of the searchable archives.
The CPAN testers ( http://testers.cpan.org/ ) are a group of
volunteers who test CPAN modules on a variety of platforms.
Perl Smokers (
http://www.nntp.perl.org/group/perl.daily-build and
http://www.nntp.perl.org/group/perl.daily-build.reports/ )
automatically test Perl source releases on platforms with
various configurations. Both efforts welcome volunteers. In
order to get involved in smoke testing of the perl itself
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visit http://search.cpan.org/dist/Test-Smoke
<http://search.cpan.org/dist/Test-Smoke>. In order to start
smoke testing CPAN modules visit
http://search.cpan.org/dist/CPANPLUS-YACSmoke/
<http://search.cpan.org/dist/CPANPLUS-YACSmoke/> or
<http://search.cpan.org/dist/minismokebox/> or
http://search.cpan.org/dist/CPAN-Reporter/
<http://search.cpan.org/dist/CPAN-Reporter/>.
It's a good idea to read and lurk for a while before
chipping in. That way you'll get to see the dynamic of the
conversations, learn the personalities of the players, and
hopefully be better prepared to make a useful contribution
when do you speak up.
If after all this you still think you want to join the
perl5-porters mailing list, send mail to
perl5-porters-subscribe@perl.org. To unsubscribe, send mail
to perl5-porters-unsubscribe@perl.org.
To hack on the Perl guts, you'll need to read the following
things:
perlguts
This is of paramount importance, since it's the
documentation of what goes where in the Perl source. Read
it over a couple of times and it might start to make
sense - don't worry if it doesn't yet, because the best
way to study it is to read it in conjunction with poking
at Perl source, and we'll do that later on.
Gisle Aas's "illustrated perlguts", also known as
illguts, has very helpful pictures:
<http://search.cpan.org/dist/illguts/>
perlxstut and perlxs
A working knowledge of XSUB programming is incredibly
useful for core hacking; XSUBs use techniques drawn from
the PP code, the portion of the guts that actually
executes a Perl program. It's a lot gentler to learn
those techniques from simple examples and explanation
than from the core itself.
perlapi
The documentation for the Perl API explains what some of
the internal functions do, as well as the many macros
used in the source.
Porting/pumpkin.pod
This is a collection of words of wisdom for a Perl
porter; some of it is only useful to the pumpkin holder,
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but most of it applies to anyone wanting to go about Perl
development.
The perl5-porters FAQ
This should be available from
http://dev.perl.org/perl5/docs/p5p-faq.html . It
contains hints on reading perl5-porters, information on
how perl5-porters works and how Perl development in
general works.
Finding Your Way Around
Perl maintenance can be split into a number of areas, and
certain people (pumpkins) will have responsibility for each
area. These areas sometimes correspond to files or
directories in the source kit. Among the areas are:
Core modules
Modules shipped as part of the Perl core live in various
subdirectories, where two are dedicated to core-only
modules, and two are for the dual-life modules which live
on CPAN and may be maintained separately with respect to
the Perl core:
lib/ is for pure-Perl modules, which exist in the core only.
ext/ is for XS extensions, and modules with special Makefile.PL
requirements, which exist in the core only.
cpan/ is for dual-life modules, where the CPAN module is
canonical (should be patched first).
dist/ is for dual-life modules, where the blead source is
canonical.
For some dual-life modules it has not been discussed if
the CPAN version or the blead source is canonical. Until
that is done, those modules should be in cpan/.
Tests
There are tests for nearly all the modules, built-ins and
major bits of functionality. Test files all have a .t
suffix. Module tests live in the lib/ and ext/
directories next to the module being tested. Others live
in t/. See "Writing a test"
Documentation
Documentation maintenance includes looking after
everything in the pod/ directory, (as well as
contributing new documentation) and the documentation to
the modules in core.
Configure
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The Configure process is the way we make Perl portable
across the myriad of operating systems it supports.
Responsibility for the Configure, build and installation
process, as well as the overall portability of the core
code rests with the Configure pumpkin - others help out
with individual operating systems.
The three files that fall under his/her responsibility
are Configure, config_h.SH, and Porting/Glossary (and a
whole bunch of small related files that are less
important here). The Configure pumpkin decides how
patches to these are dealt with. Currently, the Configure
pumpkin will accept patches in most common formats, even
directly to these files. Other committers are allowed to
commit to these files under the strict condition that
they will inform the Configure pumpkin, either on IRC (if
he/she happens to be around) or through (personal)
e-mail.
The files involved are the operating system directories,
(win32/, os2/, vms/ and so on) the shell scripts which
generate config.h and Makefile, as well as the metaconfig
files which generate Configure. (metaconfig isn't
included in the core distribution.)
See
http://perl5.git.perl.org/metaconfig.git/blob/HEAD:/README
for a description of the full process involved.
Interpreter
And of course, there's the core of the Perl interpreter
itself. Let's have a look at that in a little more
detail.
Before we leave looking at the layout, though, don't forget
that MANIFEST contains not only the file names in the Perl
distribution, but short descriptions of what's in them, too.
For an overview of the important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST
Elements of the interpreter
The work of the interpreter has two main stages: compiling
the code into the internal representation, or bytecode, and
then executing it. "Compiled code" in perlguts explains
exactly how the compilation stage happens.
Here is a short breakdown of perl's operation:
Startup
The action begins in perlmain.c. (or miniperlmain.c for
miniperl) This is very high-level code, enough to fit on
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a single screen, and it resembles the code found in
perlembed; most of the real action takes place in perl.c
perlmain.c is generated by writemain from miniperlmain.c
at make time, so you should make perl to follow this
along.
First, perlmain.c allocates some memory and constructs a
Perl interpreter, along these lines:
1 PERL_SYS_INIT3(&argc,&argv,&env);
2
3 if (!PL_do_undump) {
4 my_perl = perl_alloc();
5 if (!my_perl)
6 exit(1);
7 perl_construct(my_perl);
8 PL_perl_destruct_level = 0;
9 }
Line 1 is a macro, and its definition is dependent on
your operating system. Line 3 references "PL_do_undump",
a global variable - all global variables in Perl start
with "PL_". This tells you whether the current running
program was created with the "-u" flag to perl and then
undump, which means it's going to be false in any sane
context.
Line 4 calls a function in perl.c to allocate memory for
a Perl interpreter. It's quite a simple function, and the
guts of it looks like this:
my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
Here you see an example of Perl's system abstraction,
which we'll see later: "PerlMem_malloc" is either your
system's "malloc", or Perl's own "malloc" as defined in
malloc.c if you selected that option at configure time.
Next, in line 7, we construct the interpreter using
perl_construct, also in perl.c; this sets up all the
special variables that Perl needs, the stacks, and so on.
Now we pass Perl the command line options, and tell it to
go:
exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
if (!exitstatus)
perl_run(my_perl);
exitstatus = perl_destruct(my_perl);
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perl_free(my_perl);
"perl_parse" is actually a wrapper around "S_parse_body",
as defined in perl.c, which processes the command line
options, sets up any statically linked XS modules, opens
the program and calls "yyparse" to parse it.
Parsing
The aim of this stage is to take the Perl source, and
turn it into an op tree. We'll see what one of those
looks like later. Strictly speaking, there's three things
going on here.
"yyparse", the parser, lives in perly.c, although you're
better off reading the original YACC input in perly.y.
(Yes, Virginia, there is a YACC grammar for Perl!) The
job of the parser is to take your code and "understand"
it, splitting it into sentences, deciding which operands
go with which operators and so on.
The parser is nobly assisted by the lexer, which chunks
up your input into tokens, and decides what type of thing
each token is: a variable name, an operator, a bareword,
a subroutine, a core function, and so on. The main point
of entry to the lexer is "yylex", and that and its
associated routines can be found in toke.c. Perl isn't
much like other computer languages; it's highly context
sensitive at times, it can be tricky to work out what
sort of token something is, or where a token ends. As
such, there's a lot of interplay between the tokeniser
and the parser, which can get pretty frightening if
you're not used to it.
As the parser understands a Perl program, it builds up a
tree of operations for the interpreter to perform during
execution. The routines which construct and link together
the various operations are to be found in op.c, and will
be examined later.
Optimization
Now the parsing stage is complete, and the finished tree
represents the operations that the Perl interpreter needs
to perform to execute our program. Next, Perl does a dry
run over the tree looking for optimisations: constant
expressions such as "3 + 4" will be computed now, and the
optimizer will also see if any multiple operations can be
replaced with a single one. For instance, to fetch the
variable $foo, instead of grabbing the glob *foo and
looking at the scalar component, the optimizer fiddles
the op tree to use a function which directly looks up the
scalar in question. The main optimizer is "peep" in op.c,
and many ops have their own optimizing functions.
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Running
Now we're finally ready to go: we have compiled Perl byte
code, and all that's left to do is run it. The actual
execution is done by the "runops_standard" function in
run.c; more specifically, it's done by these three
innocent looking lines:
while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
PERL_ASYNC_CHECK();
}
You may be more comfortable with the Perl version of
that:
PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
Well, maybe not. Anyway, each op contains a function
pointer, which stipulates the function which will
actually carry out the operation. This function will
return the next op in the sequence - this allows for
things like "if" which choose the next op dynamically at
run time. The "PERL_ASYNC_CHECK" makes sure that things
like signals interrupt execution if required.
The actual functions called are known as PP code, and
they're spread between four files: pp_hot.c contains the
"hot" code, which is most often used and highly
optimized, pp_sys.c contains all the system-specific
functions, pp_ctl.c contains the functions which
implement control structures ("if", "while" and the like)
and pp.c contains everything else. These are, if you
like, the C code for Perl's built-in functions and
operators.
Note that each "pp_" function is expected to return a
pointer to the next op. Calls to perl subs (and eval
blocks) are handled within the same runops loop, and do
not consume extra space on the C stack. For example,
"pp_entersub" and "pp_entertry" just push a "CxSUB" or
"CxEVAL" block struct onto the context stack which
contain the address of the op following the sub call or
eval. They then return the first op of that sub or eval
block, and so execution continues of that sub or block.
Later, a "pp_leavesub" or "pp_leavetry" op pops the
"CxSUB" or "CxEVAL", retrieves the return op from it, and
returns it.
Exception handing
Perl's exception handing (i.e. "die" etc.) is built on
top of the low-level "setjmp()"/"longjmp()" C-library
functions. These basically provide a way to capture the
current PC and SP registers and later restore them; i.e.
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a "longjmp()" continues at the point in code where a
previous "setjmp()" was done, with anything further up on
the C stack being lost. This is why code should always
save values using "SAVE_FOO" rather than in auto
variables.
The perl core wraps "setjmp()" etc in the macros
"JMPENV_PUSH" and "JMPENV_JUMP". The basic rule of perl
exceptions is that "exit", and "die" (in the absence of
"eval") perform a JMPENV_JUMP(2), while "die" within
"eval" does a JMPENV_JUMP(3).
At entry points to perl, such as "perl_parse()",
"perl_run()" and "call_sv(cv, G_EVAL)" each does a
"JMPENV_PUSH", then enter a runops loop or whatever, and
handle possible exception returns. For a 2 return, final
cleanup is performed, such as popping stacks and calling
"CHECK" or "END" blocks. Amongst other things, this is
how scope cleanup still occurs during an "exit".
