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PERLXSTUT(1pm)         Perl Programmers Reference Guide         PERLXSTUT(1pm)


       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:

         #define PERL_NO_GET_CONTEXT

       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          => '-I',        # e.g., '-I. -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:

           #define PERL_NO_GET_CONTEXT
           #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:

               my $i;

               $i = -1.5;
               is( $i, -2.0, 'Rounding -1.5 to -2.0' );

               $i = -1.1;
               is( $i, -1.0, 'Rounding -1.1 to -1.0' );

               $i = 0.0;
               is( $i, 0.0, 'Rounding 0.0 to 0.0' );

               $i = 0.5;
               is( $i, 1.0, 'Rounding 0.5 to 1.0' );

               $i = 1.2;
               is( $i, 1.0, 'Rounding 1.2 to 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 by appending the following three


       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 "5",
       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":

           my @a;

               @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;
                       SV * path = *av_fetch((AV *)SvRV(paths), n, 0);
                       char * fn = SvPVbyte(path, 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":

               my $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  /* For co-existence with stdio only */
         #define PERL_NO_GET_CONTEXT /* This is more efficient */
         #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.

       For more details, see "Co-existence with stdio" in perlapio.

       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(1), perlapi(1), perlxs(1),
       perlmod(1), perlapio(1), and perlpod(1)


       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

       This document is now maintained as part of Perl itself.

   Last Changed

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