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perlxstut (1)


perlxstut - Tutorial for writing XSUBs


Please see following description for synopsis


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:

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         eval { require 5.007 }
             or die <<EOD;
         ### This module uses frobnication framework which is not available before
         ### version 5.007 of Perl.  Upgrade your Perl before installing Kara::Mba.

  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""

     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.

     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:

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         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"

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         #include "ppport.h"

         MODULE = Mytest             PACKAGE = Mytest

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

                 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

         % 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.

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     If you are on a Win32 system, and the build process fails
     with linker errors for functions in the C library, check if
     your Perl is configured to use PerlCRT (running perl -V:libc
     should show you if this is the case).  If Perl is configured
     to use PerlCRT, you have to make sure PerlCRT.lib is copied
     to the same location that msvcrt.lib lives in, so that the
     compiler can find it on its own.  msvcrt.lib is usually
     found in the Visual C compiler's lib directory (e.g.

     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 0 if the number
     is even or 1 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.

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     Now perform the same steps as before, generating a Makefile
     from the Makefile.PL file, and running make.

     In order to test that our extension works, we now need to
     look at the file Mytest.t.  This file is set up to imitate
     the same kind of testing structure that Perl itself has.
     Within the test script, you perform a number of tests to
     confirm the behavior of the extension, printing "ok" when
     the test is correct, "not ok" when it is not.

         use Test::More tests => 4;
         BEGIN { use_ok('Mytest') };


         # Insert your test code below, the Test::More module is use()ed here so read
         # its man page ( perldoc Test::More ) for help writing this test script.

         is(&Mytest::is_even(0), 1);
         is(&Mytest::is_even(1), 0);
         is(&Mytest::is_even(2), 1);

     We will be calling the test script through the command
     ""make test"".  You should see output that looks something
     like this:

         %make test
         PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
         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

     h2xs creates a number of files in the extension directory.
     The file Makefile.PL is a perl script which will generate a
     true Makefile to build the extension.  We'll take a closer
     look at it later.

     The .pm and .xs files contain the meat of the extension.
     The .xs file holds the C routines that make up the
     extension.  The .pm file contains routines that tell Perl
     how to load your extension.

     Generating the Makefile and running "make" created a
     directory called blib (which stands for "build library") in
     the current working directory.  This directory will contain

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

     Invoking the test script via ""make test"" did something
     very important.  It invoked perl with all those "-I"
     arguments so that it could find the various files that are
     part of the extension.  It is very important that while you
     are still testing extensions that you use ""make test"".  If
     you try to run the test script all by itself, you will get a
     fatal error.  Another reason it is important to use ""make
     test"" to run your test script is that if you are testing an
     upgrade to an already-existing version, using ""make test""
     ensures that you will test your new extension, not the
     already-existing version.

     When Perl sees a "use extension;", it searches for a file
     with the same name as the "use"'d extension that has a .pm
     suffix.  If that file cannot be found, Perl dies with a
     fatal error.  The default search path is contained in the
     @INC array.

     In our case, 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

     The two arrays @ISA and @EXPORT are very important.  The
     @ISA array contains a list of other packages in which to
     search for methods (or subroutines) that do not exist in the
     current package.  This is usually only important for object-
     oriented extensions (which we will talk about much later),
     and so usually doesn't need to be modified.

     The @EXPORT array tells Perl which of the extension's
     variables and subroutines should be placed into the calling
     package's namespace.  Because you don't know if the user has
     already used your variable and subroutine names, it's
     vitally important to carefully select what to export.  Do
     not export method or variable names by default without a
     good reason.

     As a general rule, if the module is trying to be object-
     oriented then don't export anything.  If it's just a
     collection of functions and variables, then you can export
     them via another array, called @EXPORT_OK.  This array does
     not automatically place its subroutine and variable names
     into the namespace unless the user specifically requests
     that this be done.

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     See perlmod for more information.

