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perlxstut



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 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.


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 sub­
       stitute 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 ver­
       sions 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 ver­
       sions.

       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 proba­
       bly put this information into the README file, but nowa­
       days installation of extensions may be performed automati­
       cally, 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 }

       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 exe­
       cutable 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, possibly under ext/ if that directory exists in
       the current working directory.  Several files will be cre­
       ated in the Mytest dir, including MANIFEST, 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'
               );

               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 provid­
       ing 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
       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 some­
       thing 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
               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 behav­
       ior" - 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 Perl­
       CRT.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

       file 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 exten­
       sions.  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 exten­
       sion.  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
       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 call­
       ing 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 col­
       lection 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 exten­
       sion.  If you have many test cases, you might want to copy
       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?

       ·   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.

       nating 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 suf­
       fix 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 ($PERL­
       LIB/ExtUtils/typemap).  This file is split into three
       parts.

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

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

       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 exam­
       ple).  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 directory Mytest.  In the Mytest2 directory, create
       another directory called mylib, and cd into that direc­
       tory.

       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"

               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(

               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 begin­
       ning 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 follow­
       ing 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 WriteMake­
       file 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 char­
       acter 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
       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 direc­
       tory 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.

       ·   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.

       ·   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
           these would not have been processed by h2xs.  There is
           no good solution to this right now.

       ·   We've also told Perl about the library that we built
           in the mylib subdirectory.  That required only the
           addition of the "MYEXTLIB" variable to the WriteMake­
           file call and the replacement of the postamble subrou­
           tine 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 postam­
           ble subroutine to insert our own code.  This code sim­
           ply 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 documentation skipped over
       (see perlpod) it goes into the generated output C file as
       is.

       Anything after this line is the description of XSUB func­
       tions.  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 exter­
       nal 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.

       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 func­
       tion 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 gener­
       ated 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 auto­
       generate 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 func­
       tion "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)
               {

       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 ver­
       sion 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 argu­
       ment of this C function would be a pointer to the data.
       Thus two C functions with declarations

               int string_length(char *s);
               int upper_case_char(char *cp);

       may have completely different semantics: the first one may
       inspect an array of chars pointed by s, and the second one
       may immediately dereference "cp" and manipulate *cp only
       (using the return value as, say, a success indicator).
       From Perl one would use these functions in a completely
       different manner.

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

               int
               string_length(s)
                       char *  s

       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 what­
       ever 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 actu­
       ally a macro that points to the n'th argument on the argu­
       ment 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:

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

       While experts may argue about these idioms, a novice to
       Perl guts may prefer a way which is as little Perl-guts-
       specific as possible, meaning automatic conversion and
       automatic call generation, as in "Getting the fat out of
       XSUBs".  This approach has the additional benefit of 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 simpler or easier to understand.  These
       routines should live in the .pm file.  Whether they are
       automatically loaded when the extension itself is loaded
       or only loaded when called depends on where in the .pm
       file the subroutine definition is placed.  You can also
       consult AutoLoader for an alternate way to store and load
       your extra subroutines.

       Documenting your Extension

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

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

       single value.  We'll now create an extension that returns
       an array.

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

       Return to the Mytest directory and add the following code
       to the end of 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 <sys/vfs.h>

       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";

       ·   This routine also returns a different number of argu­
           ments 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.

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

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

       ·   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 pro­
           gram, 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.

       ·   If we were interested in performance, not in code com­
           pactness, 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 con­
           sumption).

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

       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");

       ·   When dealing with references, it is important to han­
           dle 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 ref­
           erenced 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.

       ·   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 neces­
           sary.

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

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

                   av_extend(results, numpaths);

       ·   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.

       ·   To create a reference, we use the "newRV" function.
           Note that you can cast an AV* or an HV* to type SV* in
           this case (and many others).  This allows you to take
           references to arrays, hashes and scalars with the same
           function.  Conversely, the "SvRV" function always

       Setting $!

       EXAMPLE 9 Passing open files to XSes

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

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

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

               #include <stdio.h>

               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 lay­
       ers. 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 "Output­
       Stream" (T_OUT). A bare "PerlIO *" is considered a
       T_INOUT. If it matters in your code (see below for why it
       might) #define or typedef one of the specific names and
       use that as the argument or result type in your XS file.

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

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

       For streams being handed to perl a new file handle is cre­
       ated (i.e. a reference to a new glob) and associated with
       the PerlIO * provided. If the read/write state of the Per­
       lIO * 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".
               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 arguments reversed compared to "fputs()", and we
       want to keep the arguments the same.

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

               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 "Per­
       lIO_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. "Per­
       lIO()"_findFILE will retrieve the stdio layer once it has
       been generated by "PerlIO_exportFILE()".

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

       Troubleshooting these Examples

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

       ·   In versions of 5.002 prior to the gamma version, the
           above line, you will need to use the following line:

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

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


See also

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


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

perl v5.8.1                 2003-09-02               PERLXSTUT(1)

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