perlxstut
(1)
Name
perlxstut - Tutorial for writing XSUBs
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLXSTUT(1)
NAME
perlXStut - Tutorial for writing XSUBs
DESCRIPTION
This tutorial will educate the reader on the steps involved
in creating a Perl extension. The reader is assumed to have
access to perlguts, perlapi and perlxs.
This tutorial starts with very simple examples and becomes
more complex, with each new example adding new features.
Certain concepts may not be completely explained until later
in the tutorial in order to slowly ease the reader into
building extensions.
This tutorial was written from a Unix point of view. Where
I know them to be otherwise different for other platforms
(e.g. Win32), I will list them. If you find something that
was missed, please let me know.
SPECIAL NOTES
make
This tutorial assumes that the make program that Perl is
configured to use is called "make". Instead of running
"make" in the examples that follow, you may have to
substitute whatever make program Perl has been configured to
use. Running perl -V:make should tell you what it is.
Version caveat
When writing a Perl extension for general consumption, one
should expect that the extension will be used with versions
of Perl different from the version available on your
machine. Since you are reading this document, the version
of Perl on your machine is probably 5.005 or later, but the
users of your extension may have more ancient versions.
To understand what kinds of incompatibilities one may
expect, and in the rare case that the version of Perl on
your machine is older than this document, see the section on
"Troubleshooting these Examples" for more information.
If your extension uses some features of Perl which are not
available on older releases of Perl, your users would
appreciate an early meaningful warning. You would probably
put this information into the README file, but nowadays
installation of extensions may be performed automatically,
guided by CPAN.pm module or other tools.
In MakeMaker-based installations, Makefile.PL provides the
earliest opportunity to perform version checks. One can put
something like this in Makefile.PL for this purpose:
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eval { require 5.007 }
or die <<EOD;
############
### This module uses frobnication framework which is not available before
### version 5.007 of Perl. Upgrade your Perl before installing Kara::Mba.
############
EOD
Dynamic Loading versus Static Loading
It is commonly thought that if a system does not have the
capability to dynamically load a library, you cannot build
XSUBs. This is incorrect. You can build them, but you must
link the XSUBs subroutines with the rest of Perl, creating a
new executable. This situation is similar to Perl 4.
This tutorial can still be used on such a system. The XSUB
build mechanism will check the system and build a
dynamically-loadable library if possible, or else a static
library and then, optionally, a new statically-linked
executable with that static library linked in.
Should you wish to build a statically-linked executable on a
system which can dynamically load libraries, you may, in all
the following examples, where the command ""make"" with no
arguments is executed, run the command ""make perl""
instead.
If you have generated such a statically-linked executable by
choice, then instead of saying ""make test"", you should say
""make test_static"". On systems that cannot build
dynamically-loadable libraries at all, simply saying ""make
test"" is sufficient.
TUTORIAL
Now let's go on with the show!
EXAMPLE 1
Our first extension will be very simple. When we call the
routine in the extension, it will print out a well-known
message and return.
Run ""h2xs -A -n Mytest"". This creates a directory named
Mytest, possibly under ext/ if that directory exists in the
current working directory. Several files will be created
under the Mytest dir, including MANIFEST, Makefile.PL,
lib/Mytest.pm, Mytest.xs, t/Mytest.t, and Changes.
The MANIFEST file contains the names of all the files just
created in the Mytest directory.
The file Makefile.PL should look something like this:
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use ExtUtils::MakeMaker;
# See lib/ExtUtils/MakeMaker.pm for details of how to influence
# the contents of the Makefile that is written.
WriteMakefile(
NAME => 'Mytest',
VERSION_FROM => 'Mytest.pm', # finds $VERSION
LIBS => [''], # e.g., '-lm'
DEFINE => '', # e.g., '-DHAVE_SOMETHING'
INC => '', # e.g., '-I/usr/include/other'
);
The file Mytest.pm should start with something like this:
package Mytest;
use 5.008008;
use strict;
use warnings;
require Exporter;
our @ISA = qw(Exporter);
our %EXPORT_TAGS = ( 'all' => [ qw(
) ] );
our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
our @EXPORT = qw(
);
our $VERSION = '0.01';
require XSLoader;
XSLoader::load('Mytest', $VERSION);
# Preloaded methods go here.
1;
__END__
# Below is the stub of documentation for your module. You better edit it!
The rest of the .pm file contains sample code for providing
documentation for the extension.
Finally, the Mytest.xs file should look something like this:
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
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#include "ppport.h"
MODULE = Mytest PACKAGE = Mytest
Let's edit the .xs file by adding this to the end of the
file:
void
hello()
CODE:
printf("Hello, world!\n");
It is okay for the lines starting at the "CODE:" line to not
be indented. However, for readability purposes, it is
suggested that you indent CODE: one level and the lines
following one more level.
Now we'll run ""perl Makefile.PL"". This will create a real
Makefile, which make needs. Its output looks something
like:
% perl Makefile.PL
Checking if your kit is complete...