If a "die" can find a "CxEVAL" block on the context
stack, then the stack is popped to that level and the
return op in that block is assigned to "PL_restartop";
then a JMPENV_JUMP(3) is performed. This normally passes
control back to the guard. In the case of "perl_run" and
"call_sv", a non-null "PL_restartop" triggers re-entry to
the runops loop. The is the normal way that "die" or
"croak" is handled within an "eval".
Sometimes ops are executed within an inner runops loop,
such as tie, sort or overload code. In this case,
something like
sub FETCH { eval { die } }
would cause a longjmp right back to the guard in
"perl_run", popping both runops loops, which is clearly
incorrect. One way to avoid this is for the tie code to
do a "JMPENV_PUSH" before executing "FETCH" in the inner
runops loop, but for efficiency reasons, perl in fact
just sets a flag, using "CATCH_SET(TRUE)". The
"pp_require", "pp_entereval" and "pp_entertry" ops check
this flag, and if true, they call "docatch", which does a
"JMPENV_PUSH" and starts a new runops level to execute
the code, rather than doing it on the current loop.
As a further optimisation, on exit from the eval block in
the "FETCH", execution of the code following the block is
still carried on in the inner loop. When an exception is
raised, "docatch" compares the "JMPENV" level of the
"CxEVAL" with "PL_top_env" and if they differ, just re-
throws the exception. In this way any inner loops get
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popped.
Here's an example.
1: eval { tie @a, 'A' };
2: sub A::TIEARRAY {
3: eval { die };
4: die;
5: }
To run this code, "perl_run" is called, which does a
"JMPENV_PUSH" then enters a runops loop. This loop
executes the eval and tie ops on line 1, with the eval
pushing a "CxEVAL" onto the context stack.
The "pp_tie" does a "CATCH_SET(TRUE)", then starts a
second runops loop to execute the body of "TIEARRAY".
When it executes the entertry op on line 3, "CATCH_GET"
is true, so "pp_entertry" calls "docatch" which does a
"JMPENV_PUSH" and starts a third runops loop, which then
executes the die op. At this point the C call stack looks
like this:
Perl_pp_die
Perl_runops # third loop
S_docatch_body
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
and the context and data stacks, as shown by "-Dstv",
look like:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
CX 1: EVAL => *
retop=nextstate
The die pops the first "CxEVAL" off the context stack,
sets "PL_restartop" from it, does a JMPENV_JUMP(3), and
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control returns to the top "docatch". This then starts
another third-level runops level, which executes the
nextstate, pushmark and die ops on line 4. At the point
that the second "pp_die" is called, the C call stack
looks exactly like that above, even though we are no
longer within an inner eval; this is because of the
optimization mentioned earlier. However, the context
stack now looks like this, ie with the top CxEVAL popped:
STACK 0: MAIN
CX 0: BLOCK =>
CX 1: EVAL => AV() PV("A"\0)
retop=leave
STACK 1: MAGIC
CX 0: SUB =>
retop=(null)
The die on line 4 pops the context stack back down to the
CxEVAL, leaving it as:
STACK 0: MAIN
CX 0: BLOCK =>
As usual, "PL_restartop" is extracted from the "CxEVAL",
and a JMPENV_JUMP(3) done, which pops the C stack back to
the docatch:
S_docatch
Perl_pp_entertry
Perl_runops # second loop
S_call_body
Perl_call_sv
Perl_pp_tie
Perl_runops # first loop
S_run_body
perl_run
main
In this case, because the "JMPENV" level recorded in the
"CxEVAL" differs from the current one, "docatch" just
does a JMPENV_JUMP(3) and the C stack unwinds to:
perl_run
main
Because "PL_restartop" is non-null, "run_body" starts a
new runops loop and execution continues.
Internal Variable Types
You should by now have had a look at perlguts, which tells
you about Perl's internal variable types: SVs, HVs, AVs and
the rest. If not, do that now.
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These variables are used not only to represent Perl-space
variables, but also any constants in the code, as well as
some structures completely internal to Perl. The symbol
table, for instance, is an ordinary Perl hash. Your code is
represented by an SV as it's read into the parser; any
program files you call are opened via ordinary Perl
filehandles, and so on.
The core Devel::Peek module lets us examine SVs from a Perl
program. Let's see, for instance, how Perl treats the
constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")'
1 SV = PV(0xa041450) at 0xa04ecbc
2 REFCNT = 1
3 FLAGS = (POK,READONLY,pPOK)
4 PV = 0xa0484e0 "hello"\0
5 CUR = 5
6 LEN = 6
Reading "Devel::Peek" output takes a bit of practise, so
let's go through it line by line.
Line 1 tells us we're looking at an SV which lives at
0xa04ecbc in memory. SVs themselves are very simple
structures, but they contain a pointer to a more complex
structure. In this case, it's a PV, a structure which holds
a string value, at location 0xa041450. Line 2 is the
reference count; there are no other references to this data,
so it's 1.
Line 3 are the flags for this SV - it's OK to use it as a
PV, it's a read-only SV (because it's a constant) and the
data is a PV internally. Next we've got the contents of the
string, starting at location 0xa0484e0.
Line 5 gives us the current length of the string - note that
this does not include the null terminator. Line 6 is not the
length of the string, but the length of the currently
allocated buffer; as the string grows, Perl automatically
extends the available storage via a routine called "SvGROW".
You can get at any of these quantities from C very easily;
just add "Sv" to the name of the field shown in the snippet,
and you've got a macro which will return the value:
"SvCUR(sv)" returns the current length of the string,
"SvREFCOUNT(sv)" returns the reference count, "SvPV(sv,
len)" returns the string itself with its length, and so on.
More macros to manipulate these properties can be found in
perlguts.
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Let's take an example of manipulating a PV, from
"sv_catpvn", in sv.c
1 void
2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len)
3 {
4 STRLEN tlen;
5 char *junk;
6 junk = SvPV_force(sv, tlen);
7 SvGROW(sv, tlen + len + 1);
8 if (ptr == junk)
9 ptr = SvPVX(sv);
10 Move(ptr,SvPVX(sv)+tlen,len,char);
11 SvCUR(sv) += len;
12 *SvEND(sv) = '\0';
13 (void)SvPOK_only_UTF8(sv); /* validate pointer */
14 SvTAINT(sv);
15 }
This is a function which adds a string, "ptr", of length
"len" onto the end of the PV stored in "sv". The first thing
we do in line 6 is make sure that the SV has a valid PV, by
calling the "SvPV_force" macro to force a PV. As a side
effect, "tlen" gets set to the current value of the PV, and
the PV itself is returned to "junk".
In line 7, we make sure that the SV will have enough room to
accommodate the old string, the new string and the null
terminator. If "LEN" isn't big enough, "SvGROW" will
reallocate space for us.
Now, if "junk" is the same as the string we're trying to
add, we can grab the string directly from the SV; "SvPVX" is
the address of the PV in the SV.
Line 10 does the actual catenation: the "Move" macro moves a
chunk of memory around: we move the string "ptr" to the end
of the PV - that's the start of the PV plus its current
length. We're moving "len" bytes of type "char". After doing
so, we need to tell Perl we've extended the string, by
altering "CUR" to reflect the new length. "SvEND" is a macro
which gives us the end of the string, so that needs to be a
"\0".
Line 13 manipulates the flags; since we've changed the PV,
any IV or NV values will no longer be valid: if we have
"$a=10; $a.="6";" we don't want to use the old IV of 10.
"SvPOK_only_utf8" is a special UTF-8-aware version of
"SvPOK_only", a macro which turns off the IOK and NOK flags
and turns on POK. The final "SvTAINT" is a macro which
launders tainted data if taint mode is turned on.
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AVs and HVs are more complicated, but SVs are by far the
most common variable type being thrown around. Having seen
something of how we manipulate these, let's go on and look
at how the op tree is constructed.
Op Trees
First, what is the op tree, anyway? The op tree is the
parsed representation of your program, as we saw in our
section on parsing, and it's the sequence of operations that
Perl goes through to execute your program, as we saw in
"Running".
An op is a fundamental operation that Perl can perform: all
the built-in functions and operators are ops, and there are
a series of ops which deal with concepts the interpreter
needs internally - entering and leaving a block, ending a
statement, fetching a variable, and so on.
The op tree is connected in two ways: you can imagine that
there are two "routes" through it, two orders in which you
can traverse the tree. First, parse order reflects how the
parser understood the code, and secondly, execution order
tells perl what order to perform the operations in.
The easiest way to examine the op tree is to stop Perl after
it has finished parsing, and get it to dump out the tree.
This is exactly what the compiler backends B::Terse,
B::Concise and B::Debug do.
Let's have a look at how Perl sees "$a = $b + $c":
% perl -MO=Terse -e '$a=$b+$c'
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
4 BINOP (0x8179828) sassign
5 BINOP (0x8179800) add [1]
6 UNOP (0x81796e0) null [15]
7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b
8 UNOP (0x81797e0) null [15]
9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c
10 UNOP (0x816b4f0) null [15]
11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a
Let's start in the middle, at line 4. This is a BINOP, a
binary operator, which is at location 0x8179828. The
specific operator in question is "sassign" - scalar
assignment - and you can find the code which implements it
in the function "pp_sassign" in pp_hot.c. As a binary
operator, it has two children: the add operator, providing
the result of "$b+$c", is uppermost on line 5, and the left
hand side is on line 10.
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Line 10 is the null op: this does exactly nothing. What is
that doing there? If you see the null op, it's a sign that
something has been optimized away after parsing. As we
mentioned in "Optimization", the optimization stage
sometimes converts two operations into one, for example when
fetching a scalar variable. When this happens, instead of
rewriting the op tree and cleaning up the dangling pointers,
it's easier just to replace the redundant operation with the
null op. Originally, the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15]
11 SVOP (0x816dcf0) gv GV (0x80fa460) *a
That is, fetch the "a" entry from the main symbol table, and
then look at the scalar component of it: "gvsv" ("pp_gvsv"
into pp_hot.c) happens to do both these things.
The right hand side, starting at line 5 is similar to what
we've just seen: we have the "add" op ("pp_add" also in
pp_hot.c) add together two "gvsv"s.
Now, what's this about?
1 LISTOP (0x8179888) leave
2 OP (0x81798b0) enter
3 COP (0x8179850) nextstate
"enter" and "leave" are scoping ops, and their job is to
perform any housekeeping every time you enter and leave a
block: lexical variables are tidied up, unreferenced
variables are destroyed, and so on. Every program will have
those first three lines: "leave" is a list, and its children
are all the statements in the block. Statements are
delimited by "nextstate", so a block is a collection of
"nextstate" ops, with the ops to be performed for each
statement being the children of "nextstate". "enter" is a
single op which functions as a marker.
That's how Perl parsed the program, from top to bottom:
Program
|
Statement
|
=
/ \
/ \
$a +
/ \
$b $c
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However, it's impossible to perform the operations in this
order: you have to find the values of $b and $c before you
add them together, for instance. So, the other thread that
runs through the op tree is the execution order: each op has
a field "op_next" which points to the next op to be run, so
following these pointers tells us how perl executes the
code. We can traverse the tree in this order using the
"exec" option to "B::Terse":
% perl -MO=Terse,exec -e '$a=$b+$c'
1 OP (0x8179928) enter
2 COP (0x81798c8) nextstate
3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b
4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c
5 BINOP (0x8179878) add [1]
6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a
7 BINOP (0x81798a0) sassign
8 LISTOP (0x8179900) leave
This probably makes more sense for a human: enter a block,
start a statement. Get the values of $b and $c, and add them
together. Find $a, and assign one to the other. Then leave.
