Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
PERLXSTUT(1)	       Perl Programmers	Reference Guide		  PERLXSTUT(1)

       perlxstut - Tutorial for	writing	XSUBs

       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.

       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 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:

	   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.

   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.

   Threads and PERL_NO_GET_CONTEXT
       For threaded builds, perl requires the context pointer for the current
       thread, without "PERL_NO_GET_CONTEXT", perl will	call a function	to
       retrieve	the context.

       For improved performance, include:


       as shown	below.

       For more	details, see perlguts.

       Now let's go on with the	show!

       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.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:

	   use ExtUtils::MakeMaker;
	   # See lib/ExtUtils/ for details of how to influence
	   # the contents of the Makefile that is written.
	       NAME	    => 'Mytest',
	       VERSION_FROM => '', # finds $VERSION
	       LIBS	    => [''],   # e.g., '-lm'
	       DEFINE	    => '',     # e.g., '-DHAVE_SOMETHING'
	       INC	    => '',     # e.g., '-I/usr/include/other'

       The file 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.

	   # 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"

	   #include "ppport.h"

	   MODULE = Mytest	       PACKAGE = Mytest

       Let's edit the .xs file by adding this to the end of the	file:

		   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/ blib/lib/
	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
	rm -f blib/arch/auto/Mytest/
	cc -shared -L/usr/local/lib Mytest.o -o	blib/arch/auto/Mytest/

	chmod 755 blib/arch/auto/Mytest/
	cp blib/arch/auto/Mytest/
	chmod 644 blib/arch/auto/Mytest/
	Manifying blib/man3/Mytest.3pm

       You can safely ignore the line about "prototyping behavior" - it	is
       explained in "The PROTOTYPES: Keyword" in perlxs.

       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;


       Now we make the script executable ("chmod +x hello"), run the script
       and we should see the following output:

	   % ./hello
	   Hello, world!

       Now let's add to	our extension a	subroutine that	will take a single
       numeric argument	as input and return 1 if the number is even or 0 if
       the number is odd.

       Add the following to the	end of Mytest.xs:

		   int input
		   RETVAL = (input % 2 == 0);

       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.

       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
	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 the shared library that we will
       build.  Once we have tested it, we can install it into its final

       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, 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.

       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

   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

       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:

		       double  arg
		       if (arg > 0.0) {
			       arg = floor(arg + 0.5);
		       } else if (arg <	0.0) {
			       arg = ceil(arg -	0.5);
		       } else {
			       arg = 0.0;

       Edit the	Makefile.PL file so that the corresponding line	looks like

	       '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:

	       $i = -1.5; &Mytest::round($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

       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


       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.

   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).  There's a
       brief discussion	below, but all the nitty-gritty	details	can be found
       in perlxstypemap.  If you have a	new-enough version of perl (5.16 and
       up) or an upgraded XS compiler ("ExtUtils::ParseXS" 3.13_01 or better),
       then you	can inline typemaps in your XS instead of writing separate
       files.  Either way, this	typemap	thing 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

       Let's take a look at a portion of the .c	file created for our
       extension.  The file name is Mytest.c:

		   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	*/

       Notice the two lines commented with "XXXXX".  If	you check the first
       part of the typemap file	(or section), you'll see that doubles are of
       type T_DOUBLE.  In the INPUT part of the	typemap, 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

   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, 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.

       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"

	       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;
	       $Verbose	= 1;
		   NAME	  => 'Mytest2::mylib',
		   SKIP	  => [qw(all static static_lib dynamic dynamic_lib)],
		   clean  => {'FILES' => 'libmylib$(LIB_EXT)'},

	       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:

		   'NAME'      => 'Mytest2',
		   'VERSION_FROM' => '', # 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"!

	       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:


       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

		       int	       a
		       long	       b
		       const char *    c

       Now we also need	to create a typemap because the	default	Perl doesn't
       currently support the "const char *" type.  Include a new TYPEMAP
       section in your XS code before the above	function:

	       TYPEMAP:	<<END
	       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 );

       (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

   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

       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

   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:

		       int	       a
		       long	       b
		       const char *    c

       Note that in contrast with "EXAMPLE 1", "EXAMPLE	2" and "EXAMPLE	3",
       this description	does not contain the actual code for what is done
       during a	call to	Perl function foo().  To understand what is going on
       here, one can add a CODE	section	to this	XSUB:

		       int	       a
		       long	       b
		       const char *    c
		       RETVAL =	foo(a,b,c);

       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:

		       int	       a
		       long	       b
		       const char *    c

       Can we do the same with an XSUB

		       int     input
		       RETVAL =	(input % 2 == 0);

       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

	       is_even(int arg)
		       return (arg % 2 == 0);

       is probably overkill for	this.  Something as simple as a	"#define" will
       do too:

	       #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     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 passed to a library
       function	by its address.	 The above two function	may be XSUB-ified as

		       char *  s

		       char    &cp

       For example, consider:

		       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

       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 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 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.

       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.

       Return to the Mytest directory and add the following code to the	end of

		       char *  path
		       int i;
		       struct statfs buf;

		       i = statfs(path,	&buf);
		       if (i ==	0) {
		       } else {

       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

       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 is returned as a single-element array.  If
	   the call is successful, then	a 7-element array is returned.	Since
	   only	one argument is	passed into this function, we need room	on the
	   stack to hold the 7 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

		   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.

       In this example,	we will	accept a reference to an array as 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

       Return to the Mytest directory and add the following code to the	end of

	   SV *
		   SV *	paths
		   AV *	results;
		   SSize_t numpaths = 0, n;
		   int i;
		   struct statfs buf;

		   if ((!SvROK(paths))
		       || (SvTYPE(SvRV(paths)) != SVt_PVAV)
		       || ((numpaths = av_top_index((AV	*)SvRV(paths)))	< 0))
		   results = (AV *)sv_2mortal((SV *)newAV());
		   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));

		       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_inc((SV *)rh));
		   RETVAL = newRV_inc((SV *)results);

       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 calls SvGETMAGIC(paths), in case
	   paths is a tied variable.  Then it checks that "SvROK" returns
	   true, which indicates that paths is a valid reference.  (Simply
	   checking "SvROK" won't trigger FETCH	on a tied variable.)  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_top_index" 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_top_index" 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 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_inc" 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
	       #define PERL_NO_GET_CONTEXT
	       #include	"EXTERN.h"
	       #include	"perl.h"
	       #include	"XSUB.h"

	       #include	<stdio.h>

	       fputs(s,	stream)
		       char *	       s
		       FILE *	       stream

       The real	work is	done in	the standard typemap.

       But you lose 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 *
	       typedef PerlIO *	       OutputStream;

       And this	is the XS code:

	       perlioputs(s, stream)
		       char *	       s
		       OutputStream    stream
		       RETVAL =	PerlIO_puts(stream, s);

       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 *":

	       perliofputs(s, stream)
		       char *	       s
		       OutputStream    stream
		       FILE *fp	= PerlIO_findFILE(stream);
		       if (fp != (FILE*) 0) {
			       RETVAL =	fputs(s, fp);
		       } else {
			       RETVAL =	-1;

       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

       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 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:

		   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

See also
       For more	information, consult perlguts, perlapi,	perlxs,	perlmod, and

       Jeff Okamoto <>

       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

perl v5.28.3			  2020-05-14			  PERLXSTUT(1)


Want to link to this manual page? Use this URL:

home | help