Looks good
Writing Makefile for Mytest
%
Now, running make will produce output that looks something
like this (some long lines have been shortened for clarity
and some extraneous lines have been deleted):
% make
cp lib/Mytest.pm blib/lib/Mytest.pm
perl xsubpp -typemap typemap Mytest.xs > Mytest.xsc && mv Mytest.xsc Mytest.c
Please specify prototyping behavior for Mytest.xs (see perlxs manual)
cc -c Mytest.c
Running Mkbootstrap for Mytest ()
chmod 644 Mytest.bs
rm -f blib/arch/auto/Mytest/Mytest.so
cc -shared -L/usr/local/lib Mytest.o -o blib/arch/auto/Mytest/Mytest.so \
\
chmod 755 blib/arch/auto/Mytest/Mytest.so
cp Mytest.bs blib/arch/auto/Mytest/Mytest.bs
chmod 644 blib/arch/auto/Mytest/Mytest.bs
Manifying blib/man3/Mytest.3pm
%
You can safely ignore the line about "prototyping behavior"
- it is explained in "The PROTOTYPES: Keyword" in perlxs.
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If you are on a Win32 system, and the build process fails
with linker errors for functions in the C library, check if
your Perl is configured to use PerlCRT (running perl -V:libc
should show you if this is the case). If Perl is configured
to use PerlCRT, you have to make sure PerlCRT.lib is copied
to the same location that msvcrt.lib lives in, so that the
compiler can find it on its own. msvcrt.lib is usually
found in the Visual C compiler's lib directory (e.g.
C:/DevStudio/VC/lib).
Perl has its own special way of easily writing test scripts,
but for this example only, we'll create our own test script.
Create a file called hello that looks like this:
#! /opt/perl5/bin/perl
use ExtUtils::testlib;
use Mytest;
Mytest::hello();
Now we make the script executable ("chmod +x hello"), run
the script and we should see the following output:
% ./hello
Hello, world!
%
EXAMPLE 2
Now let's add to our extension a subroutine that will take a
single numeric argument as input and return 0 if the number
is even or 1 if the number is odd.
Add the following to the end of Mytest.xs:
int
is_even(input)
int input
CODE:
RETVAL = (input % 2 == 0);
OUTPUT:
RETVAL
There does not need to be whitespace at the start of the
""int input"" line, but it is useful for improving
readability. Placing a semi-colon at the end of that line
is also optional. Any amount and kind of whitespace may be
placed between the ""int"" and ""input"".
Now re-run make to rebuild our new shared library.
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Now perform the same steps as before, generating a Makefile
from the Makefile.PL file, and running make.
In order to test that our extension works, we now need to
look at the file Mytest.t. This file is set up to imitate
the same kind of testing structure that Perl itself has.
Within the test script, you perform a number of tests to
confirm the behavior of the extension, printing "ok" when
the test is correct, "not ok" when it is not.
use Test::More tests => 4;
BEGIN { use_ok('Mytest') };
#########################
# Insert your test code below, the Test::More module is use()ed here so read
# its man page ( perldoc Test::More ) for help writing this test script.
is(&Mytest::is_even(0), 1);
is(&Mytest::is_even(1), 0);
is(&Mytest::is_even(2), 1);
We will be calling the test script through the command
""make test"". You should see output that looks something
like this:
%make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/Mytest....ok
All tests successful.
Files=1, Tests=4, 0 wallclock secs ( 0.03 cusr + 0.00 csys = 0.03 CPU)
%
What has gone on?
The program h2xs is the starting point for creating
extensions. In later examples we'll see how we can use h2xs
to read header files and generate templates to connect to C
routines.
h2xs creates a number of files in the extension directory.
The file Makefile.PL is a perl script which will generate a
true Makefile to build the extension. We'll take a closer
look at it later.
The .pm and .xs files contain the meat of the extension.
The .xs file holds the C routines that make up the
extension. The .pm file contains routines that tell Perl
how to load your extension.
Generating the Makefile and running "make" created a
directory called blib (which stands for "build library") in
the current working directory. This directory will contain
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the shared library that we will build. Once we have tested
it, we can install it into its final location.
Invoking the test script via ""make test"" did something
very important. It invoked perl with all those "-I"
arguments so that it could find the various files that are
part of the extension. It is very important that while you
are still testing extensions that you use ""make test"". If
you try to run the test script all by itself, you will get a
fatal error. Another reason it is important to use ""make
test"" to run your test script is that if you are testing an
upgrade to an already-existing version, using ""make test""
ensures that you will test your new extension, not the
already-existing version.
When Perl sees a "use extension;", it searches for a file
with the same name as the "use"'d extension that has a .pm
suffix. If that file cannot be found, Perl dies with a
fatal error. The default search path is contained in the
@INC array.
In our case, Mytest.pm tells perl that it will need the
Exporter and Dynamic Loader extensions. It then sets the
@ISA and @EXPORT arrays and the $VERSION scalar; finally it
tells perl to bootstrap the module. Perl will call its
dynamic loader routine (if there is one) and load the shared
library.
The two arrays @ISA and @EXPORT are very important. The
@ISA array contains a list of other packages in which to
search for methods (or subroutines) that do not exist in the
current package. This is usually only important for object-
oriented extensions (which we will talk about much later),
and so usually doesn't need to be modified.
The @EXPORT array tells Perl which of the extension's
variables and subroutines should be placed into the calling
package's namespace. Because you don't know if the user has
already used your variable and subroutine names, it's
vitally important to carefully select what to export. Do
not export method or variable names by default without a
good reason.
As a general rule, if the module is trying to be object-
oriented then don't export anything. If it's just a
collection of functions and variables, then you can export
them via another array, called @EXPORT_OK. This array does
not automatically place its subroutine and variable names
into the namespace unless the user specifically requests
that this be done.