The way Perl builds up these op trees in the parsing process
can be unravelled by examining perly.y, the YACC grammar.
Let's take the piece we need to construct the tree for "$a =
$b + $c"
1 term : term ASSIGNOP term
2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); }
3 | term ADDOP term
4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
If you're not used to reading BNF grammars, this is how it
works: You're fed certain things by the tokeniser, which
generally end up in upper case. Here, "ADDOP", is provided
when the tokeniser sees "+" in your code. "ASSIGNOP" is
provided when "=" is used for assigning. These are "terminal
symbols", because you can't get any simpler than them.
The grammar, lines one and three of the snippet above, tells
you how to build up more complex forms. These complex forms,
"non-terminal symbols" are generally placed in lower case.
"term" here is a non-terminal symbol, representing a single
expression.
The grammar gives you the following rule: you can make the
thing on the left of the colon if you see all the things on
the right in sequence. This is called a "reduction", and
the aim of parsing is to completely reduce the input. There
are several different ways you can perform a reduction,
separated by vertical bars: so, "term" followed by "="
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followed by "term" makes a "term", and "term" followed by
"+" followed by "term" can also make a "term".
So, if you see two terms with an "=" or "+", between them,
you can turn them into a single expression. When you do
this, you execute the code in the block on the next line: if
you see "=", you'll do the code in line 2. If you see "+",
you'll do the code in line 4. It's this code which
contributes to the op tree.
| term ADDOP term
{ $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
What this does is creates a new binary op, and feeds it a
number of variables. The variables refer to the tokens: $1
is the first token in the input, $2 the second, and so on -
think regular expression backreferences. $$ is the op
returned from this reduction. So, we call "newBINOP" to
create a new binary operator. The first parameter to
"newBINOP", a function in op.c, is the op type. It's an
addition operator, so we want the type to be "ADDOP". We
could specify this directly, but it's right there as the
second token in the input, so we use $2. The second
parameter is the op's flags: 0 means "nothing special". Then
the things to add: the left and right hand side of our
expression, in scalar context.
Stacks
When perl executes something like "addop", how does it pass
on its results to the next op? The answer is, through the
use of stacks. Perl has a number of stacks to store things
it's currently working on, and we'll look at the three most
important ones here.
Argument stack
Arguments are passed to PP code and returned from PP code
using the argument stack, "ST". The typical way to handle
arguments is to pop them off the stack, deal with them
how you wish, and then push the result back onto the
stack. This is how, for instance, the cosine operator
works:
NV value;
value = POPn;
value = Perl_cos(value);
XPUSHn(value);
We'll see a more tricky example of this when we consider
Perl's macros below. "POPn" gives you the NV (floating
point value) of the top SV on the stack: the $x in
"cos($x)". Then we compute the cosine, and push the
result back as an NV. The "X" in "XPUSHn" means that the
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stack should be extended if necessary - it can't be
necessary here, because we know there's room for one more
item on the stack, since we've just removed one! The
"XPUSH*" macros at least guarantee safety.
Alternatively, you can fiddle with the stack directly:
"SP" gives you the first element in your portion of the
stack, and "TOP*" gives you the top SV/IV/NV/etc. on the
stack. So, for instance, to do unary negation of an
integer:
SETi(-TOPi);
Just set the integer value of the top stack entry to its
negation.
Argument stack manipulation in the core is exactly the
same as it is in XSUBs - see perlxstut, perlxs and
perlguts for a longer description of the macros used in
stack manipulation.
Mark stack
I say "your portion of the stack" above because PP code
doesn't necessarily get the whole stack to itself: if
your function calls another function, you'll only want to
expose the arguments aimed for the called function, and
not (necessarily) let it get at your own data. The way we
do this is to have a "virtual" bottom-of-stack, exposed
to each function. The mark stack keeps bookmarks to
locations in the argument stack usable by each function.
For instance, when dealing with a tied variable,
(internally, something with "P" magic) Perl has to call
methods for accesses to the tied variables. However, we
need to separate the arguments exposed to the method to
the argument exposed to the original function - the store
or fetch or whatever it may be. Here's roughly how the
tied "push" is implemented; see "av_push" in av.c:
1 PUSHMARK(SP);
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
5 PUTBACK;
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
Let's examine the whole implementation, for practice:
1 PUSHMARK(SP);
Push the current state of the stack pointer onto the mark
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stack. This is so that when we've finished adding items
to the argument stack, Perl knows how many things we've
added recently.
2 EXTEND(SP,2);
3 PUSHs(SvTIED_obj((SV*)av, mg));
4 PUSHs(val);
We're going to add two more items onto the argument
stack: when you have a tied array, the "PUSH" subroutine
receives the object and the value to be pushed, and
that's exactly what we have here - the tied object,
retrieved with "SvTIED_obj", and the value, the SV "val".
5 PUTBACK;
Next we tell Perl to update the global stack pointer from
our internal variable: "dSP" only gave us a local copy,
not a reference to the global.
6 ENTER;
7 call_method("PUSH", G_SCALAR|G_DISCARD);
8 LEAVE;
"ENTER" and "LEAVE" localise a block of code - they make
sure that all variables are tidied up, everything that
has been localised gets its previous value returned, and
so on. Think of them as the "{" and "}" of a Perl block.
To actually do the magic method call, we have to call a
subroutine in Perl space: "call_method" takes care of
that, and it's described in perlcall. We call the "PUSH"
method in scalar context, and we're going to discard its
return value. The call_method() function removes the top
element of the mark stack, so there is nothing for the
caller to clean up.
Save stack
C doesn't have a concept of local scope, so perl provides
one. We've seen that "ENTER" and "LEAVE" are used as
scoping braces; the save stack implements the C
equivalent of, for example:
{
local $foo = 42;
...
}
See "Localising Changes" in perlguts for how to use the
save stack.
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Millions of Macros
One thing you'll notice about the Perl source is that it's
full of macros. Some have called the pervasive use of macros
the hardest thing to understand, others find it adds to
clarity. Let's take an example, the code which implements
the addition operator:
1 PP(pp_add)
2 {
3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
4 {
5 dPOPTOPnnrl_ul;
6 SETn( left + right );
7 RETURN;
8 }
9 }
Every line here (apart from the braces, of course) contains
a macro. The first line sets up the function declaration as
Perl expects for PP code; line 3 sets up variable
declarations for the argument stack and the target, the
return value of the operation. Finally, it tries to see if
the addition operation is overloaded; if so, the appropriate
subroutine is called.
Line 5 is another variable declaration - all variable
declarations start with "d" - which pops from the top of the
argument stack two NVs (hence "nn") and puts them into the
variables "right" and "left", hence the "rl". These are the
two operands to the addition operator. Next, we call "SETn"
to set the NV of the return value to the result of adding
the two values. This done, we return - the "RETURN" macro
makes sure that our return value is properly handled, and we
pass the next operator to run back to the main run loop.
Most of these macros are explained in perlapi, and some of
the more important ones are explained in perlxs as well. Pay
special attention to "Background and PERL_IMPLICIT_CONTEXT"
in perlguts for information on the "[pad]THX_?" macros.
The .i Targets
You can expand the macros in a foo.c file by saying
make foo.i
which will expand the macros using cpp. Don't be scared by
the results.
SOURCE CODE STATIC ANALYSIS
Various tools exist for analysing C source code statically,
as opposed to dynamically, that is, without executing the
code. It is possible to detect resource leaks, undefined
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behaviour, type mismatches, portability problems, code paths
that would cause illegal memory accesses, and other similar
problems by just parsing the C code and looking at the
resulting graph, what does it tell about the execution and
data flows. As a matter of fact, this is exactly how C
compilers know to give warnings about dubious code.
lint, splint
The good old C code quality inspector, "lint", is available
in several platforms, but please be aware that there are
several different implementations of it by different
vendors, which means that the flags are not identical across
different platforms.
There is a lint variant called "splint" (Secure Programming
Lint) available from http://www.splint.org/ that should
compile on any Unix-like platform.
There are "lint" and <splint> targets in Makefile, but you
may have to diddle with the flags (see above).
Coverity
Coverity (http://www.coverity.com/) is a product similar to
lint and as a testbed for their product they periodically
check several open source projects, and they give out
accounts to open source developers to the defect databases.
cpd (cut-and-paste detector)
The cpd tool detects cut-and-paste coding. If one instance
of the cut-and-pasted code changes, all the other spots
should probably be changed, too. Therefore such code should
probably be turned into a subroutine or a macro.
cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd
project (http://pmd.sourceforge.net/). pmd was originally
written for static analysis of Java code, but later the cpd
part of it was extended to parse also C and C++.
Download the pmd-bin-X.Y.zip () from the SourceForge site,
extract the pmd-X.Y.jar from it, and then run that on source
code thusly:
java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
You may run into memory limits, in which case you should use
the -Xmx option:
java -Xmx512M ...
gcc warnings
Though much can be written about the inconsistency and
coverage problems of gcc warnings (like "-Wall" not meaning
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"all the warnings", or some common portability problems not
being covered by "-Wall", or "-ansi" and "-pedantic" both
being a poorly defined collection of warnings, and so
forth), gcc is still a useful tool in keeping our coding
nose clean.
The "-Wall" is by default on.
The "-ansi" (and its sidekick, "-pedantic") would be nice to
be on always, but unfortunately they are not safe on all
platforms, they can for example cause fatal conflicts with
the system headers (Solaris being a prime example). If
Configure "-Dgccansipedantic" is used, the "cflags" frontend
selects "-ansi -pedantic" for the platforms where they are
known to be safe.
Starting from Perl 5.9.4 the following extra flags are
added:
o "-Wendif-labels"
o "-Wextra"
o "-Wdeclaration-after-statement"
The following flags would be nice to have but they would
first need their own Augean stablemaster:
o "-Wpointer-arith"
o "-Wshadow"
o "-Wstrict-prototypes"
The "-Wtraditional" is another example of the annoying
tendency of gcc to bundle a lot of warnings under one switch
(it would be impossible to deploy in practice because it
would complain a lot) but it does contain some warnings that
would be beneficial to have available on their own, such as
the warning about string constants inside macros containing
the macro arguments: this behaved differently pre-ANSI than
it does in ANSI, and some C compilers are still in
transition, AIX being an example.
Warnings of other C compilers
Other C compilers (yes, there are other C compilers than
gcc) often have their "strict ANSI" or "strict ANSI with
some portability extensions" modes on, like for example the
Sun Workshop has its "-Xa" mode on (though implicitly), or
the DEC (these days, HP...) has its "-std1" mode on.
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DEBUGGING
You can compile a special debugging version of Perl, which
allows you to use the "-D" option of Perl to tell more about
what Perl is doing. But sometimes there is no alternative
than to dive in with a debugger, either to see the stack
trace of a core dump (very useful in a bug report), or
trying to figure out what went wrong before the core dump
happened, or how did we end up having wrong or unexpected
results.
