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   1  =head1 NAME
   3  perlXStut - Tutorial for writing XSUBs
   5  =head1 DESCRIPTION
   7  This tutorial will educate the reader on the steps involved in creating
   8  a Perl extension.  The reader is assumed to have access to L<perlguts>,
   9  L<perlapi> and L<perlxs>.
  11  This tutorial starts with very simple examples and becomes more complex,
  12  with each new example adding new features.  Certain concepts may not be
  13  completely explained until later in the tutorial in order to slowly ease
  14  the reader into building extensions.
  16  This tutorial was written from a Unix point of view.  Where I know them
  17  to be otherwise different for other platforms (e.g. Win32), I will list
  18  them.  If you find something that was missed, please let me know.
  20  =head1 SPECIAL NOTES
  22  =head2 make
  24  This tutorial assumes that the make program that Perl is configured to
  25  use is called C<make>.  Instead of running "make" in the examples that
  26  follow, you may have to substitute whatever make program Perl has been
  27  configured to use.  Running B<perl -V:make> should tell you what it is.
  29  =head2 Version caveat
  31  When writing a Perl extension for general consumption, one should expect that
  32  the extension will be used with versions of Perl different from the
  33  version available on your machine.  Since you are reading this document,
  34  the version of Perl on your machine is probably 5.005 or later, but the users
  35  of your extension may have more ancient versions.
  37  To understand what kinds of incompatibilities one may expect, and in the rare
  38  case that the version of Perl on your machine is older than this document,
  39  see the section on "Troubleshooting these Examples" for more information.
  41  If your extension uses some features of Perl which are not available on older
  42  releases of Perl, your users would appreciate an early meaningful warning.
  43  You would probably put this information into the F<README> file, but nowadays
  44  installation of extensions may be performed automatically, guided by F<CPAN.pm>
  45  module or other tools.
  47  In MakeMaker-based installations, F<Makefile.PL> provides the earliest
  48  opportunity to perform version checks.  One can put something like this
  49  in F<Makefile.PL> for this purpose:
  51      eval { require 5.007 }
  52          or die <<EOD;
  53      ############
  54      ### This module uses frobnication framework which is not available before
  55      ### version 5.007 of Perl.  Upgrade your Perl before installing Kara::Mba.
  56      ############
  57      EOD
  59  =head2 Dynamic Loading versus Static Loading
  61  It is commonly thought that if a system does not have the capability to
  62  dynamically load a library, you cannot build XSUBs.  This is incorrect.
  63  You I<can> build them, but you must link the XSUBs subroutines with the
  64  rest of Perl, creating a new executable.  This situation is similar to
  65  Perl 4.
  67  This tutorial can still be used on such a system.  The XSUB build mechanism
  68  will check the system and build a dynamically-loadable library if possible,
  69  or else a static library and then, optionally, a new statically-linked
  70  executable with that static library linked in.
  72  Should you wish to build a statically-linked executable on a system which
  73  can dynamically load libraries, you may, in all the following examples,
  74  where the command "C<make>" with no arguments is executed, run the command
  75  "C<make perl>" instead.
  77  If you have generated such a statically-linked executable by choice, then
  78  instead of saying "C<make test>", you should say "C<make test_static>".
  79  On systems that cannot build dynamically-loadable libraries at all, simply
  80  saying "C<make test>" is sufficient.
  82  =head1 TUTORIAL
  84  Now let's go on with the show!
  86  =head2 EXAMPLE 1
  88  Our first extension will be very simple.  When we call the routine in the
  89  extension, it will print out a well-known message and return.
  91  Run "C<h2xs -A -n Mytest>".  This creates a directory named Mytest,
  92  possibly under ext/ if that directory exists in the current working
  93  directory.  Several files will be created in the Mytest dir, including
  94  MANIFEST, Makefile.PL, Mytest.pm, Mytest.xs, Mytest.t, and Changes.
  96  The MANIFEST file contains the names of all the files just created in the
  97  Mytest directory.
  99  The file Makefile.PL should look something like this:
 101      use ExtUtils::MakeMaker;
 102      # See lib/ExtUtils/MakeMaker.pm for details of how to influence
 103      # the contents of the Makefile that is written.