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See perlmod for more information.
The $VERSION variable is used to ensure that the .pm file
and the shared library are "in sync" with each other. Any
time you make changes to the .pm or .xs files, you should
increment the value of this variable.
Writing good test scripts
The importance of writing good test scripts cannot be over-
emphasized. You should closely follow the "ok/not ok" style
that Perl itself uses, so that it is very easy and
unambiguous to determine the outcome of each test case.
When you find and fix a bug, make sure you add a test case
for it.
By running ""make test"", you ensure that your Mytest.t
script runs and uses the correct version of your extension.
If you have many test cases, save your test files in the "t"
directory and use the suffix ".t". When you run ""make
test"", all of these test files will be executed.
EXAMPLE 3
Our third extension will take one argument as its input,
round off that value, and set the argument to the rounded
value.
Add the following to the end of Mytest.xs:
void
round(arg)
double arg
CODE:
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
arg = ceil(arg - 0.5);
} else {
arg = 0.0;
}
OUTPUT:
arg
Edit the Makefile.PL file so that the corresponding line
looks like this:
'LIBS' => ['-lm'], # e.g., '-lm'
Generate the Makefile and run make. Change the test number
in Mytest.t to "9" and add the following tests:
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$i = -1.5; &Mytest::round($i); is( $i, -2.0 );
$i = -1.1; &Mytest::round($i); is( $i, -1.0 );
$i = 0.0; &Mytest::round($i); is( $i, 0.0 );
$i = 0.5; &Mytest::round($i); is( $i, 1.0 );
$i = 1.2; &Mytest::round($i); is( $i, 1.0 );
Running ""make test"" should now print out that all nine
tests are okay.
Notice that in these new test cases, the argument passed to
round was a scalar variable. You might be wondering if you
can round a constant or literal. To see what happens,
temporarily add the following line to Mytest.t:
&Mytest::round(3);
Run ""make test"" and notice that Perl dies with a fatal
error. Perl won't let you change the value of constants!
What's new here?
o We've made some changes to Makefile.PL. In this case,
we've specified an extra library to be linked into the
extension's shared library, the math library libm in
this case. We'll talk later about how to write XSUBs
that can call every routine in a library.
o The value of the function is not being passed back as
the function's return value, but by changing the value
of the variable that was passed into the function. You
might have guessed that when you saw that the return
value of round is of type "void".
Input and Output Parameters
You specify the parameters that will be passed into the XSUB
on the line(s) after you declare the function's return value
and name. Each input parameter line starts with optional
whitespace, and may have an optional terminating semicolon.
The list of output parameters occurs at the very end of the
function, just after the OUTPUT: directive. The use of
RETVAL tells Perl that you wish to send this value back as
the return value of the XSUB function. In Example 3, we
wanted the "return value" placed in the original variable
which we passed in, so we listed it (and not RETVAL) in the
OUTPUT: section.
The XSUBPP Program
The xsubpp program takes the XS code in the .xs file and
translates it into C code, placing it in a file whose suffix
is .c. The C code created makes heavy use of the C
functions within Perl.
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The TYPEMAP file
The xsubpp program uses rules to convert from Perl's data
types (scalar, array, etc.) to C's data types (int, char,
etc.). These rules are stored in the typemap file
($PERLLIB/ExtUtils/typemap). This file is split into three
parts.
The first section maps various C data types to a name, which
corresponds somewhat with the various Perl types. The
second section contains C code which xsubpp uses to handle
input parameters. The third section contains C code which
xsubpp uses to handle output parameters.
Let's take a look at a portion of the .c file created for
our extension. The file name is Mytest.c:
XS(XS_Mytest_round)
{
dXSARGS;
if (items != 1)
Perl_croak(aTHX_ "Usage: Mytest::round(arg)");
PERL_UNUSED_VAR(cv); /* -W */
{
double arg = (double)SvNV(ST(0)); /* XXXXX */
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
arg = ceil(arg - 0.5);
} else {
arg = 0.0;
}
sv_setnv(ST(0), (double)arg); /* XXXXX */
SvSETMAGIC(ST(0));
}
XSRETURN_EMPTY;
}
Notice the two lines commented with "XXXXX". If you check
the first section of the typemap file, you'll see that
doubles are of type T_DOUBLE. In the INPUT section, an
argument that is T_DOUBLE is assigned to the variable arg by
calling the routine SvNV on something, then casting it to
double, then assigned to the variable arg. Similarly, in
the OUTPUT section, once arg has its final value, it is
passed to the sv_setnv function to be passed back to the
calling subroutine. These two functions are explained in
perlguts; we'll talk more later about what that "ST(0)"
means in the section on the argument stack.
Warning about Output Arguments
In general, it's not a good idea to write extensions that
modify their input parameters, as in Example 3. Instead,
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you should probably return multiple values in an array and
let the caller handle them (we'll do this in a later
example). However, in order to better accommodate calling
pre-existing C routines, which often do modify their input
parameters, this behavior is tolerated.
EXAMPLE 4
In this example, we'll now begin to write XSUBs that will
interact with pre-defined C libraries. To begin with, we
will build a small library of our own, then let h2xs write
our .pm and .xs files for us.
Create a new directory called Mytest2 at the same level as
the directory Mytest. In the Mytest2 directory, create
another directory called mylib, and cd into that directory.
