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do
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done
dprofpp
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elif
else
enc
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endif
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od
onintr
open
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pack
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perlxstut
 
PERLXSTUT(1)	       Perl Programmers Reference Guide 	  PERLXSTUT(1)



NAME
       perlXStut - Tutorial for writing XSUBs

DESCRIPTION
       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 com-
       plex, 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.

SPECIAL NOTES
       make

       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 CPAN.pm 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 <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 mech-
       anism will check the system and build a dynamically-loadable library if
       possible, or else a static library and then, optionally, a new stati-
       cally-linked executable with that static library linked in.

       Should you wish to build a statically-linked executable on a system
       which can dynamically load libraries, you may, in all the following
       examples, where the command ""make"" with no arguments is executed, run
       the command ""make perl"" instead.

       If you have generated such a statically-linked executable by choice,
       then instead of saying ""make test"", you should say ""make
       test_static"".  On systems that cannot build dynamically-loadable
       libraries at all, simply saying ""make test"" is sufficient.

TUTORIAL
       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, pos-
       sibly under ext/ if that directory exists in the current working direc-
       tory.  Several files will be created in the Mytest dir, including MANI-
       FEST, Makefile.PL, Mytest.pm, Mytest.xs, test.pl, 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/MakeMaker.pm for details of how to influence
	       # the contents of the Makefile that is written.
	       WriteMakefile(
		   NAME 	=> 'Mytest',
		   VERSION_FROM => 'Mytest.pm', # finds $VERSION
		   LIBS 	=> [''],   # e.g., '-lm'
		   DEFINE	=> '',	   # e.g., '-DHAVE_SOMETHING'
		   INC		=> '',	   # e.g., '-I/usr/include/other'
	       );

       The file Mytest.pm 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.

	       1;
	       __END__
	       # Below is the stub of documentation for your module. You better edit it!

       The rest of the .pm file contains sample code for providing documenta-
       tion 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 file:

	       void
	       hello()
		   CODE:
		       printf("Hello, world!\n");

       It is okay for the lines starting at the "CODE:" line to not be
       indented.  However, for readability purposes, it is suggested that you
       indent CODE: one level and the lines following one more level.

       Now we'll run ""perl Makefile.PL"".  This will create a real Makefile,
       which make needs.  Its output looks something like:

	       % perl Makefile.PL
	       Checking if your kit is complete...
	       Looks good
	       Writing Makefile for Mytest
	       %

       Now, running make will produce output that looks something like this
       (some long lines have been shortened for clarity and some extraneous
       lines have been deleted):

	       % make
	       umask 0 && cp Mytest.pm ./blib/Mytest.pm
	       perl xsubpp -typemap typemap Mytest.xs >Mytest.tc && mv Mytest.tc Mytest.c
	       Please specify prototyping behavior for Mytest.xs (see perlxs manual)
	       cc -c Mytest.c
	       Running Mkbootstrap for Mytest ()
	       chmod 644 Mytest.bs
	       LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl -b Mytest.o
	       chmod 755 ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl
	       cp Mytest.bs ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
	       chmod 644 ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
	       Manifying ./blib/man3/Mytest.3
	       %

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

       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. C:/DevStudio/VC/lib).

       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;

	       Mytest::hello();

       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
	       is_even(input)
		       int     input
		   CODE:
		       RETVAL = (input % 2 == 0);
		   OUTPUT:
		       RETVAL

       There does not need to be whitespace at the start of the ""int input""
       line, but it is useful for improving readability.  Placing a semi-colon
       at the end of that line is also optional.  Any amount and kind of
       whitespace may be placed between the ""int"" and ""input"".

       Now re-run make to rebuild our new shared library.

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

       In order to test that our extension works, we now need to look at the
       file test.pl.  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) test.pl
	       1..4
	       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 routines.

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

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

       Generating the Makefile and running "make" created a directory called
       blib (which stands for "build library") in the current working direc-
       tory.  This directory will contain the shared library that we will
       build.  Once we have tested it, we can install it into its final loca-
       tion.