 104      WriteMakefile(
 105      NAME         => 'Mytest',
 106      VERSION_FROM => 'Mytest.pm', # finds $VERSION
 107      LIBS         => [''],   # e.g., '-lm'
 108      DEFINE       => '',     # e.g., '-DHAVE_SOMETHING'
 109      INC          => '',     # e.g., '-I/usr/include/other'
 110      );
 112  The file Mytest.pm should start with something like this:
 114      package Mytest;
 116      use 5.008008;
 117      use strict;
 118      use warnings;
 120      require Exporter;
 122      our @ISA = qw(Exporter);
 123      our %EXPORT_TAGS = ( 'all' => [ qw(
 125      ) ] );
 127      our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
 129      our @EXPORT = qw(
 131      );
 133      our $VERSION = '0.01';
 135      require XSLoader;
 136      XSLoader::load('Mytest', $VERSION);
 138      # Preloaded methods go here.
 140      1;
 141      __END__
 142      # Below is the stub of documentation for your module. You better edit it!
 144  The rest of the .pm file contains sample code for providing documentation for
 145  the extension.
 147  Finally, the Mytest.xs file should look something like this:
 149      #include "EXTERN.h"
 150      #include "perl.h"
 151      #include "XSUB.h"
 153      #include "ppport.h"
 155      MODULE = Mytest        PACKAGE = Mytest
 157  Let's edit the .xs file by adding this to the end of the file:
 159      void
 160      hello()
 161      CODE:
 162          printf("Hello, world!\n");
 164  It is okay for the lines starting at the "CODE:" line to not be indented.
 165  However, for readability purposes, it is suggested that you indent CODE:
 166  one level and the lines following one more level.
 168  Now we'll run "C<perl Makefile.PL>".  This will create a real Makefile,
 169  which make needs.  Its output looks something like:
 171      % perl Makefile.PL
 172      Checking if your kit is complete...
 173      Looks good
 174      Writing Makefile for Mytest
 175      %
 177  Now, running make will produce output that looks something like this (some
 178  long lines have been shortened for clarity and some extraneous lines have
 179  been deleted):
 181      % make
 182      cp lib/Mytest.pm blib/lib/Mytest.pm
 183      perl xsubpp  -typemap typemap  Mytest.xs > Mytest.xsc && mv Mytest.xsc Mytest.c
 184      Please specify prototyping behavior for Mytest.xs (see perlxs manual)
 185      cc -c     Mytest.c
 186      Running Mkbootstrap for Mytest ()
 187      chmod 644 Mytest.bs
 188      rm -f blib/arch/auto/Mytest/Mytest.so
 189      cc  -shared -L/usr/local/lib Mytest.o  -o blib/arch/auto/Mytest/Mytest.so   \
 190                  \
 192      chmod 755 blib/arch/auto/Mytest/Mytest.so
 193      cp Mytest.bs blib/arch/auto/Mytest/Mytest.bs
 194      chmod 644 blib/arch/auto/Mytest/Mytest.bs
 195      Manifying blib/man3/Mytest.3pm
 196      %
 198  You can safely ignore the line about "prototyping behavior" - it is
 199  explained in the section "The PROTOTYPES: Keyword" in L<perlxs>.
 201  If you are on a Win32 system, and the build process fails with linker
 202  errors for functions in the C library, check if your Perl is configured
 203  to use PerlCRT (running B<perl -V:libc> should show you if this is the
 204  case).  If Perl is configured to use PerlCRT, you have to make sure
 205  PerlCRT.lib is copied to the same location that msvcrt.lib lives in,
 206  so that the compiler can find it on its own.  msvcrt.lib is usually
 207  found in the Visual C compiler's lib directory (e.g. C:/DevStudio/VC/lib).
 209  Perl has its own special way of easily writing test scripts, but for this
 210  example only, we'll create our own test script.  Create a file called hello
 211  that looks like this:
 213      #! /opt/perl5/bin/perl
 215      use ExtUtils::testlib;
 217      use Mytest;
 219      Mytest::hello();
 221  Now we make the script executable (C<chmod +x hello>), run the script
 222  and we should see the following output:
 224      % ./hello
 225      Hello, world!
 226      %
 228  =head2 EXAMPLE 2
 230  Now let's add to our extension a subroutine that will take a single numeric
 231  argument as input and return 0 if the number is even or 1 if the number
 232  is odd.