Here we'll create some files that will generate a test
library. These will include a C source file and a header
file. We'll also create a Makefile.PL in this directory.
Then we'll make sure that running make at the Mytest2 level
will automatically run this Makefile.PL file and the
resulting Makefile.
In the mylib directory, create a file mylib.h that looks
like this:
#define TESTVAL 4
extern double foo(int, long, const char*);
Also create a file mylib.c that looks like this:
#include <stdlib.h>
#include "./mylib.h"
double
foo(int a, long b, const char *c)
{
return (a + b + atof(c) + TESTVAL);
}
And finally create a file Makefile.PL that looks like this:
use ExtUtils::MakeMaker;
$Verbose = 1;
WriteMakefile(
NAME => 'Mytest2::mylib',
SKIP => [qw(all static static_lib dynamic dynamic_lib)],
clean => {'FILES' => 'libmylib$(LIB_EXT)'},
);
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sub MY::top_targets {
'
all :: static
pure_all :: static
static :: libmylib$(LIB_EXT)
libmylib$(LIB_EXT): $(O_FILES)
$(AR) cr libmylib$(LIB_EXT) $(O_FILES)
$(RANLIB) libmylib$(LIB_EXT)
';
}
Make sure you use a tab and not spaces on the lines
beginning with "$(AR)" and "$(RANLIB)". Make will not
function properly if you use spaces. It has also been
reported that the "cr" argument to $(AR) is unnecessary on
Win32 systems.
We will now create the main top-level Mytest2 files. Change
to the directory above Mytest2 and run the following
command:
% h2xs -O -n Mytest2 ./Mytest2/mylib/mylib.h
This will print out a warning about overwriting Mytest2, but
that's okay. Our files are stored in Mytest2/mylib, and
will be untouched.
The normal Makefile.PL that h2xs generates doesn't know
about the mylib directory. We need to tell it that there is
a subdirectory and that we will be generating a library in
it. Let's add the argument MYEXTLIB to the WriteMakefile
call so that it looks like this:
WriteMakefile(
'NAME' => 'Mytest2',
'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
'LIBS' => [''], # e.g., '-lm'
'DEFINE' => '', # e.g., '-DHAVE_SOMETHING'
'INC' => '', # e.g., '-I/usr/include/other'
'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
);
and then at the end add a subroutine (which will override
the pre-existing subroutine). Remember to use a tab
character to indent the line beginning with "cd"!
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sub MY::postamble {
'
$(MYEXTLIB): mylib/Makefile
cd mylib && $(MAKE) $(PASSTHRU)
';
}
Let's also fix the MANIFEST file so that it accurately
reflects the contents of our extension. The single line
that says "mylib" should be replaced by the following three
lines:
mylib/Makefile.PL
mylib/mylib.c
mylib/mylib.h
To keep our namespace nice and unpolluted, edit the .pm file
and change the variable @EXPORT to @EXPORT_OK. Finally, in
the .xs file, edit the #include line to read:
#include "mylib/mylib.h"
And also add the following function definition to the end of
the .xs file:
double
foo(a,b,c)
int a
long b
const char * c
OUTPUT:
RETVAL
Now we also need to create a typemap file because the
default Perl doesn't currently support the const char *
type. Create a file called typemap in the Mytest2 directory
and place the following in it:
const char * T_PV
Now run perl on the top-level Makefile.PL. Notice that it
also created a Makefile in the mylib directory. Run make
and watch that it does cd into the mylib directory and run
make in there as well.
Now edit the Mytest2.t script and change the number of tests
to "4", and add the following lines to the end of the
script:
is( &Mytest2::foo(1, 2, "Hello, world!"), 7 );
is( &Mytest2::foo(1, 2, "0.0"), 7 );
ok( abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 );
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(When dealing with floating-point comparisons, it is best to
not check for equality, but rather that the difference
between the expected and actual result is below a certain
amount (called epsilon) which is 0.01 in this case)
Run ""make test"" and all should be well. There are some
warnings on missing tests for the Mytest2::mylib extension,
but you can ignore them.
What has happened here?
Unlike previous examples, we've now run h2xs on a real
include file. This has caused some extra goodies to appear
in both the .pm and .xs files.
o In the .xs file, there's now a #include directive with
the absolute path to the mylib.h header file. We
changed this to a relative path so that we could move
the extension directory if we wanted to.
o There's now some new C code that's been added to the .xs
file. The purpose of the "constant" routine is to make
the values that are #define'd in the header file
accessible by the Perl script (by calling either
"TESTVAL" or &Mytest2::TESTVAL). There's also some XS
code to allow calls to the "constant" routine.
o The .pm file originally exported the name "TESTVAL" in
the @EXPORT array. This could lead to name clashes. A
good rule of thumb is that if the #define is only going
to be used by the C routines themselves, and not by the
user, they should be removed from the @EXPORT array.
Alternately, if you don't mind using the "fully
qualified name" of a variable, you could move most or
all of the items from the @EXPORT array into the
@EXPORT_OK array.
o If our include file had contained #include directives,
these would not have been processed by h2xs. There is
no good solution to this right now.
o We've also told Perl about the library that we built in
the mylib subdirectory. That required only the addition
of the "MYEXTLIB" variable to the WriteMakefile call and
the replacement of the postamble subroutine to cd into
the subdirectory and run make. The Makefile.PL for the
library is a bit more complicated, but not excessively
so. Again we replaced the postamble subroutine to
insert our own code. This code simply specified that
the library to be created here was a static archive
library (as opposed to a dynamically loadable library)
and provided the commands to build it.