       Invoking the test script via ""make test"" did something very impor-
       tant.  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 can-
       not be found, Perl dies with a fatal error.  The default search path is
       contained in the @INC array.

       In our case, Mytest.pm tells perl that it will need the Exporter and
       Dynamic Loader extensions.  It then sets the @ISA and @EXPORT arrays
       and the $VERSION scalar; finally it tells perl to bootstrap the module.
       Perl will call its dynamic loader routine (if there is one) and load
       the shared library.

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

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

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

       See perlmod for more information.

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

       Add the following to the end of Mytest.xs:

	       void
	       round(arg)
		       double  arg
		   CODE:
		       if (arg > 0.0) {
			       arg = floor(arg + 0.5);
		       } else if (arg < 0.0) {
			       arg = ceil(arg - 0.5);
		       } else {
			       arg = 0.0;
		       }
		   OUTPUT:
		       arg

       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 test.pl:

	       $i = -1.5; &Mytest::round($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
       test.pl:

	       &Mytest::round(3);

       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 speci-
	   fied 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 func-
	   tion'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 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 cre-
       ated 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 parts.

       The first section maps various C data types to a name, which corre-
       sponds somewhat with the various Perl types.  The second section con-
       tains 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 exten-
       sion.  The file name is Mytest.c:

	       XS(XS_Mytest_round)
	       {
		   dXSARGS;
		   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 */
		   }
		   XSRETURN(1);
	       }

       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_DOU-
       BLE.  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, 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.

       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 direc-
       tory 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 
	       #include "./mylib.h"

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

	       sub MY::top_targets {
		       '
	       all :: static

	       pure_all :: static

	       static ::       libmylib$(LIB_EXT)

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

	       ';
	       }

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

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

	       % h2xs -O -n Mytest2 ./Mytest2/mylib/mylib.h

       This will print out a warning about overwriting Mytest2, but that's
       okay.  Our files are stored in Mytest2/mylib, and will be untouched.

       The normal Makefile.PL that h2xs generates doesn't know about the mylib
       directory.  We need to tell it that there is a subdirectory and that we
       will be generating a library in it.  Let's add the argument MYEXTLIB to
       the WriteMakefile call so that it looks like this:

	       WriteMakefile(
		   'NAME'      => 'Mytest2',
		   'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
		   'LIBS'      => [''],   # e.g., '-lm'
		   'DEFINE'    => '',	  # e.g., '-DHAVE_SOMETHING'
		   'INC'       => '',	  # e.g., '-I/usr/include/other'
		   'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
	       );

       and then at the end add a subroutine (which will override the pre-
       existing subroutine).  Remember to use a tab character to indent the
       line beginning with "cd"!

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

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

	       mylib/Makefile.PL
	       mylib/mylib.c
	       mylib/mylib.h

       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:

	       double
	       foo(a,b,c)
		       int	       a
		       long	       b
		       const char *    c
		   OUTPUT:
		       RETVAL

       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 test.pl 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 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 post-
	   amble 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 "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 documenta-
       tion skipped over (see perlpod) it goes into the generated output C
       file as is.

       Anything after this line is the description of XSUB functions.  These
       descriptions are translated by xsubpp into C code which implements
       these functions using Perl calling conventions, and which makes these
       functions visible from Perl interpreter.

       Pay a special attention to the function "constant".  This name appears
       twice in the generated .xs file: once in the first part, as a static C
       function, then another time in the second part, when an XSUB interface
       to this static C function is defined.

       This is quite typical for .xs files: usually the .xs file provides an
       interface to an existing C function.  Then this C function is defined
       somewhere (either in an external library, or in the first part of .xs
       file), and a Perl interface to this function (i.e. "Perl glue") is
       described in the second part of .xs file.  The situation in "EXAMPLE
       1", "EXAMPLE 2", and "EXAMPLE 3", when all the work is done inside the
       "Perl glue", is somewhat of an exception rather than the rule.