Poking at Perl
To really poke around with Perl, you'll probably want to
build Perl for debugging, like this:
./Configure -d -D optimize=-g
make
"-g" is a flag to the C compiler to have it produce
debugging information which will allow us to step through a
running program, and to see in which C function we are at
(without the debugging information we might see only the
numerical addresses of the functions, which is not very
helpful).
Configure will also turn on the "DEBUGGING" compilation
symbol which enables all the internal debugging code in
Perl. There are a whole bunch of things you can debug with
this: perlrun lists them all, and the best way to find out
about them is to play about with them. The most useful
options are probably
l Context (loop) stack processing
t Trace execution
o Method and overloading resolution
c String/numeric conversions
Some of the functionality of the debugging code can be
achieved using XS modules.
-Dr => use re 'debug'
-Dx => use O 'Debug'
Using a source-level debugger
If the debugging output of "-D" doesn't help you, it's time
to step through perl's execution with a source-level
debugger.
o We'll use "gdb" for our examples here; the principles
will apply to any debugger (many vendors call their
debugger "dbx"), but check the manual of the one you're
using.
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To fire up the debugger, type
gdb ./perl
Or if you have a core dump:
gdb ./perl core
You'll want to do that in your Perl source tree so the
debugger can read the source code. You should see the
copyright message, followed by the prompt.
(gdb)
"help" will get you into the documentation, but here are the
most useful commands:
run [args]
Run the program with the given arguments.
break function_name
break source.c:xxx
Tells the debugger that we'll want to pause execution
when we reach either the named function (but see
"Internal Functions" in perlguts!) or the given line in
the named source file.
step
Steps through the program a line at a time.
next
Steps through the program a line at a time, without
descending into functions.
continue
Run until the next breakpoint.
finish
Run until the end of the current function, then stop
again.
'enter'
Just pressing Enter will do the most recent operation
again - it's a blessing when stepping through miles of
source code.
print
Execute the given C code and print its results. WARNING:
Perl makes heavy use of macros, and gdb does not
necessarily support macros (see later "gdb macro
support"). You'll have to substitute them yourself, or
to invoke cpp on the source code files (see "The .i
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Targets") So, for instance, you can't say
print SvPV_nolen(sv)
but you have to say
print Perl_sv_2pv_nolen(sv)
You may find it helpful to have a "macro dictionary", which
you can produce by saying "cpp -dM perl.c | sort". Even
then, cpp won't recursively apply those macros for you.
gdb macro support
Recent versions of gdb have fairly good macro support, but
in order to use it you'll need to compile perl with macro
definitions included in the debugging information. Using
gcc version 3.1, this means configuring with
"-Doptimize=-g3". Other compilers might use a different
switch (if they support debugging macros at all).
Dumping Perl Data Structures
One way to get around this macro hell is to use the dumping
functions in dump.c; these work a little like an internal
Devel::Peek, but they also cover OPs and other structures
that you can't get at from Perl. Let's take an example.
We'll use the "$a = $b + $c" we used before, but give it a
bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good
place to stop and poke around?
What about "pp_add", the function we examined earlier to
implement the "+" operator:
(gdb) break Perl_pp_add
Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
Notice we use "Perl_pp_add" and not "pp_add" - see "Internal
Functions" in perlguts. With the breakpoint in place, we
can run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
Lots of junk will go past as gdb reads in the relevant
source files and libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309
309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
(gdb) step
311 dPOPTOPnnrl_ul;
(gdb)
We looked at this bit of code before, and we said that
"dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into
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"left" and "right" - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \
SV *leftsv = TOPs; \
NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
"POPn" takes the SV from the top of the stack and obtains
its NV either directly (if "SvNOK" is set) or by calling the
"sv_2nv" function. "TOPs" takes the next SV from the top of
the stack - yes, "POPn" uses "TOPs" - but doesn't remove it.
We then use "SvNV" to get the NV from "leftsv" in the same
way as before - yes, "POPn" uses "SvNV".
Since we don't have an NV for $b, we'll have to use "sv_2nv"
to convert it. If we step again, we'll find ourselves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
1669 if (!sv)
(gdb)
We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0xa06a510 "6XXXX"\0
CUR = 5
LEN = 6
$1 = void
We know we're going to get 6 from this, so let's finish the
subroutine:
(gdb) finish
Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
0x462669 in Perl_pp_add () at pp_hot.c:311
311 dPOPTOPnnrl_ul;
We can also dump out this op: the current op is always
stored in "PL_op", and we can dump it with "Perl_op_dump".
This'll give us similar output to B::Debug.
{
13 TYPE = add ===> 14
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (12)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
11 TYPE = gvsv ===> 12
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FLAGS = (SCALAR)
GV = main::b
}
}
# finish this later #
Patching
All right, we've now had a look at how to navigate the Perl
sources and some things you'll need to know when fiddling
with them. Let's now get on and create a simple patch.
Here's something Larry suggested: if a "U" is the first
active format during a "pack", (for example, "pack "U3C8",
@stuff") then the resulting string should be treated as
UTF-8 encoded.
If you are working with a git clone of the Perl repository,
you will want to create a branch for your changes. This will
make creating a proper patch much simpler. See the
perlrepository for details on how to do this.
How do we prepare to fix this up? First we locate the code
in question - the "pack" happens at runtime, so it's going
to be in one of the pp files. Sure enough, "pp_pack" is in
pp.c. Since we're going to be altering this file, let's copy
it to pp.c~.
[Well, it was in pp.c when this tutorial was written. It has
now been split off with "pp_unpack" to its own file,
pp_pack.c]
Now let's look over "pp_pack": we take a pattern into "pat",
and then loop over the pattern, taking each format character
in turn into "datum_type". Then for each possible format
character, we swallow up the other arguments in the pattern
(a field width, an asterisk, and so on) and convert the next
chunk input into the specified format, adding it onto the
output SV "cat".
How do we know if the "U" is the first format in the "pat"?
Well, if we have a pointer to the start of "pat" then, if we
see a "U" we can test whether we're still at the start of
the string. So, here's where "pat" is set up:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
register I32 len;
I32 datumtype;
SV *fromstr;
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We'll have another string pointer in there:
STRLEN fromlen;
register char *pat = SvPVx(*++MARK, fromlen);
register char *patend = pat + fromlen;
+ char *patcopy;
register I32 len;
I32 datumtype;
SV *fromstr;
And just before we start the loop, we'll set "patcopy" to be
the start of "pat":
items = SP - MARK;
MARK++;
sv_setpvn(cat, "", 0);
+ patcopy = pat;
while (pat < patend) {
Now if we see a "U" which was at the start of the string, we
turn on the "UTF8" flag for the output SV, "cat":
+ if (datumtype == 'U' && pat==patcopy+1)
+ SvUTF8_on(cat);
if (datumtype == '#') {
while (pat < patend && *pat != '\n')
pat++;
Remember that it has to be "patcopy+1" because the first
character of the string is the "U" which has been swallowed
into "datumtype!"
Oops, we forgot one thing: what if there are spaces at the
start of the pattern? "pack(" U*", @stuff)" will have "U"
as the first active character, even though it's not the
first thing in the pattern. In this case, we have to advance
"patcopy" along with "pat" when we see spaces:
if (isSPACE(datumtype))
continue;
needs to become
if (isSPACE(datumtype)) {
patcopy++;
continue;
}
OK. That's the C part done. Now we must do two additional
things before this patch is ready to go: we've changed the
behaviour of Perl, and so we must document that change. We
must also provide some more regression tests to make sure
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our patch works and doesn't create a bug somewhere else
along the line.
The regression tests for each operator live in t/op/, and so
we make a copy of t/op/pack.t to t/op/pack.t~. Now we can
add our tests to the end. First, we'll test that the "U"
does indeed create Unicode strings.
t/op/pack.t has a sensible ok() function, but if it didn't
we could use the one from t/test.pl.
require './test.pl';
plan( tests => 159 );
so instead of this:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
pack("U*",1,20,300,4000);
print "ok $test\n"; $test++;
we can write the more sensible (see Test::More for a full
explanation of is() and other testing functions).
is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
"U* produces Unicode" );
Now we'll test that we got that space-at-the-beginning
business right:
is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000),
" with spaces at the beginning" );
And finally we'll test that we don't make Unicode strings if
"U" is not the first active format:
isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
"U* not first isn't Unicode" );
Mustn't forget to change the number of tests which appears
at the top, or else the automated tester will get confused.
This will either look like this:
print "1..156\n";
or this:
plan( tests => 156 );
We now compile up Perl, and run it through the test suite.
Our new tests pass, hooray!
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Finally, the documentation. The job is never done until the
paperwork is over, so let's describe the change we've just
made. The relevant place is pod/perlfunc.pod; again, we make
a copy, and then we'll insert this text in the description
of "pack":
=item *
If the pattern begins with a C<U>, the resulting string will be treated
as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string
with an initial C<U0>, and the bytes that follow will be interpreted as
Unicode characters. If you don't want this to happen, you can begin
your pattern with C<C0> (or anything else) to force Perl not to UTF-8
encode your string, and then follow this with a C<U*> somewhere in your
pattern.
Patching a core module
This works just like patching anything else, with an extra
consideration. Many core modules also live on CPAN. If
this is so, patch the CPAN version instead of the core and
send the patch off to the module maintainer (with a copy to
p5p). This will help the module maintainer keep the CPAN
version in sync with the core version without constantly
scanning p5p.
The list of maintainers of core modules is usefully
documented in Porting/Maintainers.pl.
Adding a new function to the core
If, as part of a patch to fix a bug, or just because you
have an especially good idea, you decide to add a new
function to the core, discuss your ideas on p5p well before
you start work. It may be that someone else has already
attempted to do what you are considering and can give lots
of good advice or even provide you with bits of code that
they already started (but never finished).
You have to follow all of the advice given above for
patching. It is extremely important to test any addition
thoroughly and add new tests to explore all boundary
conditions that your new function is expected to handle. If
your new function is used only by one module (e.g. toke),
then it should probably be named S_your_function (for
static); on the other hand, if you expect it to accessible
from other functions in Perl, you should name it
Perl_your_function. See "Internal Functions" in perlguts
for more details.
The location of any new code is also an important
consideration. Don't just create a new top level .c file
and put your code there; you would have to make changes to
Configure (so the Makefile is created properly), as well as
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possibly lots of include files. This is strictly pumpking
business.
It is better to add your function to one of the existing top
level source code files, but your choice is complicated by
the nature of the Perl distribution. Only the files that
are marked as compiled static are located in the perl
executable. Everything else is located in the shared
library (or DLL if you are running under WIN32). So, for
example, if a function was only used by functions located in
toke.c, then your code can go in toke.c. If, however, you
want to call the function from universal.c, then you should
put your code in another location, for example util.c.
In addition to writing your c-code, you will need to create
an appropriate entry in embed.pl describing your function,
then run 'make regen_headers' to create the entries in the
numerous header files that perl needs to compile correctly.
See "Internal Functions" in perlguts for information on the
various options that you can set in embed.pl. You will
forget to do this a few (or many) times and you will get
warnings during the compilation phase. Make sure that you
mention this when you post your patch to P5P; the pumpking
needs to know this.
When you write your new code, please be conscious of
existing code conventions used in the perl source files.
See perlstyle for details. Although most of the guidelines
discussed seem to focus on Perl code, rather than c, they
all apply (except when they don't ;). Also see
perlrepository for lots of details about both formatting and
submitting patches of your changes.