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Anatomy of .xs file
The .xs file of "EXAMPLE 4" contained some new elements. To
understand the meaning of these elements, pay attention to
the line which reads
MODULE = Mytest2 PACKAGE = Mytest2
Anything before this line is plain C code which describes
which headers to include, and defines some convenience
functions. No translations are performed on this part,
apart from having embedded POD documentation skipped over
(see perlpod) it goes into the generated output C file as
is.
Anything after this line is the description of XSUB
functions. These descriptions are translated by xsubpp into
C code which implements these functions using Perl calling
conventions, and which makes these functions visible from
Perl interpreter.
Pay a special attention to the function "constant". This
name appears twice in the generated .xs file: once in the
first part, as a static C function, then another time in the
second part, when an XSUB interface to this static C
function is defined.
This is quite typical for .xs files: usually the .xs file
provides an interface to an existing C function. Then this
C function is defined somewhere (either in an external
library, or in the first part of .xs file), and a Perl
interface to this function (i.e. "Perl glue") is described
in the second part of .xs file. The situation in "EXAMPLE
1", "EXAMPLE 2", and "EXAMPLE 3", when all the work is done
inside the "Perl glue", is somewhat of an exception rather
than the rule.
Getting the fat out of XSUBs
In "EXAMPLE 4" the second part of .xs file contained the
following description of an XSUB:
double
foo(a,b,c)
int a
long b
const char * c
OUTPUT:
RETVAL
Note that in contrast with "EXAMPLE 1", "EXAMPLE 2" and
"EXAMPLE 3", this description does not contain the actual
code for what is done is done during a call to Perl function
foo(). To understand what is going on here, one can add a
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CODE section to this XSUB:
double
foo(a,b,c)
int a
long b
const char * c
CODE:
RETVAL = foo(a,b,c);
OUTPUT:
RETVAL
However, these two XSUBs provide almost identical generated
C code: xsubpp compiler is smart enough to figure out the
"CODE:" section from the first two lines of the description
of XSUB. What about "OUTPUT:" section? In fact, that is
absolutely the same! The "OUTPUT:" section can be removed
as well, as far as "CODE:" section or "PPCODE:" section is
not specified: xsubpp can see that it needs to generate a
function call section, and will autogenerate the OUTPUT
section too. Thus one can shortcut the XSUB to become:
double
foo(a,b,c)
int a
long b
const char * c
Can we do the same with an XSUB
int
is_even(input)
int input
CODE:
RETVAL = (input % 2 == 0);
OUTPUT:
RETVAL
of "EXAMPLE 2"? To do this, one needs to define a C
function "int is_even(int input)". As we saw in "Anatomy of
.xs file", a proper place for this definition is in the
first part of .xs file. In fact a C function
int
is_even(int arg)
{
return (arg % 2 == 0);
}
is probably overkill for this. Something as simple as a
"#define" will do too:
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#define is_even(arg) ((arg) % 2 == 0)
After having this in the first part of .xs file, the "Perl
glue" part becomes as simple as
int
is_even(input)
int input
This technique of separation of the glue part from the
workhorse part has obvious tradeoffs: if you want to change
a Perl interface, you need to change two places in your
code. However, it removes a lot of clutter, and makes the
workhorse part independent from idiosyncrasies of Perl
calling convention. (In fact, there is nothing Perl-
specific in the above description, a different version of
xsubpp might have translated this to TCL glue or Python glue
as well.)
More about XSUB arguments
With the completion of Example 4, we now have an easy way to
simulate some real-life libraries whose interfaces may not
be the cleanest in the world. We shall now continue with a
discussion of the arguments passed to the xsubpp compiler.
When you specify arguments to routines in the .xs file, you
are really passing three pieces of information for each
argument listed. The first piece is the order of that
argument relative to the others (first, second, etc). The
second is the type of argument, and consists of the type
declaration of the argument (e.g., int, char*, etc). The
third piece is the calling convention for the argument in
the call to the library function.
While Perl passes arguments to functions by reference, C
passes arguments by value; to implement a C function which
modifies data of one of the "arguments", the actual argument
of this C function would be a pointer to the data. Thus two
C functions with declarations
int string_length(char *s);
int upper_case_char(char *cp);
may have completely different semantics: the first one may
inspect an array of chars pointed by s, and the second one
may immediately dereference "cp" and manipulate *cp only
(using the return value as, say, a success indicator). From
Perl one would use these functions in a completely different
manner.
One conveys this info to xsubpp by replacing "*" before the
argument by "&". "&" means that the argument should be
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passed to a library function by its address. The above two
function may be XSUB-ified as
int
string_length(s)
char * s
int
upper_case_char(cp)
char &cp
For example, consider:
int
foo(a,b)
char &a
char * b
The first Perl argument to this function would be treated as
a char and assigned to the variable a, and its address would
be passed into the function foo. The second Perl argument
would be treated as a string pointer and assigned to the
variable b. The value of b would be passed into the
function foo. The actual call to the function foo that
xsubpp generates would look like this:
foo(&a, b);
xsubpp will parse the following function argument lists
identically:
char &a
char&a
char & a
However, to help ease understanding, it is suggested that
you place a "&" next to the variable name and away from the
variable type), and place a "*" near the variable type, but
away from the variable name (as in the call to foo above).