     The $VERSION variable is used to ensure that the .pm file
     and the shared library are "in sync" with each other.  Any
     time you make changes to the .pm or .xs files, you should
     increment the value of this variable.

  Writing good test scripts
     The importance of writing good test scripts cannot be over-
     emphasized.  You should closely follow the "ok/not ok" style
     that Perl itself uses, so that it is very easy and
     unambiguous to determine the outcome of each test case.
     When you find and fix a bug, make sure you add a test case
     for it.

     By running ""make test"", you ensure that your Mytest.t
     script runs and uses the correct version of your extension.
     If you have many test cases, save your test files in the "t"
     directory and use the suffix ".t".  When you run ""make
     test"", all of these test files will be executed.

     Our third extension will take one argument as its input,
     round off that value, and set the argument to the rounded

     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 this:

             'LIBS'      => ['-lm'],   # e.g., '-lm'

     Generate the Makefile and run make.  Change the test number
     in Mytest.t to "9" and add the following tests:

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             $i = -1.5; &Mytest::round($i); is( $i, -2.0 );
             $i = -1.1; &Mytest::round($i); is( $i, -1.0 );
             $i = 0.0; &Mytest::round($i);  is( $i,  0.0 );
             $i = 0.5; &Mytest::round($i);  is( $i,  1.0 );
             $i = 1.2; &Mytest::round($i);  is( $i,  1.0 );

     Running ""make test"" should now print out that all nine
     tests are okay.

     Notice that in these new test cases, the argument passed to
     round was a scalar variable.  You might be wondering if you
     can round a constant or literal.  To see what happens,
     temporarily add the following line to Mytest.t:


     Run ""make test"" and notice that Perl dies with a fatal
     error.  Perl won't let you change the value of constants!

  What's new here?
     o   We've made some changes to Makefile.PL.  In this case,
         we've specified an extra library to be linked into the
         extension's shared library, the math library libm in
         this case.  We'll talk later about how to write XSUBs
         that can call every routine in a library.

     o   The value of the function is not being passed back as
         the function's return value, but by changing the value
         of the variable that was passed into the function.  You
         might have guessed that when you saw that the return
         value of round is of type "void".

  Input and Output Parameters
     You specify the parameters that will be passed into the XSUB
     on the line(s) after you declare the function's return value
     and name.  Each input parameter line starts with optional
     whitespace, and may have an optional terminating semicolon.

     The list of output parameters occurs at the very end of the
     function, just after the OUTPUT: directive.  The use of
     RETVAL tells Perl that you wish to send this value back as
     the return value of the XSUB function.  In Example 3, we
     wanted the "return value" placed in the original variable
     which we passed in, so we listed it (and not RETVAL) in the
     OUTPUT: section.

  The XSUBPP Program
     The xsubpp program takes the XS code in the .xs file and
     translates it into C code, placing it in a file whose suffix
     is .c.  The C code created makes heavy use of the C
     functions within Perl.

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  The TYPEMAP file
     The xsubpp program uses rules to convert from Perl's data
     types (scalar, array, etc.) to C's data types (int, char,
     etc.).  These rules are stored in the typemap file
     ($PERLLIB/ExtUtils/typemap).  This file is split into three

     The first section maps various C data types to a name, which
     corresponds somewhat with the various Perl types.  The
     second section contains C code which xsubpp uses to handle
     input parameters.  The third section contains C code which
     xsubpp uses to handle output parameters.

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

                 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 section of the typemap file, you'll see that
     doubles are of type T_DOUBLE.  In the INPUT section, an
     argument that is T_DOUBLE is assigned to the variable arg by
     calling the routine SvNV on something, then casting it to
     double, then assigned to the variable arg.  Similarly, in
     the OUTPUT section, once arg has its final value, it is
     passed to the sv_setnv function to be passed back to the
     calling subroutine.  These two functions are explained in
     perlguts; we'll talk more later about what that "ST(0)"
     means in the section on the argument stack.