       Getting the fat out of XSUBs

       In "EXAMPLE 4" the second part of .xs file contained the following
       description of an XSUB:

	       double
	       foo(a,b,c)
		       int	       a
		       long	       b
		       const char *    c
		   OUTPUT:
		       RETVAL

       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 CODE section to this XSUB:

	       double
	       foo(a,b,c)
		       int	       a
		       long	       b
		       const char *    c
		   CODE:
		       RETVAL = foo(a,b,c);
		   OUTPUT:
		       RETVAL

       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:

	       double
	       foo(a,b,c)
		       int	       a
		       long	       b
		       const char *    c

       Can we do the same with an XSUB

	       int
	       is_even(input)
		       int     input
		   CODE:
		       RETVAL = (input % 2 == 0);
		   OUTPUT:
		       RETVAL

       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

	       int
	       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
	       is_even(input)
		       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 clut-
       ter, 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 trans-
       lated 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 argu-
       ments 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 deref-
       erence "cp" and manipulate *cp only (using the return value as, say, a
       success indicator).  From Perl one would use these functions in a com-
       pletely different manner.

       One conveys this info to xsubpp by replacing "*" before the argument by
       "&".  "&" means that the argument should be passed to a library func-
       tion by its address.  The above two function may be XSUB-ified as

	       int
	       string_length(s)
		       char *  s

	       int
	       upper_case_char(cp)
		       char    &cp

       For example, consider:

	       int
	       foo(a,b)
		       char    &a
		       char *  b

       The first Perl argument to this function would be treated as a char and
       assigned to the variable a, and its address would be passed into the
       function foo.  The second Perl argument would be treated as a string
       pointer and assigned to the variable b.	The value of b would be passed
       into the function foo.  The actual call to the function foo that xsubpp
       generates would look like this:

	       foo(&a, b);

       xsubpp will parse the following function argument lists identically:

	       char    &a
	       char&a
	       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 sec-
       ond 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 argu-
       ment) is changed.  You can verify this by looking at the C code gener-
       ated for Example 3.  The code for the round() XSUB routine contains
       lines that look like this:

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

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

       XSUBs are also allowed to return lists, not just scalars.  This must be
       done by manipulating stack values ST(0), ST(1), etc, in a subtly dif-
       ferent 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 peo-
       ple 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 "work-
       horse" 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 pro-
       tecting 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 sim-
       pler or easier to understand.  These routines should live in the .pm
       file.  Whether they are automatically loaded when the extension itself
       is loaded or only loaded when called depends on where in the .pm file
       the subroutine definition is placed.  You can also consult AutoLoader
       for an alternate way to store and load your extra subroutines.

       Documenting your Extension

       There is absolutely no excuse for not documenting your extension.  Doc-
       umentation belongs in the .pm file.  This file will be fed to pod2man,
       and the embedded documentation will be converted to the manpage format,
       then placed in the blib directory.  It will be copied to Perl's manpage
       directory when the extension is installed.

       You may intersperse documentation and Perl code within the .pm file.
       In fact, if you want to use method autoloading, you must do this, as
       the comment inside the .pm file explains.

       See perlpod for more information about the pod format.

       Installing your Extension

       Once your extension is complete and passes all its tests, installing it
       is quite simple: you simply run "make install".	You will either need
       to have write permission into the directories where Perl is installed,
       or ask your system administrator to run the make for you.

       Alternately, you can specify the exact directory to place the exten-
       sion'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 dis-
       tribute 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 array.

       This extension is very Unix-oriented (struct statfs and the statfs sys-
       tem 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 exam-
       ple.  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:

	       void
	       statfs(path)
		       char *  path
		   INIT:
		       int i;
		       struct statfs buf;

		   PPCODE:
		       i = statfs(path, &buf);
		       if (i == 0) {
			       XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[0])));
			       XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[1])));
		       } else {
			       XPUSHs(sv_2mortal(newSVnv(errno)));
		       }

       You'll also need to add the following code to the top of the .xs file,
       just after the include of "XSUB.h":

	       #include 

       Also add the following code segment to test.pl while incrementing the
       "1..9" string in the BEGIN block to "1..11":

	       @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 returned.