By doing so, it is easy to understand exactly what will be
passed to the C function; it will be whatever is in the
"last column".
You should take great pains to try to pass the function the
type of variable it wants, when possible. It will save you
a lot of trouble in the long run.
The Argument Stack
If we look at any of the C code generated by any of the
examples except example 1, you will notice a number of
references to ST(n), where n is usually 0. "ST" is actually
a macro that points to the n'th argument on the argument
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stack. ST(0) is thus the first argument on the stack and
therefore the first argument passed to the XSUB, ST(1) is
the second argument, and so on.
When you list the arguments to the XSUB in the .xs file,
that tells xsubpp which argument corresponds to which of the
argument stack (i.e., the first one listed is the first
argument, and so on). You invite disaster if you do not
list them in the same order as the function expects them.
The actual values on the argument stack are pointers to the
values passed in. When an argument is listed as being an
OUTPUT value, its corresponding value on the stack (i.e.,
ST(0) if it was the first argument) is changed. You can
verify this by looking at the C code generated for Example
3. The code for the round() XSUB routine contains lines
that look like this:
double arg = (double)SvNV(ST(0));
/* Round the contents of the variable arg */
sv_setnv(ST(0), (double)arg);
The arg variable is initially set by taking the value from
ST(0), then is stored back into ST(0) at the end of the
routine.
XSUBs are also allowed to return lists, not just scalars.
This must be done by manipulating stack values ST(0), ST(1),
etc, in a subtly different way. See perlxs for details.
XSUBs are also allowed to avoid automatic conversion of Perl
function arguments to C function arguments. See perlxs for
details. Some people prefer manual conversion by inspecting
ST(i) even in the cases when automatic conversion will do,
arguing that this makes the logic of an XSUB call clearer.
Compare with "Getting the fat out of XSUBs" for a similar
tradeoff of a complete separation of "Perl glue" and
"workhorse" parts of an XSUB.
While experts may argue about these idioms, a novice to Perl
guts may prefer a way which is as little Perl-guts-specific
as possible, meaning automatic conversion and automatic call
generation, as in "Getting the fat out of XSUBs". This
approach has the additional benefit of protecting the XSUB
writer from future changes to the Perl API.
Extending your Extension
Sometimes you might want to provide some extra methods or
subroutines to assist in making the interface between Perl
and your extension simpler or easier to understand. These
routines should live in the .pm file. Whether they are
automatically loaded when the extension itself is loaded or
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only loaded when called depends on where in the .pm file the
subroutine definition is placed. You can also consult
AutoLoader for an alternate way to store and load your extra
subroutines.
Documenting your Extension
There is absolutely no excuse for not documenting your
extension. Documentation belongs in the .pm file. This
file will be fed to pod2man, and the embedded documentation
will be converted to the manpage format, then placed in the
blib directory. It will be copied to Perl's manpage
directory when the extension is installed.
You may intersperse documentation and Perl code within the
.pm file. In fact, if you want to use method autoloading,
you must do this, as the comment inside the .pm file
explains.
See perlpod for more information about the pod format.
Installing your Extension
Once your extension is complete and passes all its tests,
installing it is quite simple: you simply run "make
install". You will either need to have write permission
into the directories where Perl is installed, or ask your
system administrator to run the make for you.
Alternately, you can specify the exact directory to place
the extension's files by placing a
"PREFIX=/destination/directory" after the make install. (or
in between the make and install if you have a brain-dead
version of make). This can be very useful if you are
building an extension that will eventually be distributed to
multiple systems. You can then just archive the files in
the destination directory and distribute them to your
destination systems.
EXAMPLE 5
In this example, we'll do some more work with the argument
stack. The previous examples have all returned only a
single value. We'll now create an extension that returns an
array.
This extension is very Unix-oriented (struct statfs and the
statfs system call). If you are not running on a Unix
system, you can substitute for statfs any other function
that returns multiple values, you can hard-code values to be
returned to the caller (although this will be a bit harder
to test the error case), or you can simply not do this
example. If you change the XSUB, be sure to fix the test
cases to match the changes.
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Return to the Mytest directory and add the following code to
the end of Mytest.xs:
void
statfs(path)
char * path
INIT:
int i;
struct statfs buf;
PPCODE:
i = statfs(path, &buf);
if (i == 0) {
XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
} else {
XPUSHs(sv_2mortal(newSVnv(errno)));
}
You'll also need to add the following code to the top of the
.xs file, just after the include of "XSUB.h":
#include <sys/vfs.h>
Also add the following code segment to Mytest.t while
incrementing the "9" tests to "11":
@a = &Mytest::statfs("/blech");
ok( scalar(@a) == 1 && $a[0] == 2 );
@a = &Mytest::statfs("/");
is( scalar(@a), 7 );
New Things in this Example
This example added quite a few new concepts. We'll take
them one at a time.
o The INIT: directive contains code that will be placed
immediately after the argument stack is decoded. C does
not allow variable declarations at arbitrary locations
inside a function, so this is usually the best way to
declare local variables needed by the XSUB.