 234  Add the following to the end of Mytest.xs:
 236      int
 237      is_even(input)
 238          int input
 239      CODE:
 240          RETVAL = (input % 2 == 0);
 241      OUTPUT:
 242          RETVAL
 244  There does not need to be whitespace at the start of the "C<int input>"
 245  line, but it is useful for improving readability.  Placing a semi-colon at
 246  the end of that line is also optional.  Any amount and kind of whitespace
 247  may be placed between the "C<int>" and "C<input>".
 249  Now re-run make to rebuild our new shared library.
 251  Now perform the same steps as before, generating a Makefile from the
 252  Makefile.PL file, and running make.
 254  In order to test that our extension works, we now need to look at the
 255  file Mytest.t.  This file is set up to imitate the same kind of testing
 256  structure that Perl itself has.  Within the test script, you perform a
 257  number of tests to confirm the behavior of the extension, printing "ok"
 258  when the test is correct, "not ok" when it is not.
 260      use Test::More tests => 4;
 261      BEGIN { use_ok('Mytest') };
 263      #########################
 265      # Insert your test code below, the Test::More module is use()ed here so read
 266      # its man page ( perldoc Test::More ) for help writing this test script.
 268      is(&Mytest::is_even(0), 1);
 269      is(&Mytest::is_even(1), 0);
 270      is(&Mytest::is_even(2), 1);
 272  We will be calling the test script through the command "C<make test>".  You
 273  should see output that looks something like this:
 275      %make test
 276      PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
 277      t/Mytest....ok
 278      All tests successful.
 279      Files=1, Tests=4,  0 wallclock secs ( 0.03 cusr +  0.00 csys =  0.03 CPU)
 280      %
 282  =head2 What has gone on?
 284  The program h2xs is the starting point for creating extensions.  In later
 285  examples we'll see how we can use h2xs to read header files and generate
 286  templates to connect to C routines.
 288  h2xs creates a number of files in the extension directory.  The file
 289  Makefile.PL is a perl script which will generate a true Makefile to build
 290  the extension.  We'll take a closer look at it later.
 292  The .pm and .xs files contain the meat of the extension.  The .xs file holds
 293  the C routines that make up the extension.  The .pm file contains routines
 294  that tell Perl how to load your extension.
 296  Generating the Makefile and running C<make> created a directory called blib
 297  (which stands for "build library") in the current working directory.  This
 298  directory will contain the shared library that we will build.  Once we have
 299  tested it, we can install it into its final location.
 301  Invoking the test script via "C<make test>" did something very important.
 302  It invoked perl with all those C<-I> arguments so that it could find the
 303  various files that are part of the extension.  It is I<very> important that
 304  while you are still testing extensions that you use "C<make test>".  If you
 305  try to run the test script all by itself, you will get a fatal error.
 306  Another reason it is important to use "C<make test>" to run your test
 307  script is that if you are testing an upgrade to an already-existing version,
 308  using "C<make test>" ensures that you will test your new extension, not the
 309  already-existing version.
 311  When Perl sees a C<use extension;>, it searches for a file with the same name
 312  as the C<use>'d extension that has a .pm suffix.  If that file cannot be found,
 313  Perl dies with a fatal error.  The default search path is contained in the
 314  C<@INC> array.
 316  In our case, Mytest.pm tells perl that it will need the Exporter and Dynamic
 317  Loader extensions.  It then sets the C<@ISA> and C<@EXPORT> arrays and the
 318  C<$VERSION> scalar; finally it tells perl to bootstrap the module.  Perl
 319  will call its dynamic loader routine (if there is one) and load the shared
 320  library.
 322  The two arrays C<@ISA> and C<@EXPORT> are very important.  The C<@ISA>
 323  array contains a list of other packages in which to search for methods (or
 324  subroutines) that do not exist in the current package.  This is usually
 325  only important for object-oriented extensions (which we will talk about
 326  much later), and so usually doesn't need to be modified.
 328  The C<@EXPORT> array tells Perl which of the extension's variables and
 329  subroutines should be placed into the calling package's namespace.  Because
 330  you don't know if the user has already used your variable and subroutine
 331  names, it's vitally important to carefully select what to export.  Do I<not>
 332  export method or variable names I<by default> without a good reason.