	   We do this by using the PPCODE: directive, rather than the CODE:
	   directive.  This tells xsubpp that we will be managing the return
	   values that will be put on the argument stack by ourselves.

       o   When we want to place values to be returned to the caller onto the
	   stack, we use the series of macros that begin with "XPUSH".	There
	   are five different versions, for placing integers, unsigned inte-
	   gers, doubles, strings, and Perl scalars on the stack.  In our
	   example, we placed a Perl scalar onto the stack.  (In fact this is
	   the only macro which can be used to return multiple values.)

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

       o   The values pushed onto the return stack of the XSUB are actually
	   mortal SV's.  They are made mortal so that once the values are
	   copied by the calling program, the SV's that held the returned val-
	   ues 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 typi-
	   cally hurt anything but memory consumption).

	   Similarly, in the failure branch we could use "PUSHs" without
	   extending the stack: the Perl function reference comes to an XSUB
	   on the stack, thus the stack is always large enough to take one
	   return value.

       EXAMPLE 6

       In this example, we will accept a reference to an array as an input
       parameter, and return a reference to an array of hashes.  This will
       demonstrate manipulation of complex Perl data types from an XSUB.

       This extension is somewhat contrived.  It is based on the code in the
       previous example.  It calls the statfs function multiple times, accept-
       ing a reference to an array of filenames as input, and returning a ref-
       erence 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 *
	       multi_statfs(paths)
		       SV * paths
		   INIT:
		       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))
		       {
			   XSRETURN_UNDEF;
		       }
		       results = (AV *)sv_2mortal((SV *)newAV());
		   CODE:
		       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));
			       continue;
			   }

			   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));
		       }
		       RETVAL = newRV((SV *)results);
		   OUTPUT:
		       RETVAL

       And add the following code to test.pl, while incrementing the "1..11"
       string in the BEGIN block to "1..13":

	       $results = Mytest::multi_statfs([ '/', '/blech' ]);
	       print ((ref $results->[0]) ? "ok 12\n" : "not ok 12\n");
	       print ((! ref $results->[1]) ? "ok 13\n" : "not ok 13\n");

       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 met.

       o   We manipulate several arrays in this XSUB.  Note that an array is
	   represented internally by an AV* pointer.  The functions and macros
	   for manipulating arrays are similar to the functions in Perl:
	   "av_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 structure.

	   As with the return stack, it would be possible (and a small perfor-
	   mance 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 oth-
	   ers).  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 type-
       globs and stuff. Well, it isn't.

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

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

	       #include 

	       int
	       fputs(s, stream)
		       char *	       s
		       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 back-
       wards 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 *
	   or
	       typedef PerlIO *        OutputStream;

       And this is the XS code:

	       int
	       perlioputs(s, stream)
		       char *	       s
		       OutputStream    stream
	       CODE:
		       RETVAL = PerlIO_puts(stream, s);
	       OUTPUT:
		       RETVAL

       We have to use a "CODE" section because "PerlIO_puts()" has the argu-
       ments 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 *":

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

       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_export-
       FILE()".

       This applies to the perlio system only. For versions before 5.7, "Per-
       lIO_exportFILE()" is equivalent to "PerlIO_findFILE()".

       Troubleshooting these Examples

       As mentioned at the top of this document, if you are having problems
       with these example extensions, you might see if any of these help you.

       o   In versions of 5.002 prior to the gamma version, the test script in
	   Example 1 will not function properly.  You need to change the "use
	   lib" line to read:

		   use lib './blib';

       o   In versions of 5.002 prior to version 5.002b1h, the test.pl 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 ver-
	   sion 5.  Some systems may have installed Perl version 5 as "perl5".

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

Author
       Jeff Okamoto <okamoto@corp.hp.com>

       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.

       Last Changed

       2002/05/08



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