(Alternatively, one could put the whole "PPCODE:"
section into braces, and put these declarations on top.)
o This routine also returns a different number of
arguments depending on the success or failure of the
call to statfs. If there is an error, the error number
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is returned as a single-element array. If the call is
successful, then a 9-element array is returned. Since
only one argument is passed into this function, we need
room on the stack to hold the 9 values which may be
returned.
We do this by using the PPCODE: directive, rather than
the CODE: directive. This tells xsubpp that we will be
managing the return values that will be put on the
argument stack by ourselves.
o When we want to place values to be returned to the
caller onto the stack, we use the series of macros that
begin with "XPUSH". There are five different versions,
for placing integers, unsigned integers, doubles,
strings, and Perl scalars on the stack. In our example,
we placed a Perl scalar onto the stack. (In fact this
is the only macro which can be used to return multiple
values.)
The XPUSH* macros will automatically extend the return
stack to prevent it from being overrun. You push values
onto the stack in the order you want them seen by the
calling program.
o The values pushed onto the return stack of the XSUB are
actually mortal SV's. They are made mortal so that once
the values are copied by the calling program, the SV's
that held the returned values can be deallocated. If
they were not mortal, then they would continue to exist
after the XSUB routine returned, but would not be
accessible. This is a memory leak.
o If we were interested in performance, not in code
compactness, in the success branch we would not use
"XPUSHs" macros, but "PUSHs" macros, and would pre-
extend the stack before pushing the return values:
EXTEND(SP, 7);
The tradeoff is that one needs to calculate the number
of return values in advance (though overextending the
stack will not typically hurt anything but memory
consumption).
Similarly, in the failure branch we could use "PUSHs"
without extending the stack: the Perl function reference
comes to an XSUB on the stack, thus the stack is always
large enough to take one return value.
EXAMPLE 6
In this example, we will accept a reference to an array as
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an input parameter, and return a reference to an array of
hashes. This will demonstrate manipulation of complex Perl
data types from an XSUB.
This extension is somewhat contrived. It is based on the
code in the previous example. It calls the statfs function
multiple times, accepting a reference to an array of
filenames as input, and returning a reference to an array of
hashes containing the data for each of the filesystems.
Return to the Mytest directory and add the following code to
the end of Mytest.xs:
SV *
multi_statfs(paths)
SV * paths
INIT:
AV * results;
I32 numpaths = 0;
int i, n;
struct statfs buf;
if ((!SvROK(paths))
|| (SvTYPE(SvRV(paths)) != SVt_PVAV)
|| ((numpaths = av_len((AV *)SvRV(paths))) < 0))
{
XSRETURN_UNDEF;
}
results = (AV *)sv_2mortal((SV *)newAV());
CODE:
for (n = 0; n <= numpaths; n++) {
HV * rh;
STRLEN l;
char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
i = statfs(fn, &buf);
if (i != 0) {
av_push(results, newSVnv(errno));
continue;
}
rh = (HV *)sv_2mortal((SV *)newHV());
hv_store(rh, "f_bavail", 8, newSVnv(buf.f_bavail), 0);
hv_store(rh, "f_bfree", 7, newSVnv(buf.f_bfree), 0);
hv_store(rh, "f_blocks", 8, newSVnv(buf.f_blocks), 0);
hv_store(rh, "f_bsize", 7, newSVnv(buf.f_bsize), 0);
hv_store(rh, "f_ffree", 7, newSVnv(buf.f_ffree), 0);
hv_store(rh, "f_files", 7, newSVnv(buf.f_files), 0);
hv_store(rh, "f_type", 6, newSVnv(buf.f_type), 0);
av_push(results, newRV((SV *)rh));
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}
RETVAL = newRV((SV *)results);
OUTPUT:
RETVAL
And add the following code to Mytest.t, while incrementing
the "11" tests to "13":
$results = Mytest::multi_statfs([ '/', '/blech' ]);
ok( ref $results->[0]) );
ok( ! ref $results->[1] );
New Things in this Example
There are a number of new concepts introduced here,
described below:
o This function does not use a typemap. Instead, we
declare it as accepting one SV* (scalar) parameter, and
returning an SV* value, and we take care of populating
these scalars within the code. Because we are only
returning one value, we don't need a "PPCODE:" directive
- instead, we use "CODE:" and "OUTPUT:" directives.
o When dealing with references, it is important to handle
them with caution. The "INIT:" block first checks that
"SvROK" returns true, which indicates that paths is a
valid reference. It then verifies that the object
referenced by paths is an array, using "SvRV" to
dereference paths, and "SvTYPE" to discover its type.
As an added test, it checks that the array referenced by
paths is non-empty, using the "av_len" function (which
returns -1 if the array is empty). The XSRETURN_UNDEF
macro is used to abort the XSUB and return the undefined
value whenever all three of these conditions are not
met.
o We manipulate several arrays in this XSUB. Note that an
array is represented internally by an AV* pointer. The
functions and macros for manipulating arrays are similar
to the functions in Perl: "av_len" returns the highest
index in an AV*, much like $#array; "av_fetch" fetches a
single scalar value from an array, given its index;
"av_push" pushes a scalar value onto the end of the
array, automatically extending the array as necessary.
Specifically, we read pathnames one at a time from the
input array, and store the results in an output array
(results) in the same order. If statfs fails, the
element pushed onto the return array is the value of
errno after the failure. If statfs succeeds, though,
the value pushed onto the return array is a reference to
a hash containing some of the information in the statfs
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structure.