Lastly, TEST TEST TEST TEST TEST any code before posting to
p5p. Test on as many platforms as you can find. Test as
many perl Configure options as you can (e.g. MULTIPLICITY).
If you have profiling or memory tools, see "EXTERNAL TOOLS
FOR DEBUGGING PERL" below for how to use them to further
test your code. Remember that most of the people on P5P are
doing this on their own time and don't have the time to
debug your code.
Writing a test
Every module and built-in function has an associated test
file (or should...). If you add or change functionality,
you have to write a test. If you fix a bug, you have to
write a test so that bug never comes back. If you alter the
docs, it would be nice to test what the new documentation
says.
In short, if you submit a patch you probably also have to
patch the tests.
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For modules, the test file is right next to the module
itself. lib/strict.t tests lib/strict.pm. This is a recent
innovation, so there are some snags (and it would be
wonderful for you to brush them out), but it basically works
that way. Everything else lives in t/.
If you add a new test directory under t/, it is imperative
that you add that directory to t/HARNESS and t/TEST.
t/base/
Testing of the absolute basic functionality of Perl.
Things like "if", basic file reads and writes, simple
regexes, etc. These are run first in the test suite and
if any of them fail, something is really broken.
t/cmd/
These test the basic control structures, "if/else",
"while", subroutines, etc.
t/comp/
Tests basic issues of how Perl parses and compiles
itself.
t/io/
Tests for built-in IO functions, including command line
arguments.
t/lib/
The old home for the module tests, you shouldn't put
anything new in here. There are still some bits and
pieces hanging around in here that need to be moved.
Perhaps you could move them? Thanks!
t/mro/
Tests for perl's method resolution order implementations
(see mro).
t/op/
Tests for perl's built in functions that don't fit into
any of the other directories.
t/re/
Tests for regex related functions or behaviour. (These
used to live in t/op).
t/run/
Testing features of how perl actually runs, including
exit codes and handling of PERL* environment variables.
t/uni/
Tests for the core support of Unicode.
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t/win32/
Windows-specific tests.
t/x2p
A test suite for the s2p converter.
The core uses the same testing style as the rest of Perl, a
simple "ok/not ok" run through Test::Harness, but there are
a few special considerations.
There are three ways to write a test in the core.
Test::More, t/test.pl and ad hoc "print $test ? "ok 42\n" :
"not ok 42\n"". The decision of which to use depends on
what part of the test suite you're working on. This is a
measure to prevent a high-level failure (such as Config.pm
breaking) from causing basic functionality tests to fail.
If you write your own test, use the Test Anything Protocol.
t/base t/comp
Since we don't know if require works, or even
subroutines, use ad hoc tests for these two. Step
carefully to avoid using the feature being tested.
t/cmd t/run t/io t/op
Now that basic require() and subroutines are tested, you
can use the t/test.pl library which emulates the
important features of Test::More while using a minimum
of core features.
You can also conditionally use certain libraries like
Config, but be sure to skip the test gracefully if it's
not there.
t/lib ext lib
Now that the core of Perl is tested, Test::More can be
used. You can also use the full suite of core modules
in the tests.
When you say "make test" Perl uses the t/TEST program to run
the test suite (except under Win32 where it uses t/harness
instead.) All tests are run from the t/ directory, not the
directory which contains the test. This causes some
problems with the tests in lib/, so here's some opportunity
for some patching.
You must be triply conscious of cross-platform concerns.
This usually boils down to using File::Spec and avoiding
things like "fork()" and "system()" unless absolutely
necessary.
Special Make Test Targets
There are various special make targets that can be used to
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test Perl slightly differently than the standard "test"
target. Not all them are expected to give a 100% success
rate. Many of them have several aliases, and many of them
are not available on certain operating systems.
coretest
Run perl on all core tests (t/* and lib/[a-z]* pragma
tests).
(Not available on Win32)
test.deparse
Run all the tests through B::Deparse. Not all tests
will succeed.
(Not available on Win32)
test.taintwarn
Run all tests with the -t command-line switch. Not all
tests are expected to succeed (until they're
specifically fixed, of course).
(Not available on Win32)
minitest
Run miniperl on t/base, t/comp, t/cmd, t/run, t/io,
t/op, t/uni and t/mro tests.
test.valgrind check.valgrind utest.valgrind ucheck.valgrind
(Only in Linux) Run all the tests using the memory leak
+ naughty memory access tool "valgrind". The log files
will be named testname.valgrind.
test.third check.third utest.third ucheck.third
(Only in Tru64) Run all the tests using the memory leak
+ naughty memory access tool "Third Degree". The log
files will be named perl.3log.testname.
test.torture torturetest
Run all the usual tests and some extra tests. As of
Perl 5.8.0 the only extra tests are Abigail's JAPHs,
t/japh/abigail.t.
You can also run the torture test with t/harness by
giving "-torture" argument to t/harness.
utest ucheck test.utf8 check.utf8
Run all the tests with -Mutf8. Not all tests will
succeed.
(Not available on Win32)
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minitest.utf16 test.utf16
Runs the tests with UTF-16 encoded scripts, encoded with
different versions of this encoding.
"make utest.utf16" runs the test suite with a
combination of "-utf8" and "-utf16" arguments to t/TEST.
(Not available on Win32)
test_harness
Run the test suite with the t/harness controlling
program, instead of t/TEST. t/harness is more
sophisticated, and uses the Test::Harness module, thus
using this test target supposes that perl mostly works.
The main advantage for our purposes is that it prints a
detailed summary of failed tests at the end. Also,
unlike t/TEST, it doesn't redirect stderr to stdout.
Note that under Win32 t/harness is always used instead
of t/TEST, so there is no special "test_harness" target.
Under Win32's "test" target you may use the
TEST_SWITCHES and TEST_FILES environment variables to
control the behaviour of t/harness. This means you can
say
nmake test TEST_FILES="op/*.t"
nmake test TEST_SWITCHES="-torture" TEST_FILES="op/*.t"
Parallel tests
The core distribution can now run its regression tests
in parallel on Unix-like platforms. Instead of running
"make test", set "TEST_JOBS" in your environment to the
number of tests to run in parallel, and run "make
test_harness". On a Bourne-like shell, this can be done
as
TEST_JOBS=3 make test_harness # Run 3 tests in parallel
An environment variable is used, rather than parallel
make itself, because TAP::Harness needs to be able to
schedule individual non-conflicting test scripts itself,
and there is no standard interface to "make" utilities
to interact with their job schedulers.
Note that currently some test scripts may fail when run
in parallel (most notably "ext/IO/t/io_dir.t"). If
necessary run just the failing scripts again
sequentially and see if the failures go away. =item
test-notty test_notty
Sets PERL_SKIP_TTY_TEST to true before running normal
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test.
Running tests by hand
You can run part of the test suite by hand by using one the
following commands from the t/ directory :
./perl -I../lib TEST list-of-.t-files
or
./perl -I../lib harness list-of-.t-files
(if you don't specify test scripts, the whole test suite
will be run.)
Using t/harness for testing
If you use "harness" for testing you have several command
line options available to you. The arguments are as follows,
and are in the order that they must appear if used together.
harness -v -torture -re=pattern LIST OF FILES TO TEST
harness -v -torture -re LIST OF PATTERNS TO MATCH
If "LIST OF FILES TO TEST" is omitted the file list is
obtained from the manifest. The file list may include shell
wildcards which will be expanded out.
-v Run the tests under verbose mode so you can see what
tests were run, and debug output.
-torture
Run the torture tests as well as the normal set.
-re=PATTERN
Filter the file list so that all the test files run
match PATTERN. Note that this form is distinct from the
-re LIST OF PATTERNS form below in that it allows the
file list to be provided as well.
-re LIST OF PATTERNS
Filter the file list so that all the test files run
match /(LIST|OF|PATTERNS)/. Note that with this form the
patterns are joined by '|' and you cannot supply a list
of files, instead the test files are obtained from the
MANIFEST.
You can run an individual test by a command similar to
./perl -I../lib patho/to/foo.t
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except that the harnesses set up some environment variables
that may affect the execution of the test :
PERL_CORE=1
indicates that we're running this test part of the perl
core test suite. This is useful for modules that have a
dual life on CPAN.
PERL_DESTRUCT_LEVEL=2
is set to 2 if it isn't set already (see
"PERL_DESTRUCT_LEVEL")
PERL
(used only by t/TEST) if set, overrides the path to the
perl executable that should be used to run the tests
(the default being ./perl).
PERL_SKIP_TTY_TEST
if set, tells to skip the tests that need a terminal.
It's actually set automatically by the Makefile, but can
also be forced artificially by running 'make
test_notty'.
Other environment variables that may influence tests
PERL_TEST_Net_Ping
Setting this variable runs all the Net::Ping modules
tests, otherwise some tests that interact with the
outside world are skipped. See perl58delta.
PERL_TEST_NOVREXX
Setting this variable skips the vrexx.t tests for
OS2::REXX.
PERL_TEST_NUMCONVERTS
This sets a variable in op/numconvert.t.
See also the documentation for the Test and Test::Harness
modules, for more environment variables that affect testing.
Common problems when patching Perl source code
Perl source plays by ANSI C89 rules: no C99 (or C++)
extensions. In some cases we have to take pre-ANSI
requirements into consideration. You don't care about some
particular platform having broken Perl? I hear there is
still a strong demand for J2EE programmers.
Perl environment problems
o Not compiling with threading
Compiling with threading (-Duseithreads) completely
rewrites the function prototypes of Perl. You better
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try your changes with that. Related to this is the
difference between "Perl_-less" and "Perl_-ly" APIs, for
example:
Perl_sv_setiv(aTHX_ ...);
sv_setiv(...);
The first one explicitly passes in the context, which is
needed for e.g. threaded builds. The second one does
that implicitly; do not get them mixed. If you are not
passing in a aTHX_, you will need to do a dTHX (or a
dVAR) as the first thing in the function.
See "How multiple interpreters and concurrency are
supported" in perlguts for further discussion about
context.
o Not compiling with -DDEBUGGING
The DEBUGGING define exposes more code to the compiler,
therefore more ways for things to go wrong. You should
try it.
o Introducing (non-read-only) globals
Do not introduce any modifiable globals, truly global or
file static. They are bad form and complicate
multithreading and other forms of concurrency. The
right way is to introduce them as new interpreter
variables, see intrpvar.h (at the very end for binary
compatibility).
Introducing read-only (const) globals is okay, as long
as you verify with e.g. "nm libperl.a|egrep -v ' [TURtr]
'" (if your "nm" has BSD-style output) that the data you
added really is read-only. (If it is, it shouldn't show
up in the output of that command.)
If you want to have static strings, make them constant:
static const char etc[] = "...";
If you want to have arrays of constant strings, note
carefully the right combination of "const"s:
static const char * const yippee[] =
{"hi", "ho", "silver"};
There is a way to completely hide any modifiable globals
(they are all moved to heap), the compilation setting
"-DPERL_GLOBAL_STRUCT_PRIVATE". It is not normally
used, but can be used for testing, read more about it in
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"Background and PERL_IMPLICIT_CONTEXT" in perlguts.
o Not exporting your new function
Some platforms (Win32, AIX, VMS, OS/2, to name a few)
require any function that is part of the public API (the
shared Perl library) to be explicitly marked as
exported. See the discussion about embed.pl in
perlguts.
o Exporting your new function
The new shiny result of either genuine new functionality
or your arduous refactoring is now ready and correctly
exported. So what could possibly go wrong?