 334  As a general rule, if the module is trying to be object-oriented then don't
 335  export anything.  If it's just a collection of functions and variables, then
 336  you can export them via another array, called C<@EXPORT_OK>.  This array
 337  does not automatically place its subroutine and variable names into the
 338  namespace unless the user specifically requests that this be done.
 340  See L<perlmod> for more information.
 342  The C<$VERSION> variable is used to ensure that the .pm file and the shared
 343  library are "in sync" with each other.  Any time you make changes to
 344  the .pm or .xs files, you should increment the value of this variable.
 346  =head2 Writing good test scripts
 348  The importance of writing good test scripts cannot be over-emphasized.  You
 349  should closely follow the "ok/not ok" style that Perl itself uses, so that
 350  it is very easy and unambiguous to determine the outcome of each test case.
 351  When you find and fix a bug, make sure you add a test case for it.
 353  By running "C<make test>", you ensure that your Mytest.t script runs and uses
 354  the correct version of your extension.  If you have many test cases,
 355  save your test files in the "t" directory and use the suffix ".t".
 356  When you run "C<make test>", all of these test files will be executed.
 358  =head2 EXAMPLE 3
 360  Our third extension will take one argument as its input, round off that
 361  value, and set the I<argument> to the rounded value.
 363  Add the following to the end of Mytest.xs:
 365      void
 366      round(arg)
 367          double  arg
 368          CODE:
 369          if (arg > 0.0) {
 370              arg = floor(arg + 0.5);
 371          } else if (arg < 0.0) {
 372              arg = ceil(arg - 0.5);
 373          } else {
 374              arg = 0.0;
 375          }
 376          OUTPUT:
 377          arg
 379  Edit the Makefile.PL file so that the corresponding line looks like this:
 381      'LIBS'      => ['-lm'],   # e.g., '-lm'
 383  Generate the Makefile and run make.  Change the test number in Mytest.t to
 384  "9" and add the following tests:
 386      $i = -1.5; &Mytest::round($i); is( $i, -2.0 );
 387      $i = -1.1; &Mytest::round($i); is( $i, -1.0 );
 388      $i = 0.0; &Mytest::round($i);  is( $i,  0.0 );
 389      $i = 0.5; &Mytest::round($i);  is( $i,  1.0 );
 390      $i = 1.2; &Mytest::round($i);  is( $i,  1.0 );
 392  Running "C<make test>" should now print out that all nine tests are okay.
 394  Notice that in these new test cases, the argument passed to round was a
 395  scalar variable.  You might be wondering if you can round a constant or
 396  literal.  To see what happens, temporarily add the following line to Mytest.t:
 398      &Mytest::round(3);
 400  Run "C<make test>" and notice that Perl dies with a fatal error.  Perl won't
 401  let you change the value of constants!
 403  =head2 What's new here?
 405  =over 4
 407  =item *
 409  We've made some changes to Makefile.PL.  In this case, we've specified an
 410  extra library to be linked into the extension's shared library, the math
 411  library libm in this case.  We'll talk later about how to write XSUBs that
 412  can call every routine in a library.
 414  =item *
 416  The value of the function is not being passed back as the function's return
 417  value, but by changing the value of the variable that was passed into the
 418  function.  You might have guessed that when you saw that the return value
 419  of round is of type "void".
 421  =back
 423  =head2 Input and Output Parameters
 425  You specify the parameters that will be passed into the XSUB on the line(s)
 426  after you declare the function's return value and name.  Each input parameter
 427  line starts with optional whitespace, and may have an optional terminating
 428  semicolon.
 430  The list of output parameters occurs at the very end of the function, just
 431  before after the OUTPUT: directive.  The use of RETVAL tells Perl that you
 432  wish to send this value back as the return value of the XSUB function.  In
 433  Example 3, we wanted the "return value" placed in the original variable
 434  which we passed in, so we listed it (and not RETVAL) in the OUTPUT: section.
 436  =head2 The XSUBPP Program
 438  The B<xsubpp> program takes the XS code in the .xs file and translates it into
 439  C code, placing it in a file whose suffix is .c.  The C code created makes
 440  heavy use of the C functions within Perl.