  Warning about Output Arguments
     In general, it's not a good idea to write extensions that
     modify their input parameters, as in Example 3.  Instead,

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     you should probably return multiple values in an array and
     let the caller handle them (we'll do this in a later
     example).  However, in order to better accommodate calling
     pre-existing C routines, which often do modify their input
     parameters, this behavior is tolerated.

     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)'},

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             sub MY::top_targets {
             all :: static

             pure_all :: static

             static ::       libmylib$(LIB_EXT)

             libmylib$(LIB_EXT): $(O_FILES)
                     $(AR) cr libmylib$(LIB_EXT) $(O_FILES)
                     $(RANLIB) libmylib$(LIB_EXT)


     Make sure you use a tab and not spaces on the lines
     beginning with "$(AR)" and "$(RANLIB)".  Make will not
     function properly if you use spaces.  It has also been
     reported that the "cr" argument to $(AR) is unnecessary on
     Win32 systems.

     We will now create the main top-level Mytest2 files.  Change
     to the directory above Mytest2 and run the following

             % 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"!

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             sub MY::postamble {
             $(MYEXTLIB): mylib/Makefile
                     cd mylib && $(MAKE) $(PASSTHRU)

     Let's also fix the MANIFEST file so that it accurately
     reflects the contents of our extension.  The single line
     that says "mylib" should be replaced by the following three


     To keep our namespace nice and unpolluted, edit the .pm file
     and change the variable @EXPORT to @EXPORT_OK.  Finally, in
     the .xs file, edit the #include line to read:

             #include "mylib/mylib.h"

     And also add the following function definition to the end of
     the .xs file:

                     int             a
                     long            b
                     const char *    c

     Now we also need to create a typemap file because the
     default Perl doesn't currently support the const char *
     type.  Create a file called typemap in the Mytest2 directory
     and place the following in it:

             const char *    T_PV

     Now run perl on the top-level Makefile.PL.  Notice that it
     also created a Makefile in the mylib directory.  Run make
     and watch that it does cd into the mylib directory and run
     make in there as well.

     Now edit the Mytest2.t script and change the number of tests
     to "4", and add the following lines to the end of the

             is( &Mytest2::foo(1, 2, "Hello, world!"), 7 );
             is( &Mytest2::foo(1, 2, "0.0"), 7 );
             ok( abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 );

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     (When dealing with floating-point comparisons, it is best to
     not check for equality, but rather that the difference
     between the expected and actual result is below a certain
     amount (called epsilon) which is 0.01 in this case)

     Run ""make test"" and all should be well. There are some
     warnings on missing tests for the Mytest2::mylib extension,
     but you can ignore them.

  What has happened here?
     Unlike previous examples, we've now run h2xs on a real
     include file.  This has caused some extra goodies to appear
     in both the .pm and .xs files.

     o   In the .xs file, there's now a #include directive with
         the absolute path to the mylib.h header file.  We
         changed this to a relative path so that we could move
         the extension directory if we wanted to.

     o   There's now some new C code that's been added to the .xs
         file.  The purpose of the "constant" routine is to make
         the values that are #define'd in the header file
         accessible by the Perl script (by calling either
         "TESTVAL" or &Mytest2::TESTVAL).  There's also some XS
         code to allow calls to the "constant" routine.

     o   The .pm file originally exported the name "TESTVAL" in
         the @EXPORT array.  This could lead to name clashes.  A
         good rule of thumb is that if the #define is only going
         to be used by the C routines themselves, and not by the
         user, they should be removed from the @EXPORT array.
         Alternately, if you don't mind using the "fully
         qualified name" of a variable, you could move most or
         all of the items from the @EXPORT array into the
         @EXPORT_OK array.

     o   If our include file had contained #include directives,
         these would not have been processed by h2xs.  There is
         no good solution to this right now.

     o   We've also told Perl about the library that we built in
         the mylib subdirectory.  That required only the addition
         of the "MYEXTLIB" variable to the WriteMakefile call and
         the replacement of the postamble subroutine to cd into
         the subdirectory and run make.  The Makefile.PL for the
         library is a bit more complicated, but not excessively
         so.  Again we replaced the postamble subroutine to
         insert our own code.  This code simply specified that
         the library to be created here was a static archive
         library (as opposed to a dynamically loadable library)
         and provided the commands to build it.