As with the return stack, it would be possible (and a
small performance win) to pre-extend the return array
before pushing data into it, since we know how many
elements we will return:
av_extend(results, numpaths);
o We are performing only one hash operation in this
function, which is storing a new scalar under a key
using "hv_store". A hash is represented by an HV*
pointer. Like arrays, the functions for manipulating
hashes from an XSUB mirror the functionality available
from Perl. See perlguts and perlapi for details.
o To create a reference, we use the "newRV" function.
Note that you can cast an AV* or an HV* to type SV* in
this case (and many others). This allows you to take
references to arrays, hashes and scalars with the same
function. Conversely, the "SvRV" function always
returns an SV*, which may need to be cast to the
appropriate type if it is something other than a scalar
(check with "SvTYPE").
o At this point, xsubpp is doing very little work - the
differences between Mytest.xs and Mytest.c are minimal.
EXAMPLE 7 (Coming Soon)
XPUSH args AND set RETVAL AND assign return value to array
EXAMPLE 8 (Coming Soon)
Setting $!
EXAMPLE 9 Passing open files to XSes
You would think passing files to an XS is difficult, with
all the typeglobs and stuff. Well, it isn't.
Suppose that for some strange reason we need a wrapper
around the standard C library function "fputs()". This is
all we need:
#define PERLIO_NOT_STDIO 0
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <stdio.h>
int
fputs(s, stream)
char * s
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FILE * stream
The real work is done in the standard typemap.
But you loose all the fine stuff done by the perlio layers.
This calls the stdio function "fputs()", which knows nothing
about them.
The standard typemap offers three variants of PerlIO *:
"InputStream" (T_IN), "InOutStream" (T_INOUT) and
"OutputStream" (T_OUT). A bare "PerlIO *" is considered a
T_INOUT. If it matters in your code (see below for why it
might) #define or typedef one of the specific names and use
that as the argument or result type in your XS file.
The standard typemap does not contain PerlIO * before perl
5.7, but it has the three stream variants. Using a PerlIO *
directly is not backwards compatible unless you provide your
own typemap.
For streams coming from perl the main difference is that
"OutputStream" will get the output PerlIO * - which may make
a difference on a socket. Like in our example...
For streams being handed to perl a new file handle is
created (i.e. a reference to a new glob) and associated with
the PerlIO * provided. If the read/write state of the PerlIO
* is not correct then you may get errors or warnings from
when the file handle is used. So if you opened the PerlIO *
as "w" it should really be an "OutputStream" if open as "r"
it should be an "InputStream".
Now, suppose you want to use perlio layers in your XS. We'll
use the perlio "PerlIO_puts()" function as an example.
In the C part of the XS file (above the first MODULE line)
you have
#define OutputStream PerlIO *
or
typedef PerlIO * OutputStream;
And this is the XS code:
int
perlioputs(s, stream)
char * s
OutputStream stream
CODE:
RETVAL = PerlIO_puts(stream, s);
OUTPUT:
RETVAL
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We have to use a "CODE" section because "PerlIO_puts()" has
the arguments reversed compared to "fputs()", and we want to
keep the arguments the same.
Wanting to explore this thoroughly, we want to use the stdio
"fputs()" on a PerlIO *. This means we have to ask the
perlio system for a stdio "FILE *":
int
perliofputs(s, stream)
char * s
OutputStream stream
PREINIT:
FILE *fp = PerlIO_findFILE(stream);
CODE:
if (fp != (FILE*) 0) {
RETVAL = fputs(s, fp);
} else {
RETVAL = -1;
}
OUTPUT:
RETVAL
Note: "PerlIO_findFILE()" will search the layers for a stdio
layer. If it can't find one, it will call
"PerlIO_exportFILE()" to generate a new stdio "FILE". Please
only call "PerlIO_exportFILE()" if you want a new "FILE". It
will generate one on each call and push a new stdio layer.
So don't call it repeatedly on the same file.
"PerlIO()"_findFILE will retrieve the stdio layer once it
has been generated by "PerlIO_exportFILE()".
This applies to the perlio system only. For versions before
5.7, "PerlIO_exportFILE()" is equivalent to
"PerlIO_findFILE()".
Troubleshooting these Examples
As mentioned at the top of this document, if you are having
problems with these example extensions, you might see if any
of these help you.
o In versions of 5.002 prior to the gamma version, the
test script in Example 1 will not function properly.
You need to change the "use lib" line to read:
use lib './blib';
o In versions of 5.002 prior to version 5.002b1h, the
test.pl file was not automatically created by h2xs.
This means that you cannot say "make test" to run the
test script. You will need to add the following line
before the "use extension" statement:
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use lib './blib';
o In versions 5.000 and 5.001, instead of using the above
line, you will need to use the following line:
BEGIN { unshift(@INC, "./blib") }
o This document assumes that the executable named "perl"
is Perl version 5. Some systems may have installed Perl
version 5 as "perl5".
See also
For more information, consult perlguts, perlapi, perlxs,
perlmod, and perlpod.
Author
Jeff Okamoto <okamoto@corp.hp.com>
Reviewed and assisted by Dean Roehrich, Ilya Zakharevich,
Andreas Koenig, and Tim Bunce.
PerlIO material contributed by Lupe Christoph, with some
clarification by Nick Ing-Simmons.
Changes for h2xs as of Perl 5.8.x by Renee Baecker
Last Changed
2007/10/11
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
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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