Maybe simply that your function did not need to be
exported in the first place. Perl has a long and not so
glorious history of exporting functions that it should
not have.
If the function is used only inside one source code
file, make it static. See the discussion about embed.pl
in perlguts.
If the function is used across several files, but
intended only for Perl's internal use (and this should
be the common case), do not export it to the public API.
See the discussion about embed.pl in perlguts.
Portability problems
The following are common causes of compilation and/or
execution failures, not common to Perl as such. The C FAQ
is good bedtime reading. Please test your changes with as
many C compilers and platforms as possible; we will, anyway,
and it's nice to save oneself from public embarrassment.
If using gcc, you can add the "-std=c89" option which will
hopefully catch most of these unportabilities. (However it
might also catch incompatibilities in your system's header
files.)
Use the Configure "-Dgccansipedantic" flag to enable the gcc
"-ansi -pedantic" flags which enforce stricter ANSI rules.
If using the "gcc -Wall" note that not all the possible
warnings (like "-Wunitialized") are given unless you also
compile with "-O".
Note that if using gcc, starting from Perl 5.9.5 the Perl
core source code files (the ones at the top level of the
source code distribution, but not e.g. the extensions under
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ext/) are automatically compiled with as many as possible of
the "-std=c89", "-ansi", "-pedantic", and a selection of
"-W" flags (see cflags.SH).
Also study perlport carefully to avoid any bad assumptions
about the operating system, filesystems, and so forth.
You may once in a while try a "make microperl" to see
whether we can still compile Perl with just the bare minimum
of interfaces. (See README.micro.)
Do not assume an operating system indicates a certain
compiler.
o Casting pointers to integers or casting integers to
pointers
void castaway(U8* p)
{
IV i = p;
or
void castaway(U8* p)
{
IV i = (IV)p;
Both are bad, and broken, and unportable. Use the
PTR2IV() macro that does it right. (Likewise, there are
PTR2UV(), PTR2NV(), INT2PTR(), and NUM2PTR().)
o Casting between data function pointers and data pointers
Technically speaking casting between function pointers
and data pointers is unportable and undefined, but
practically speaking it seems to work, but you should
use the FPTR2DPTR() and DPTR2FPTR() macros. Sometimes
you can also play games with unions.
o Assuming sizeof(int) == sizeof(long)
There are platforms where longs are 64 bits, and
platforms where ints are 64 bits, and while we are out
to shock you, even platforms where shorts are 64 bits.
This is all legal according to the C standard. (In
other words, "long long" is not a portable way to
specify 64 bits, and "long long" is not even guaranteed
to be any wider than "long".)
Instead, use the definitions IV, UV, IVSIZE, I32SIZE,
and so forth. Avoid things like I32 because they are
not guaranteed to be exactly 32 bits, they are at least
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32 bits, nor are they guaranteed to be int or long. If
you really explicitly need 64-bit variables, use I64 and
U64, but only if guarded by HAS_QUAD.
o Assuming one can dereference any type of pointer for any
type of data
char *p = ...;
long pony = *p; /* BAD */
Many platforms, quite rightly so, will give you a core
dump instead of a pony if the p happens not be correctly
aligned.
o Lvalue casts
(int)*p = ...; /* BAD */
Simply not portable. Get your lvalue to be of the right
type, or maybe use temporary variables, or dirty tricks
with unions.
o Assume anything about structs (especially the ones you
don't control, like the ones coming from the system
headers)
o That a certain field exists in a struct
o That no other fields exist besides the ones you
know of
o That a field is of certain signedness, sizeof,
or type
o That the fields are in a certain order
o While C guarantees the ordering
specified in the struct definition,
between different platforms the
definitions might differ
o That the sizeof(struct) or the alignments are
the same everywhere
o There might be padding bytes between the
fields to align the fields - the bytes
can be anything
o Structs are required to be aligned to
the maximum alignment required by the
fields - which for native types is for
usually equivalent to sizeof() of the
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field
o Assuming the character set is ASCIIish
Perl can compile and run under EBCDIC platforms. See
perlebcdic. This is transparent for the most part, but
because the character sets differ, you shouldn't use
numeric (decimal, octal, nor hex) constants to refer to
characters. You can safely say 'A', but not 0x41. You
can safely say '\n', but not \012. If a character
doesn't have a trivial input form, you can create a
#define for it in both "utfebcdic.h" and "utf8.h", so
that it resolves to different values depending on the
character set being used. (There are three different
EBCDIC character sets defined in "utfebcdic.h", so it
might be best to insert the #define three times in that
file.)
Also, the range 'A' - 'Z' in ASCII is an unbroken
sequence of 26 upper case alphabetic characters. That
is not true in EBCDIC. Nor for 'a' to 'z'. But '0' -
'9' is an unbroken range in both systems. Don't assume
anything about other ranges.
Many of the comments in the existing code ignore the
possibility of EBCDIC, and may be wrong therefore, even
if the code works. This is actually a tribute to the
successful transparent insertion of being able to handle
EBCDIC without having to change pre-existing code.
UTF-8 and UTF-EBCDIC are two different encodings used to
represent Unicode code points as sequences of bytes.
Macros with the same names (but different definitions)
in "utf8.h" and "utfebcdic.h" are used to allow the
calling code to think that there is only one such
encoding. This is almost always referred to as "utf8",
but it means the EBCDIC version as well. Again,
comments in the code may well be wrong even if the code
itself is right. For example, the concept of "invariant
characters" differs between ASCII and EBCDIC. On ASCII
platforms, only characters that do not have the high-
order bit set (i.e. whose ordinals are strict ASCII, 0 -
127) are invariant, and the documentation and comments
in the code may assume that, often referring to
something like, say, "hibit". The situation differs and
is not so simple on EBCDIC machines, but as long as the
code itself uses the "NATIVE_IS_INVARIANT()" macro
appropriately, it works, even if the comments are wrong.
o Assuming the character set is just ASCII
ASCII is a 7 bit encoding, but bytes have 8 bits in
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them. The 128 extra characters have different meanings
depending on the locale. Absent a locale, currently
these extra characters are generally considered to be
unassigned, and this has presented some problems. This
is being changed starting in 5.12 so that these
characters will be considered to be Latin-1
(ISO-8859-1).
o Mixing #define and #ifdef
#define BURGLE(x) ... \
#ifdef BURGLE_OLD_STYLE /* BAD */
... do it the old way ... \
#else
... do it the new way ... \
#endif
You cannot portably "stack" cpp directives. For example
in the above you need two separate BURGLE() #defines,
one for each #ifdef branch.
o Adding non-comment stuff after #endif or #else
#ifdef SNOSH
...
#else !SNOSH /* BAD */
...
#endif SNOSH /* BAD */
The #endif and #else cannot portably have anything non-
comment after them. If you want to document what is
going (which is a good idea especially if the branches
are long), use (C) comments:
#ifdef SNOSH
...
#else /* !SNOSH */
...
#endif /* SNOSH */
The gcc option "-Wendif-labels" warns about the bad
variant (by default on starting from Perl 5.9.4).
o Having a comma after the last element of an enum list
enum color {
CERULEAN,
CHARTREUSE,
CINNABAR, /* BAD */
};
is not portable. Leave out the last comma.
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Also note that whether enums are implicitly morphable to
ints varies between compilers, you might need to (int).
o Using //-comments
// This function bamfoodles the zorklator. /* BAD */
That is C99 or C++. Perl is C89. Using the //-comments
is silently allowed by many C compilers but cranking up
the ANSI C89 strictness (which we like to do) causes the
compilation to fail.
o Mixing declarations and code
void zorklator()
{
int n = 3;
set_zorkmids(n); /* BAD */
int q = 4;
That is C99 or C++. Some C compilers allow that, but
you shouldn't.
The gcc option "-Wdeclaration-after-statements" scans
for such problems (by default on starting from Perl
5.9.4).
o Introducing variables inside for()
for(int i = ...; ...; ...) { /* BAD */
That is C99 or C++. While it would indeed be awfully
nice to have that also in C89, to limit the scope of the
loop variable, alas, we cannot.
o Mixing signed char pointers with unsigned char pointers
int foo(char *s) { ... }
...
unsigned char *t = ...; /* Or U8* t = ... */
foo(t); /* BAD */
While this is legal practice, it is certainly dubious,
and downright fatal in at least one platform: for
example VMS cc considers this a fatal error. One cause
for people often making this mistake is that a "naked
char" and therefore dereferencing a "naked char pointer"
have an undefined signedness: it depends on the compiler
and the flags of the compiler and the underlying
platform whether the result is signed or unsigned. For
this very same reason using a 'char' as an array index
is bad.
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o Macros that have string constants and their arguments as
substrings of the string constants
#define FOO(n) printf("number = %d\n", n) /* BAD */
FOO(10);
Pre-ANSI semantics for that was equivalent to
printf("10umber = %d\10");
which is probably not what you were expecting.
Unfortunately at least one reasonably common and modern
C compiler does "real backward compatibility" here, in
AIX that is what still happens even though the rest of
the AIX compiler is very happily C89.
o Using printf formats for non-basic C types
IV i = ...;
printf("i = %d\n", i); /* BAD */
While this might by accident work in some platform
(where IV happens to be an "int"), in general it cannot.
IV might be something larger. Even worse the situation
is with more specific types (defined by Perl's
configuration step in config.h):
Uid_t who = ...;
printf("who = %d\n", who); /* BAD */
The problem here is that Uid_t might be not only not
"int"-wide but it might also be unsigned, in which case
large uids would be printed as negative values.
There is no simple solution to this because of
printf()'s limited intelligence, but for many types the
right format is available as with either 'f' or '_f'
suffix, for example:
IVdf /* IV in decimal */
UVxf /* UV is hexadecimal */
printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
Uid_t_f /* Uid_t in decimal */
printf("who = %"Uid_t_f"\n", who);
Or you can try casting to a "wide enough" type:
printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
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Also remember that the %p format really does require a
void pointer:
U8* p = ...;
printf("p = %p\n", (void*)p);
The gcc option "-Wformat" scans for such problems.
o Blindly using variadic macros
gcc has had them for a while with its own syntax, and
C99 brought them with a standardized syntax. Don't use
the former, and use the latter only if the
HAS_C99_VARIADIC_MACROS is defined.
o Blindly passing va_list
Not all platforms support passing va_list to further
varargs (stdarg) functions. The right thing to do is to
copy the va_list using the Perl_va_copy() if the
NEED_VA_COPY is defined.
o Using gcc statement expressions
val = ({...;...;...}); /* BAD */
While a nice extension, it's not portable. The Perl
code does admittedly use them if available to gain some
extra speed (essentially as a funky form of inlining),
but you shouldn't.
o Binding together several statements in a macro
Use the macros STMT_START and STMT_END.
STMT_START {
...
