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

       perlXStut - Tutorial for XSUBs

       This tutorial will educate the reader on the steps
       involved in creating a Perl extension.  The reader is
       assumed to have access to the perlguts manpage and the
       perlxs manpage.

       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 to ease the reader slowly into
       building extensions.


       This tutorial tries hard to keep up with the latest
       development versions of Perl.  This often means that it is
       sometimes in advance of the latest released version of
       Perl, and that certain features described here might not
       work on earlier versions.  This section will keep track of
       when various features were added to Perl 5.

       o   In versions of Perl 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

                   use lib './blib';

       o   In versions of Perl 5.002 prior to version beta 3, the
           line in the .xs file about "PROTOTYPES: DISABLE" will
           cause a compiler error.  Simply remove that line from
           the file.

       o   In versions of Perl 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 "perl5".


       It is commonly thought that if a system does not have the
       capability to load a library dynamically, you cannot build
       XSUBs.  This is incorrect.  You can build them, but you
       must link the XSUB's 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.


       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 in the Mytest dir, including MANIFEST,
       Makefile.PL,, Mytest.xs,, and Changes.

       The MANIFEST file contains the names of all the files

       The file Makefile.PL should look something like this:

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

       The file should start with something like this:

               package Mytest;

               require Exporter;
               require DynaLoader;

               @ISA = qw(Exporter DynaLoader);
               # Items to export into callers namespace by default. Note: do not export
               # names by default without a very good reason. Use EXPORT_OK instead.
               # Do not simply export all your public functions/methods/constants.
               @EXPORT = qw(

               $VERSION = '0.01';

               bootstrap Mytest $VERSION;

               # Preloaded methods go here.

               # Autoload methods go after __END__, and are processed by the autosplit program.

               # Below is the stub of documentation for your module. You better edit it!

       And the Mytest.xs file should look something like this:

               #ifdef __cplusplus
               extern "C" {
               #include "EXTERN.h"
               #include "perl.h"
               #include "XSUB.h"
               #ifdef __cplusplus

               PROTOTYPES: DISABLE

               MODULE = Mytest         PACKAGE = Mytest

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

                       printf("Hello, world!\n");

       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 shortened for clarity):

               % make
               umask 0 && cp ./blib/
               perl xsubpp -typemap typemap Mytest.xs > && mv Mytest.c
               cc -c Mytest.c
               Running Mkbootstrap for Mytest ()
               chmod 644
               LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/ -b Mytest.o
               chmod 755 ./blib/PA-RISC1.1/auto/Mytest/
               cp ./blib/PA-RISC1.1/auto/Mytest/
               chmod 644 ./blib/PA-RISC1.1/auto/Mytest/

       Now, although there is already a template ready
       for us, for this example only, we'll create a special test
       script.  Create a file called hello that looks like this:

               #! /opt/perl5/bin/perl

               use ExtUtils::testlib;

               use Mytest;


       Now we run the script and we should see the following

               % perl hello
               Hello, world!


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

       Add the following to the end of Mytest.xs:

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

       There does not need to be white space at the start of the
       "int input" line, but it is useful for improving
       readability.  The semi-colon at the end of that line is
       also optional.

       Any white space may be between the "int" and "input".  It
       is also okay for the four lines starting at the "CODE:"
       line to not be indented.  However, for readability
       purposes, it is suggested that you indent them 8 spaces
       (or one normal tab stop).

       Now rerun 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.

       To test that our extension works, we now need to look at
       the file  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.  Change the print
       statement in the BEGIN block to print "1..4", and add the
       following code to the end of the file:

               print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok 2", "\n";
               print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok 3", "\n";
               print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok 4", "\n";

       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 /opt/perl5.002b2/bin/perl (lots of -I arguments)
               ok 1
               ok 2
               ok 3
               ok 4


       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 files <extension>.pm and <extension>.xs 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 and invoking the Makefile created a directory
       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 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" insures
       that you use your new extension, not the already-existing

       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 that are set in the .pm file 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.  The @EXPORT
       array tells Perl which of the extension's routines should
       be placed into the calling package's namespace.

       It's important to select what to export carefully.  Do NOT
       export method names and do NOT export anything else 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 then you can export any of the
       functions via another array, called @EXPORT_OK.

       See the perlmod manpage 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.


       The importance of writing good test scripts cannot be
       overemphasized.  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
       script runs and uses the correct version of your
       extension.  If you have many test cases, you might want to
       copy Perl's test style.  Create a directory named "t", and
       ensure all your test files end with the suffix ".t".  The
       Makefile will properly run all these test files.


       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 BEGIN
       block to print out "1..9" and add the following to

               $i = -1.5; &Mytest::round($i); print $i == -2.0 ? "ok 5" : "not ok 5", "\n";
               $i = -1.1; &Mytest::round($i); print $i == -1.0 ? "ok 6" : "not ok 6", "\n";
               $i = 0.0; &Mytest::round($i); print $i == 0.0 ? "ok 7" : "not ok 7", "\n";
               $i = 0.5; &Mytest::round($i); print $i == 1.0 ? "ok 8" : "not ok 8", "\n";
               $i = 1.2; &Mytest::round($i); print $i == 1.0 ? "ok 9" : "not ok 9", "\n";

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

       You might be wondering if you can round a constant.  To
       see what happens, 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!