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  Anatomy of .xs file
     The .xs file of "EXAMPLE 4" contained some new elements.  To
     understand the meaning of these elements, pay attention to
     the line which reads

             MODULE = Mytest2                PACKAGE = Mytest2

     Anything before this line is plain C code which describes
     which headers to include, and defines some convenience
     functions.  No translations are performed on this part,
     apart from having embedded POD documentation skipped over
     (see perlpod) it goes into the generated output C file as

     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 is done during a call to Perl function
     foo().  To understand what is going on here, one can add a

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     CODE section to this XSUB:

                     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:

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             #define is_even(arg)    ((arg) % 2 == 0)

     After having this in the first part of .xs file, the "Perl
     glue" part becomes as simple as

                     int     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

     One conveys this info to xsubpp by replacing "*" before the
     argument by "&".  "&" means that the argument should be

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

                     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

             char    &a
             char    & a

     However, to help ease understanding, it is suggested that
     you place a "&" next to the variable name and away from the
     variable type), and place a "*" near the variable type, but
     away from the variable name (as in the call to foo above).
     By doing so, it is easy to understand exactly what will be
     passed to the C function; it will be whatever is in the
     "last column".

     You should take great pains to try to pass the function the
     type of variable it wants, when possible.  It will save you
     a lot of trouble in the long run.

  The Argument Stack
     If we look at any of the C code generated by any of the
     examples except example 1, you will notice a number of
     references to ST(n), where n is usually 0.  "ST" is actually
     a macro that points to the n'th argument on the argument

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     stack.  ST(0) is thus the first argument on the stack and
     therefore the first argument passed to the XSUB, ST(1) is
     the second argument, and so on.

     When you list the arguments to the XSUB in the .xs file,
     that tells xsubpp which argument corresponds to which of the
     argument stack (i.e., the first one listed is the first
     argument, and so on).  You invite disaster if you do not
     list them in the same order as the function expects them.

     The actual values on the argument stack are pointers to the
     values passed in.  When an argument is listed as being an
     OUTPUT value, its corresponding value on the stack (i.e.,
     ST(0) if it was the first argument) is changed.  You can
     verify this by looking at the C code generated for Example
     3.  The code for the round() XSUB routine contains lines
     that look like this:

             double  arg = (double)SvNV(ST(0));
             /* Round the contents of the variable arg */
             sv_setnv(ST(0), (double)arg);

     The arg variable is initially set by taking the value from
     ST(0), then is stored back into ST(0) at the end of the

     XSUBs are also allowed to return lists, not just scalars.
     This must be done by manipulating stack values ST(0), ST(1),
     etc, in a subtly different way.  See perlxs for details.

     XSUBs are also allowed to avoid automatic conversion of Perl
     function arguments to C function arguments.  See perlxs for
     details.  Some people prefer manual conversion by inspecting
     ST(i) even in the cases when automatic conversion will do,
     arguing that this makes the logic of an XSUB call clearer.
     Compare with "Getting the fat out of XSUBs" for a similar
     tradeoff of a complete separation of "Perl glue" and
     "workhorse" parts of an XSUB.

     While experts may argue about these idioms, a novice to Perl
     guts may prefer a way which is as little Perl-guts-specific
     as possible, meaning automatic conversion and automatic call
     generation, as in "Getting the fat out of XSUBs".  This
     approach has the additional benefit of protecting the XSUB
     writer from future changes to the Perl API.