} STMT_END
o Testing for operating systems or versions when should be
testing for features
#ifdef __FOONIX__ /* BAD */
foo = quux();
#endif
Unless you know with 100% certainty that quux() is only
ever available for the "Foonix" operating system and
that is available and correctly working for all past,
present, and future versions of "Foonix", the above is
very wrong. This is more correct (though still not
perfect, because the below is a compile-time check):
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#ifdef HAS_QUUX
foo = quux();
#endif
How does the HAS_QUUX become defined where it needs to
be? Well, if Foonix happens to be Unixy enough to be
able to run the Configure script, and Configure has been
taught about detecting and testing quux(), the HAS_QUUX
will be correctly defined. In other platforms, the
corresponding configuration step will hopefully do the
same.
In a pinch, if you cannot wait for Configure to be
educated, or if you have a good hunch of where quux()
might be available, you can temporarily try the
following:
#if (defined(__FOONIX__) || defined(__BARNIX__))
# define HAS_QUUX
#endif
...
#ifdef HAS_QUUX
foo = quux();
#endif
But in any case, try to keep the features and operating
systems separate.
Problematic System Interfaces
o malloc(0), realloc(0), calloc(0, 0) are non-portable.
To be portable allocate at least one byte. (In general
you should rarely need to work at this low level, but
instead use the various malloc wrappers.)
o snprintf() - the return type is unportable. Use
my_snprintf() instead.
Security problems
Last but not least, here are various tips for safer coding.
o Do not use gets()
Or we will publicly ridicule you. Seriously.
o Do not use strcpy() or strcat() or strncpy() or
strncat()
Use my_strlcpy() and my_strlcat() instead: they either
use the native implementation, or Perl's own
implementation (borrowed from the public domain
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implementation of INN).
o Do not use sprintf() or vsprintf()
If you really want just plain byte strings, use
my_snprintf() and my_vsnprintf() instead, which will try
to use snprintf() and vsnprintf() if those safer APIs
are available. If you want something fancier than a
plain byte string, use SVs and Perl_sv_catpvf().
EXTERNAL TOOLS FOR DEBUGGING PERL
Sometimes it helps to use external tools while debugging and
testing Perl. This section tries to guide you through using
some common testing and debugging tools with Perl. This is
meant as a guide to interfacing these tools with Perl, not
as any kind of guide to the use of the tools themselves.
NOTE 1: Running under memory debuggers such as Purify,
valgrind, or Third Degree greatly slows down the execution:
seconds become minutes, minutes become hours. For example
as of Perl 5.8.1, the ext/Encode/t/Unicode.t takes
extraordinarily long to complete under e.g. Purify, Third
Degree, and valgrind. Under valgrind it takes more than six
hours, even on a snappy computer. The said test must be
doing something that is quite unfriendly for memory
debuggers. If you don't feel like waiting, that you can
simply kill away the perl process.
NOTE 2: To minimize the number of memory leak false alarms
(see "PERL_DESTRUCT_LEVEL" for more information), you have
to set the environment variable PERL_DESTRUCT_LEVEL to 2.
For csh-like shells:
setenv PERL_DESTRUCT_LEVEL 2
For Bourne-type shells:
PERL_DESTRUCT_LEVEL=2
export PERL_DESTRUCT_LEVEL
In Unixy environments you can also use the "env" command:
env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
NOTE 3: There are known memory leaks when there are compile-
time errors within eval or require, seeing "S_doeval" in the
call stack is a good sign of these. Fixing these leaks is
non-trivial, unfortunately, but they must be fixed
eventually.
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NOTE 4: DynaLoader will not clean up after itself completely
unless Perl is built with the Configure option
"-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".
Rational Software's Purify
Purify is a commercial tool that is helpful in identifying
memory overruns, wild pointers, memory leaks and other such
badness. Perl must be compiled in a specific way for
optimal testing with Purify. Purify is available under
Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.
Purify on Unix
On Unix, Purify creates a new Perl binary. To get the most
benefit out of Purify, you should create the perl to Purify
using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
-Uusemymalloc -Dusemultiplicity
where these arguments mean:
-Accflags=-DPURIFY
Disables Perl's arena memory allocation functions, as
well as forcing use of memory allocation functions
derived from the system malloc.
-Doptimize='-g'
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.
-Uusemymalloc
Disable Perl's malloc so that Purify can more closely
monitor allocations and leaks. Using Perl's malloc will
make Purify report most leaks in the "potential" leaks
category.
-Dusemultiplicity
Enabling the multiplicity option allows perl to clean up
thoroughly when the interpreter shuts down, which
reduces the number of bogus leak reports from Purify.
Once you've compiled a perl suitable for Purify'ing, then
you can just:
make pureperl
which creates a binary named 'pureperl' that has been
Purify'ed. This binary is used in place of the standard
'perl' binary when you want to debug Perl memory problems.
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As an example, to show any memory leaks produced during the
standard Perl testset you would create and run the Purify'ed
perl as:
make pureperl
cd t
../pureperl -I../lib harness
which would run Perl on test.pl and report any memory
problems.
Purify outputs messages in "Viewer" windows by default. If
you don't have a windowing environment or if you simply want
the Purify output to unobtrusively go to a log file instead
of to the interactive window, use these following options to
output to the log file "perl.log":
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
-log-file=perl.log -append-logfile=yes"
If you plan to use the "Viewer" windows, then you only need
this option:
setenv PURIFYOPTIONS "-chain-length=25"
In Bourne-type shells:
PURIFYOPTIONS="..."
export PURIFYOPTIONS
or if you have the "env" utility:
env PURIFYOPTIONS="..." ../pureperl ...
Purify on NT
Purify on Windows NT instruments the Perl binary 'perl.exe'
on the fly. There are several options in the makefile you
should change to get the most use out of Purify:
DEFINES
You should add -DPURIFY to the DEFINES line so the
DEFINES line looks something like:
DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1
to disable Perl's arena memory allocation functions, as
well as to force use of memory allocation functions
derived from the system malloc.
USE_MULTI = define
Enabling the multiplicity option allows perl to clean up
thoroughly when the interpreter shuts down, which
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reduces the number of bogus leak reports from Purify.
#PERL_MALLOC = define
Disable Perl's malloc so that Purify can more closely
monitor allocations and leaks. Using Perl's malloc will
make Purify report most leaks in the "potential" leaks
category.
CFG = Debug
Adds debugging information so that you see the exact
source statements where the problem occurs. Without
this flag, all you will see is the source filename of
where the error occurred.
As an example, to show any memory leaks produced during the
standard Perl testset you would create and run Purify as:
cd win32
make
cd ../t
purify ../perl -I../lib harness
which would instrument Perl in memory, run Perl on test.pl,
then finally report any memory problems.
valgrind
The excellent valgrind tool can be used to find out both
memory leaks and illegal memory accesses. As of version
3.3.0, Valgrind only supports Linux on x86, x86-64 and
PowerPC. The special "test.valgrind" target can be used to
run the tests under valgrind. Found errors and memory leaks
are logged in files named testfile.valgrind.
Valgrind also provides a cachegrind tool, invoked on perl
as:
VG_OPTS=--tool=cachegrind make test.valgrind
As system libraries (most notably glibc) are also triggering
errors, valgrind allows to suppress such errors using
suppression files. The default suppression file that comes
with valgrind already catches a lot of them. Some additional
suppressions are defined in t/perl.supp.
To get valgrind and for more information see
http://developer.kde.org/~sewardj/
Compaq's/Digital's/HP's Third Degree
Third Degree is a tool for memory leak detection and memory
access checks. It is one of the many tools in the ATOM
toolkit. The toolkit is only available on Tru64 (formerly
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known as Digital UNIX formerly known as DEC OSF/1).
When building Perl, you must first run Configure with
-Doptimize=-g and -Uusemymalloc flags, after that you can
use the make targets "perl.third" and "test.third". (What
is required is that Perl must be compiled using the "-g"
flag, you may need to re-Configure.)
The short story is that with "atom" you can instrument the
Perl executable to create a new executable called
perl.third. When the instrumented executable is run, it
creates a log of dubious memory traffic in file called
perl.3log. See the manual pages of atom and third for more
information. The most extensive Third Degree documentation
is available in the Compaq "Tru64 UNIX Programmer's Guide",
chapter "Debugging Programs with Third Degree".
The "test.third" leaves a lot of files named foo_bar.3log in
the t/ subdirectory. There is a problem with these files:
Third Degree is so effective that it finds problems also in
the system libraries. Therefore you should used the
Porting/thirdclean script to cleanup the *.3log files.
There are also leaks that for given certain definition of a
leak, aren't. See "PERL_DESTRUCT_LEVEL" for more
information.
PERL_DESTRUCT_LEVEL
If you want to run any of the tests yourself manually using
e.g. valgrind, or the pureperl or perl.third executables,
please note that by default perl does not explicitly cleanup
all the memory it has allocated (such as global memory
arenas) but instead lets the exit() of the whole program
"take care" of such allocations, also known as "global
destruction of objects".
There is a way to tell perl to do complete cleanup: set the
environment variable PERL_DESTRUCT_LEVEL to a non-zero
value. The t/TEST wrapper does set this to 2, and this is
what you need to do too, if you don't want to see the
"global leaks": For example, for "third-degreed" Perl:
env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t
(Note: the mod_perl apache module uses also this environment
variable for its own purposes and extended its semantics.
Refer to the mod_perl documentation for more information.
Also, spawned threads do the equivalent of setting this
variable to the value 1.)
If, at the end of a run you get the message N scalars
leaked, you can recompile with "-DDEBUG_LEAKING_SCALARS",
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which will cause the addresses of all those leaked SVs to be
dumped along with details as to where each SV was originally
allocated. This information is also displayed by
Devel::Peek. Note that the extra details recorded with each
SV increases memory usage, so it shouldn't be used in
production environments. It also converts "new_SV()" from a
macro into a real function, so you can use your favourite
debugger to discover where those pesky SVs were allocated.
If you see that you're leaking memory at runtime, but
neither valgrind nor "-DDEBUG_LEAKING_SCALARS" will find
anything, you're probably leaking SVs that are still
reachable and will be properly cleaned up during destruction
of the interpreter. In such cases, using the "-Dm" switch
can point you to the source of the leak. If the executable
was built with "-DDEBUG_LEAKING_SCALARS", "-Dm" will output
SV allocations in addition to memory allocations. Each SV
allocation has a distinct serial number that will be written
on creation and destruction of the SV. So if you're
executing the leaking code in a loop, you need to look for
SVs that are created, but never destroyed between each
cycle. If such an SV is found, set a conditional breakpoint
within "new_SV()" and make it break only when "PL_sv_serial"
is equal to the serial number of the leaking SV. Then you
will catch the interpreter in exactly the state where the
leaking SV is allocated, which is sufficient in many cases
to find the source of the leak.
As "-Dm" is using the PerlIO layer for output, it will by
itself allocate quite a bunch of SVs, which are hidden to
avoid recursion. You can bypass the PerlIO layer if you use
the SV logging provided by "-DPERL_MEM_LOG" instead.
PERL_MEM_LOG
If compiled with "-DPERL_MEM_LOG", both memory and SV
allocations go through logging functions, which is handy for
breakpoint setting.
Unless "-DPERL_MEM_LOG_NOIMPL" is also compiled, the logging
functions read $ENV{PERL_MEM_LOG} to determine whether to
log the event, and if so how:
$ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
$ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
$ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
$ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
Memory logging is somewhat similar to "-Dm" but is
independent of "-DDEBUGGING", and at a higher level; all
uses of Newx(), Renew(), and Safefree() are logged with the
caller's source code file and line number (and C function
name, if supported by the C compiler). In contrast, "-Dm"
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is directly at the point of "malloc()". SV logging is
similar.