       Two things are new here.  First, we've made some changes
       to Makefile.PL.  In this case, we've specified an extra
       library to link in, the math library libm.  We'll talk
       later about how to write XSUBs that can call every routine
       in a library.

       Second, the value of the function is being passed back not
       as the function's return value, but through the same
       variable that was passed into the function.


       You specify the parameters that will be passed into the
       XSUB just after you declare the function return value and
       name.  Each parameter line starts with optional white
       space, and may have an optional terminating semicolon.

       The list of output parameters occurs 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, the value we wanted returned was
       contained in the same variable we passed in, so we listed
       it (and not RETVAL) in the OUTPUT: section.


       The compiler xsubpp takes the XS code in the .xs file and
       converts 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 xsubpp compiler uses rules to convert from Perl's data
       types (scalar, array, etc.) to C's data types (int, char
       *, etc.).  These rules are stored in the typemap file
       ($PERLLIB/ExtUtils/typemap).  This file is split into
       three parts.

       The first part attempts to map various C data types to a
       coded flag, which has some correspondence with the various
       Perl types.  The second part contains C code which xsubpp
       uses for input parameters.  The third part contains C code
       which xsubpp uses for output parameters.  We'll talk more
       about the C code later.

       Let's now take a look at a portion of the .c file created
       for our extension.

                   if (items != 1)
                       croak("Usage: Mytest::round(arg)");
                       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 marked 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
       the perlguts manpage; we'll talk more later about what
       that "ST(0)" means in the section on the argument stack.


       In general, it's not a good idea to write extensions that
       modify their input parameters, as in Example 3.  However,
       to accommodate better calling pre-existing C routines,
       which often do modify their input parameters, this
       behavior is tolerated.  The next example will show how to
       do this.


       In this example, we'll now begin to write XSUBs that will
       interact with predefined 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

       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(a, b, c)
               int             a;
               long            b;
               const char *    c;
                       return (a + b + atof(c) + TESTVAL);

       And finally create a file Makefile.PL that looks like

               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

               static ::       libmylib$(LIB_EXT)

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


       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 following key-value pair to the
       WriteMakefile call:

               'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',

       and a new replacement subroutine too:

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

       (Note: Most makes will require that there be a tab
       character that indents the line cd mylib && $(MAKE)
       $(PASTHRU), similarly for the Makefile in the

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


       To keep our namespace nice and unpolluted, edit the .pm
       file and change the lines setting @EXPORT to @EXPORT_OK
       (there are two: one in the line beginning "use vars" and
       one setting the array itself).  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 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 see that it does cd into the mylib directory and run
       make in there as well.

       Now edit the script and change the BEGIN block to
       print "1..4", and add the following lines to the end of
       the script:

               print &Mytest2::foo(1, 2, "Hello, world!") == 7 ? "ok 2\n" : "not ok 2\n";
               print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 3\n" : "not ok 3\n";
               print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 ? "ok 4\n" : "not ok 4\n";

       (When dealing with floating-point comparisons, it is often
       useful not to check for equality, but rather the
       difference being below a certain epsilon factor, 0.01 in
       this case)

       Run "make test" and all should be well.


       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 declaration
           with the full path to the mylib.h header file.

       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
           available to the Perl script (in this case, by calling
           &main::TESTVAL).  There's also some XS code to allow
           calls to the constant routine.

       o   The .pm file has 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 going to
           be used by only 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 remove
           most or all of the items in the @EXPORT array.

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

       We've also told Perl about the library that we built in
       the mylib subdirectory.  That required the addition of
       only 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 specified simply that the library to
       be created here was a static archive (as opposed to a
       dynamically loadable library) and provided the commands to
       build it.


       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

       When you specify arguments in the .xs file, you are really
       passing three pieces of information for each one 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
       exact way in which the argument should be used in the call
       to the library function from this XSUB.  This would mean
       whether or not to place a "&" before the argument or not,
       meaning the argument expects to be passed the address of
       the specified data type.

       There is a difference between the two arguments in this
       hypothetical function:

                       char    &a
                       char *  b

       The first 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 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 identically 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 complete
       example 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.


       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.  The "ST" is
       actually a macro that points to the n'th argument on the
       argument stack.  ST(0) is thus 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


       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 loaded only when called depends on where in the .pm
       file the subroutine definition is placed.


       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 man page 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 the perlpod manpage for more information about the pod


       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.


       For more information, consult the perlguts manpage, the
       perlxs manpage, the perlmod manpage, and the perlpod


       Jeff Okamoto <>

       Reviewed and assisted by Dean Roehrich, Ilya Zakharevich,
       Andreas Koenig, and Tim Bunce.

       LLaasstt CChhaannggeedd


16/Jan/1999            perl 5.005, patch 03                     1