  Extending your Extension
     Sometimes you might want to provide some extra methods or
     subroutines to assist in making the interface between Perl
     and your extension simpler or easier to understand.  These
     routines should live in the .pm file.  Whether they are
     automatically loaded when the extension itself is loaded or

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     only loaded when called depends on where in the .pm file the
     subroutine definition is placed.  You can also consult
     AutoLoader for an alternate way to store and load your extra

  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

     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

     This extension is very Unix-oriented (struct statfs and the
     statfs system call).  If you are not running on a Unix
     system, you can substitute for statfs any other function
     that returns multiple values, you can hard-code values to be
     returned to the caller (although this will be a bit harder
     to test the error case), or you can simply not do this
     example.  If you change the XSUB, be sure to fix the test
     cases to match the changes.

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     Return to the Mytest directory and add the following code to
     the end of Mytest.xs:

                     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 time.

     o   The INIT: directive contains code that will be placed
         immediately after the argument stack is decoded.  C does
         not allow variable declarations at arbitrary locations
         inside a function, so this is usually the best way to
         declare local variables needed by the XSUB.
         (Alternatively, one could put the whole "PPCODE:"
         section into braces, and put these declarations on top.)

     o   This routine also returns a different number of
         arguments depending on the success or failure of the
         call to statfs.  If there is an error, the error number

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         is returned as a single-element array.  If the call is
         successful, then a 9-element array is returned.  Since
         only one argument is passed into this function, we need
         room on the stack to hold the 9 values which may be

         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

         The XPUSH* macros will automatically extend the return
         stack to prevent it from being overrun.  You push values
         onto the stack in the order you want them seen by the
         calling program.

     o   The values pushed onto the return stack of the XSUB are
         actually mortal SV's.  They are made mortal so that once
         the values are copied by the calling program, the SV's
         that held the returned values can be deallocated.  If
         they were not mortal, then they would continue to exist
         after the XSUB routine returned, but would not be
         accessible.  This is a memory leak.

     o   If we were interested in performance, not in code
         compactness, in the success branch we would not use
         "XPUSHs" macros, but "PUSHs" macros, and would pre-
         extend the stack before pushing the return values:

                 EXTEND(SP, 7);

         The tradeoff is that one needs to calculate the number
         of return values in advance (though overextending the
         stack will not typically hurt anything but memory

         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

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     an input parameter, and return a reference to an array of
     hashes.  This will demonstrate manipulation of complex Perl
     data types from an XSUB.

     This extension is somewhat contrived.  It is based on the
     code in the previous example.  It calls the statfs function
     multiple times, accepting a reference to an array of
     filenames as input, and returning a reference to an array of
     hashes containing the data for each of the filesystems.

     Return to the Mytest directory and add the following code to
     the end of Mytest.xs:

         SV *
                 SV * paths
                 AV * results;
                 I32 numpaths = 0;
                 int i, n;
                 struct statfs buf;

                 if ((!SvROK(paths))
                     || (SvTYPE(SvRV(paths)) != SVt_PVAV)
                     || ((numpaths = av_len((AV *)SvRV(paths))) < 0))
                 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((SV *)rh));

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                 RETVAL = newRV((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 checks that
         "SvROK" returns true, which indicates that paths is a
         valid reference.  It then verifies that the object
         referenced by paths is an array, using "SvRV" to
         dereference paths, and "SvTYPE" to discover its type.
         As an added test, it checks that the array referenced by
         paths is non-empty, using the "av_len" function (which
         returns -1 if the array is empty).  The XSRETURN_UNDEF
         macro is used to abort the XSUB and return the undefined
         value whenever all three of these conditions are not

     o   We manipulate several arrays in this XSUB.  Note that an
         array is represented internally by an AV* pointer.  The
         functions and macros for manipulating arrays are similar
         to the functions in Perl: "av_len" returns the highest
         index in an AV*, much like $#array; "av_fetch" fetches a
         single scalar value from an array, given its index;
         "av_push" pushes a scalar value onto the end of the
         array, automatically extending the array as necessary.