Since the logging doesn't use PerlIO, all SV allocations are
logged and no extra SV allocations are introduced by
enabling the logging. If compiled with
"-DDEBUG_LEAKING_SCALARS", the serial number for each SV
allocation is also logged.
Profiling
Depending on your platform there are various of profiling
Perl.
There are two commonly used techniques of profiling
executables: statistical time-sampling and basic-block
counting.
The first method takes periodically samples of the CPU
program counter, and since the program counter can be
correlated with the code generated for functions, we get a
statistical view of in which functions the program is
spending its time. The caveats are that very small/fast
functions have lower probability of showing up in the
profile, and that periodically interrupting the program
(this is usually done rather frequently, in the scale of
milliseconds) imposes an additional overhead that may skew
the results. The first problem can be alleviated by running
the code for longer (in general this is a good idea for
profiling), the second problem is usually kept in guard by
the profiling tools themselves.
The second method divides up the generated code into basic
blocks. Basic blocks are sections of code that are entered
only in the beginning and exited only at the end. For
example, a conditional jump starts a basic block. Basic
block profiling usually works by instrumenting the code by
adding enter basic block #nnnn book-keeping code to the
generated code. During the execution of the code the basic
block counters are then updated appropriately. The caveat
is that the added extra code can skew the results: again,
the profiling tools usually try to factor their own effects
out of the results.
Gprof Profiling
gprof is a profiling tool available in many Unix platforms,
it uses statistical time-sampling.
You can build a profiled version of perl called "perl.gprof"
by invoking the make target "perl.gprof" (What is required
is that Perl must be compiled using the "-pg" flag, you may
need to re-Configure). Running the profiled version of Perl
will create an output file called gmon.out is created which
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contains the profiling data collected during the execution.
The gprof tool can then display the collected data in
various ways. Usually gprof understands the following
options:
-a Suppress statically defined functions from the profile.
-b Suppress the verbose descriptions in the profile.
-e routine
Exclude the given routine and its descendants from the
profile.
-f routine
Display only the given routine and its descendants in
the profile.
-s Generate a summary file called gmon.sum which then may
be given to subsequent gprof runs to accumulate data
over several runs.
-z Display routines that have zero usage.
For more detailed explanation of the available commands and
output formats, see your own local documentation of gprof.
quick hint:
$ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
$ ./perl.gprof someprog # creates gmon.out in current directory
$ gprof ./perl.gprof > out
$ view out
GCC gcov Profiling
Starting from GCC 3.0 basic block profiling is officially
available for the GNU CC.
You can build a profiled version of perl called perl.gcov by
invoking the make target "perl.gcov" (what is required that
Perl must be compiled using gcc with the flags
"-fprofile-arcs -ftest-coverage", you may need to re-
Configure).
Running the profiled version of Perl will cause profile
output to be generated. For each source file an
accompanying ".da" file will be created.
To display the results you use the "gcov" utility (which
should be installed if you have gcc 3.0 or newer installed).
gcov is run on source code files, like this
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gcov sv.c
which will cause sv.c.gcov to be created. The .gcov files
contain the source code annotated with relative frequencies
of execution indicated by "#" markers.
Useful options of gcov include "-b" which will summarise the
basic block, branch, and function call coverage, and "-c"
which instead of relative frequencies will use the actual
counts. For more information on the use of gcov and basic
block profiling with gcc, see the latest GNU CC manual, as
of GCC 3.0 see
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html
and its section titled "8. gcov: a Test Coverage Program"
http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132
quick hint:
$ sh Configure -des -Doptimize='-g' -Accflags='-fprofile-arcs -ftest-coverage' \
-Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
$ rm -f regexec.c.gcov regexec.gcda
$ ./perl.gcov
$ gcov regexec.c
$ view regexec.c.gcov
Pixie Profiling
Pixie is a profiling tool available on IRIX and Tru64 (aka
Digital UNIX aka DEC OSF/1) platforms. Pixie does its
profiling using basic-block counting.
You can build a profiled version of perl called perl.pixie
by invoking the make target "perl.pixie" (what is required
is that Perl must be compiled using the "-g" flag, you may
need to re-Configure).
In Tru64 a file called perl.Addrs will also be silently
created, this file contains the addresses of the basic
blocks. Running the profiled version of Perl will create a
new file called "perl.Counts" which contains the counts for
the basic block for that particular program execution.
To display the results you use the prof utility. The exact
incantation depends on your operating system, "prof
perl.Counts" in IRIX, and "prof -pixie -all -L. perl" in
Tru64.
In IRIX the following prof options are available:
-h Reports the most heavily used lines in descending order
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of use. Useful for finding the hotspot lines.
-l Groups lines by procedure, with procedures sorted in
descending order of use. Within a procedure, lines are
listed in source order. Useful for finding the hotspots
of procedures.
In Tru64 the following options are available:
-p[rocedures]
Procedures sorted in descending order by the number of
cycles executed in each procedure. Useful for finding
the hotspot procedures. (This is the default option.)
-h[eavy]
Lines sorted in descending order by the number of cycles
executed in each line. Useful for finding the hotspot
lines.
-i[nvocations]
The called procedures are sorted in descending order by
number of calls made to the procedures. Useful for
finding the most used procedures.
-l[ines]
Grouped by procedure, sorted by cycles executed per
procedure. Useful for finding the hotspots of
procedures.
-testcoverage
The compiler emitted code for these lines, but the code
was unexecuted.
-z[ero]
Unexecuted procedures.
For further information, see your system's manual pages for
pixie and prof.
Miscellaneous tricks
o Those debugging perl with the DDD frontend over gdb may
find the following useful:
You can extend the data conversion shortcuts menu, so
for example you can display an SV's IV value with one
click, without doing any typing. To do that simply edit
~/.ddd/init file and add after:
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! Display shortcuts.
Ddd*gdbDisplayShortcuts: \
/t () // Convert to Bin\n\
/d () // Convert to Dec\n\
/x () // Convert to Hex\n\
/o () // Convert to Oct(\n\
the following two lines:
((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
so now you can do ivx and pvx lookups or you can plug
there the sv_peek "conversion":
Perl_sv_peek(my_perl, (SV*)()) // sv_peek
(The my_perl is for threaded builds.) Just remember
that every line, but the last one, should end with \n\
Alternatively edit the init file interactively via: 3rd
mouse button -> New Display -> Edit Menu
Note: you can define up to 20 conversion shortcuts in
the gdb section.
o If you see in a debugger a memory area mysteriously full
of 0xABABABAB or 0xEFEFEFEF, you may be seeing the
effect of the Poison() macros, see perlclib.
o Under ithreads the optree is read only. If you want to
enforce this, to check for write accesses from buggy
code, compile with "-DPL_OP_SLAB_ALLOC" to enable the OP
slab allocator and "-DPERL_DEBUG_READONLY_OPS" to enable
code that allocates op memory via "mmap", and sets it
read-only at run time. Any write access to an op
results in a "SIGBUS" and abort.
This code is intended for development only, and may not
be portable even to all Unix variants. Also, it is an
80% solution, in that it isn't able to make all ops read
only. Specifically it
1. Only sets read-only on all slabs of ops at "CHECK"
time, hence ops allocated later via "require" or
"eval" will be re-write
2. Turns an entire slab of ops read-write if the
refcount of any op in the slab needs to be
decreased.
3. Turns an entire slab of ops read-write if any op
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from the slab is freed.
It's not possible to turn the slabs to read-only after
an action requiring read-write access, as either can
happen during op tree building time, so there may still
be legitimate write access.
However, as an 80% solution it is still effective, as
currently it catches a write access during the
generation of Config.pm, which means that we can't yet
build perl with this enabled.
CONCLUSION
We've had a brief look around the Perl source, how to
maintain quality of the source code, an overview of the
stages perl goes through when it's running your code, how to
use debuggers to poke at the Perl guts, and finally how to
analyse the execution of Perl. We took a very simple problem
and demonstrated how to solve it fully - with documentation,
regression tests, and finally a patch for submission to p5p.
Finally, we talked about how to use external tools to debug
and test Perl.
I'd now suggest you read over those references again, and
then, as soon as possible, get your hands dirty. The best
way to learn is by doing, so:
o Subscribe to perl5-porters, follow the patches and try
and understand them; don't be afraid to ask if there's a
portion you're not clear on - who knows, you may unearth
a bug in the patch...
o Keep up to date with the bleeding edge Perl distributions
and get familiar with the changes. Try and get an idea of
what areas people are working on and the changes they're
making.
o Do read the README associated with your operating system,
e.g. README.aix on the IBM AIX OS. Don't hesitate to
supply patches to that README if you find anything
missing or changed over a new OS release.
o Find an area of Perl that seems interesting to you, and
see if you can work out how it works. Scan through the
source, and step over it in the debugger. Play, poke,
investigate, fiddle! You'll probably get to understand
not just your chosen area but a much wider range of
perl's activity as well, and probably sooner than you'd
think.
The Road goes ever on and on, down from the door where it
began.
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If you can do these things, you've started on the long road
to Perl porting. Thanks for wanting to help make Perl
better - and happy hacking!
Metaphoric Quotations
If you recognized the quote about the Road above, you're in
luck.
Most software projects begin each file with a literal
description of each file's purpose. Perl instead begins
each with a literary allusion to that file's purpose.
Like chapters in many books, all top-level Perl source files
(along with a few others here and there) begin with an
epigramic inscription that alludes, indirectly and
metaphorically, to the material you're about to read.
Quotations are taken from writings of J.R.R Tolkien
pertaining to his Legendarium, almost always from The Lord
of the Rings. Chapters and page numbers are given using the
following editions:
o The Hobbit, by J.R.R. Tolkien. The hardcover,
70th-anniversary edition of 2007 was used, published in
the UK by Harper Collins Publishers and in the US by the
Houghton Mifflin Company.
o The Lord of the Rings, by J.R.R. Tolkien. The
hardcover, 50th-anniversary edition of 2004 was used,
published in the UK by Harper Collins Publishers and in
the US by the Houghton Mifflin Company.
o The Lays of Beleriand, by J.R.R. Tolkien and published
posthumously by his son and literary executor, C.J.R.
Tolkien, being the 3rd of the 12 volumes in
Christopher's mammoth History of Middle Earth. Page
numbers derive from the hardcover edition, first
published in 1983 by George Allen & Unwin; no page
numbers changed for the special 3-volume omnibus edition
of 2002 or the various trade-paper editions, all again
now by Harper Collins or Houghton Mifflin.
Other JRRT books fair game for quotes would thus include The
Adventures of Tom Bombadil, The Silmarillion, Unfinished
Tales, and The Tale of the Children of Hurin, all but the
first posthumously assembled by CJRT. But The Lord of the
Rings itself is perfectly fine and probably best to quote
from, provided you can find a suitable quote there.
So if you were to supply a new, complete, top-level source
file to add to Perl, you should conform to this peculiar
practice by yourself selecting an appropriate quotation from
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Tolkien, retaining the original spelling and punctuation and
using the same format the rest of the quotes are in.
Indirect and oblique is just fine; remember, it's a
metaphor, so being meta is, after all, what it's for.
AUTHOR
This document was written by Nathan Torkington, and is
maintained by the perl5-porters mailing list.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlrepository
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
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/.
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