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perlxstut ()
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         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 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 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'"
         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!
         EXAMPLE 1
         Our first extension will be very simple.  When we call the
         routine in the extension, it will print out a well-known
         message and return.
         Run "`h2xs -A -n Mytest'".  This creates a directory named
         Mytest, possibly under ext/ if that directory exists in the
         current working directory.  Several files will be created in
         the Mytest dir, including MANIFEST, Makefile.PL,,
         Mytest.xs,, and Changes.
         The MANIFEST file contains the names of all the files just
         created in the Mytest directory.
         The file Makefile.PL should look something like this:
                 use ExtUtils::MakeMaker;
                 # See lib/ExtUtils/ for details of how to influence
                 # the contents of the Makefile that is written.
                     NAME         => 'Mytest',
                     VERSION_FROM => '', # finds $VERSION
                     LIBS         => [''],   # e.g., '-lm'
                     DEFINE       => '',     # e.g., '-DHAVE_SOMETHING'
                     INC          => '',     # e.g., '-I/usr/include/other'
         The file should start with something like this:
                 package Mytest;
                 use strict;
                 use warnings;
                 require Exporter;
                 require DynaLoader;
                 our @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.
                 our @EXPORT = qw(
                 our $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!
         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"
                 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
                 umask 0 && cp ./blib/
                 perl xsubpp -typemap typemap Mytest.xs > && mv Mytest.c
                 Please specify prototyping behavior for Mytest.xs (see perlxs manual)
                 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/
                 Manifying ./blib/man3/Mytest.3
         You can safely ignore the line about "prototyping behavior".
         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!
         EXAMPLE 2
         Now let's add to our extension a subroutine that will take a
         single numeric argument as input and return 0 if the number
         is even or 1 if the number is odd.
         Add the following to the end of Mytest.xs:
                         int     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.  Placing a semi-colon at the end of that line
         is also optional.  Any amount and kind of white space 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  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/perl5.004/bin/perl (lots of -I arguments)
                 ok 1
                 ok 2
                 ok 3
                 ok 4
         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
         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 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 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.
         Writing good test scripts
         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" in the extension's
         directory and append the suffix ".t" to the names of your
         test files.  When you run "`make test'", all of these test
         files will be executed.
         EXAMPLE 3
         Our third extension will take one argument as its input,
         round off that value, and set the argument to the rounded
         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 "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.
         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
         white space, and may have an optional terminating semicolon.
         The list of output parameters occurs at the very end of the
         function, just before 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).  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)
                         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 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
         the perlguts manpage; 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,
         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 accomodate calling
         pre-existing C routines, which often do modify their input
         parameters, this behavior is tolerated.
         EXAMPLE 4
         In this example, we'll now begin to write XSUBs that will
         interact with pre-defined C libraries.  To begin with, we
         will build a small library of our own, then let h2xs write
         our .pm and .xs files for us.
         Create a new directory called Mytest2 at the same level as
         the directory Mytest.  In the Mytest2 directory, create
         another directory called mylib, and cd into that directory.
         Here we'll create some files that will generate a test
         library.  These will include a C source file and a header
         file.  We'll also create a Makefile.PL in this directory.
         Then we'll make sure that running make at the Mytest2 level
         will automatically run this Makefile.PL file and the
         resulting Makefile.
         In the mylib directory, create a file mylib.h that looks
         like this:
                 #define TESTVAL 4
                 extern double   foo(int, long, const char*);
         Also create a file mylib.c that looks like this:
                 #include <stdlib.h>
                 #include "./mylib.h"
                 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;
                 $Verbose = 1;
                     NAME   => 'Mytest2::mylib',
                     SKIP   => [qw(all static static_lib dynamic dynamic_lib)],
                     clean  => {'FILES' => 'libmylib$(LIBEEXT)'},
                 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"!
                 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 script and change the BEGIN block to
         print "1..4", and add the following lines to the end of the
                 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 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.
         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.
         Anatomy of .xs file
         The .xs file of the section on "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, 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, the 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 the
         section on "EXAMPLE 1", the section on "EXAMPLE 2", and the
         section on "EXAMPLE 3", when all the work is done inside the
         "Perl glue", is somewhat of an exception rather than the
         Getting the fat out of XSUBs
         In the section on "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 the section on "EXAMPLE 1", the
         section on "EXAMPLE 2" and the section on "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 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 the section on "EXAMPLE 2"?  To do this, one needs to
         define a C function `int is_even(int input)'.  As we saw in
         the Anatomy of .xs file entry elsewhere in this document, 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
         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
                 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
         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 the perlxs manpage for
         XSUBs are also allowed to avoid automatic conversion of Perl
         function arguments to C function arguments.  See the perlxs
         manpage 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 the section on "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 the section on "Getting the fat out of
         XSUBs".  This approach has the additional benefit of
         protecting the XSUB writer from future changes to the Perl
         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 the
         AutoLoader manpage 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 man page 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
         Installing your Extension
         Once your extension is complete and passes all its tests,
         installing it is quite simple: you simply run "make
         install".  You will either need to have write permission
         into the directories where Perl is installed, or ask your
         system administrator to run the make for you.
         Alternately, you can specify the exact directory to place
         the extension's files by placing a
         "PREFIX=/destination/directory" after the make install.  (or
         in between the make and install if you have a brain-dead
         version of make).  This can be very useful if you are
         building an extension that will eventually be distributed to
         multiple systems.  You can then just archive the files in
         the destination directory and distribute them to your
         destination systems.
         EXAMPLE 5
         In this example, we'll do some more work with the argument
         stack.  The previous examples have all returned only a
         single value.  We'll now create an extension that returns an
         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 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 while
         incrementing the "1..9" string in the BEGIN block to
                 @a = &Mytest::statfs("/blech");
                 print ((scalar(@a) == 1 && $a[0] == 2) ? "ok 10\n" : "not ok 10\n");
                 @a = &Mytest::statfs("/");
                 print scalar(@a) == 9 ? "ok 11\n" : "not ok 11\n";
         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
             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, 9);
             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.
         EXAMPLE 6 (Coming Soon)
         Passing in and returning references to arrays and/or hashes
         EXAMPLE 7 (Coming Soon)
         XPUSH args AND set RETVAL AND assign return value to array
         EXAMPLE 8 (Coming Soon)
         Setting $!
         EXAMPLE 9 (Coming Soon)
         Getting fd's from filehandles
         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 "perl5".

    See also

         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.
         Last Changed

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