         Specifically, we read pathnames one at a time from the
         input array, and store the results in an output array
         (results) in the same order.  If statfs fails, the
         element pushed onto the return array is the value of
         errno after the failure.  If statfs succeeds, though,
         the value pushed onto the return array is a reference to
         a hash containing some of the information in the statfs

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         As with the return stack, it would be possible (and a
         small performance win) to pre-extend the return array
         before pushing data into it, since we know how many
         elements we will return:

                 av_extend(results, numpaths);

     o   We are performing only one hash operation in this
         function, which is storing a new scalar under a key
         using "hv_store".  A hash is represented by an HV*
         pointer.  Like arrays, the functions for manipulating
         hashes from an XSUB mirror the functionality available
         from Perl.  See perlguts and perlapi for details.

     o   To create a reference, we use the "newRV" function.
         Note that you can cast an AV* or an HV* to type SV* in
         this case (and many others).  This allows you to take
         references to arrays, hashes and scalars with the same
         function.  Conversely, the "SvRV" function always
         returns an SV*, which may need to be cast to the
         appropriate type if it is something other than a scalar
         (check with "SvTYPE").

     o   At this point, xsubpp is doing very little work - the
         differences between Mytest.xs and Mytest.c are minimal.

  EXAMPLE 7 (Coming Soon)
     XPUSH args AND set RETVAL AND assign return value to array

  EXAMPLE 8 (Coming Soon)
     Setting $!

  EXAMPLE 9 Passing open files to XSes
     You would think passing files to an XS is difficult, with
     all the typeglobs and stuff. Well, it isn't.

     Suppose that for some strange reason we need a wrapper
     around the standard C library function "fputs()". This is
     all we need:

             #define PERLIO_NOT_STDIO 0
             #include "EXTERN.h"
             #include "perl.h"
             #include "XSUB.h"

             #include <stdio.h>

             fputs(s, stream)
                     char *          s

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                     FILE *          stream

     The real work is done in the standard typemap.

     But you loose all the fine stuff done by the perlio layers.
     This calls the stdio function "fputs()", which knows nothing
     about them.

     The standard typemap offers three variants of PerlIO *:
     "InputStream" (T_IN), "InOutStream" (T_INOUT) and
     "OutputStream" (T_OUT). A bare "PerlIO *" is considered a
     T_INOUT. If it matters in your code (see below for why it
     might) #define or typedef one of the specific names and use
     that as the argument or result type in your XS file.

     The standard typemap does not contain PerlIO * before perl
     5.7, but it has the three stream variants. Using a PerlIO *
     directly is not backwards compatible unless you provide your
     own typemap.

     For streams coming from perl the main difference is that
     "OutputStream" will get the output PerlIO * - which may make
     a difference on a socket. Like in our example...

     For streams being handed to perl a new file handle is
     created (i.e. a reference to a new glob) and associated with
     the PerlIO * provided. If the read/write state of the PerlIO
     * is not correct then you may get errors or warnings from
     when the file handle is used.  So if you opened the PerlIO *
     as "w" it should really be an "OutputStream" if open as "r"
     it should be an "InputStream".

     Now, suppose you want to use perlio layers in your XS. We'll
     use the perlio "PerlIO_puts()" function as an example.

     In the C part of the XS file (above the first MODULE line)
     you have

             #define OutputStream    PerlIO *
             typedef PerlIO *        OutputStream;

     And this is the XS code:

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

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     We have to use a "CODE" section because "PerlIO_puts()" has
     the arguments reversed compared to "fputs()", and we want to
     keep the arguments the same.

     Wanting to explore this thoroughly, we want to use the stdio
     "fputs()" on a PerlIO *. This means we have to ask the
     perlio system for a stdio "FILE *":

             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 "PerlIO_exportFILE()".

     This applies to the perlio system only. For versions before
     5.7, "PerlIO_exportFILE()" is equivalent to

  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:

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                 use lib './blib';

     o   In versions 5.000 and 5.001, instead of using the above
         line, you will need to use the following line:

                 BEGIN { unshift(@INC, "./blib") }

     o   This document assumes that the executable named "perl"
         is Perl version 5.  Some systems may have installed Perl
         version 5 as "perl5".

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

     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

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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