 442  =head2 The TYPEMAP file
 444  The B<xsubpp> program uses rules to convert from Perl's data types (scalar,
 445  array, etc.) to C's data types (int, char, etc.).  These rules are stored
 446  in the typemap file ($PERLLIB/ExtUtils/typemap).  This file is split into
 447  three parts.
 449  The first section maps various C data types to a name, which corresponds
 450  somewhat with the various Perl types.  The second section contains C code
 451  which B<xsubpp> uses to handle input parameters.  The third section contains
 452  C code which B<xsubpp> uses to handle output parameters.
 454  Let's take a look at a portion of the .c file created for our extension.
 455  The file name is Mytest.c:
 457      XS(XS_Mytest_round)
 458      {
 459          dXSARGS;
 460          if (items != 1)
 461          Perl_croak(aTHX_ "Usage: Mytest::round(arg)");
 462          PERL_UNUSED_VAR(cv); /* -W */
 463          {
 464          double  arg = (double)SvNV(ST(0));    /* XXXXX */
 465          if (arg > 0.0) {
 466              arg = floor(arg + 0.5);
 467          } else if (arg < 0.0) {
 468              arg = ceil(arg - 0.5);
 469          } else {
 470              arg = 0.0;
 471          }
 472          sv_setnv(ST(0), (double)arg);    /* XXXXX */
 473          SvSETMAGIC(ST(0));
 474          }
 475          XSRETURN_EMPTY;
 476      }
 478  Notice the two lines commented with "XXXXX".  If you check the first section
 479  of the typemap file, you'll see that doubles are of type T_DOUBLE.  In the
 480  INPUT section, an argument that is T_DOUBLE is assigned to the variable
 481  arg by calling the routine SvNV on something, then casting it to double,
 482  then assigned to the variable arg.  Similarly, in the OUTPUT section,
 483  once arg has its final value, it is passed to the sv_setnv function to
 484  be passed back to the calling subroutine.  These two functions are explained
 485  in L<perlguts>; we'll talk more later about what that "ST(0)" means in the
 486  section on the argument stack.
 488  =head2 Warning about Output Arguments
 490  In general, it's not a good idea to write extensions that modify their input
 491  parameters, as in Example 3.  Instead, you should probably return multiple
 492  values in an array and let the caller handle them (we'll do this in a later
 493  example).  However, in order to better accommodate calling pre-existing C
 494  routines, which often do modify their input parameters, this behavior is
 495  tolerated.
 497  =head2 EXAMPLE 4
 499  In this example, we'll now begin to write XSUBs that will interact with
 500  pre-defined C libraries.  To begin with, we will build a small library of
 501  our own, then let h2xs write our .pm and .xs files for us.
 503  Create a new directory called Mytest2 at the same level as the directory
 504  Mytest.  In the Mytest2 directory, create another directory called mylib,
 505  and cd into that directory.
 507  Here we'll create some files that will generate a test library.  These will
 508  include a C source file and a header file.  We'll also create a Makefile.PL
 509  in this directory.  Then we'll make sure that running make at the Mytest2
 510  level will automatically run this Makefile.PL file and the resulting Makefile.
 512  In the mylib directory, create a file mylib.h that looks like this:
 514      #define TESTVAL    4
 516      extern double    foo(int, long, const char*);
 518  Also create a file mylib.c that looks like this:
 520      #include <stdlib.h>
 521      #include "./mylib.h"
 523      double
 524      foo(int a, long b, const char *c)
 525      {
 526          return (a + b + atof(c) + TESTVAL);
 527      }
 529  And finally create a file Makefile.PL that looks like this:
 531      use ExtUtils::MakeMaker;
 532      $Verbose = 1;
 533      WriteMakefile(
 534          NAME   => 'Mytest2::mylib',
 535          SKIP   => [qw(all static static_lib dynamic dynamic_lib)],
 536          clean  => {'FILES' => 'libmylib$(LIB_EXT)'},
 537      );
 540      sub MY::top_targets {
 541          '
 542      all :: static
 544      pure_all :: static
 546      static ::       libmylib$(LIB_EXT)
 548      libmylib$(LIB_EXT): $(O_FILES)
 549          $(AR) cr libmylib$(LIB_EXT) $(O_FILES)
 550          $(RANLIB) libmylib$(LIB_EXT)
 552      ';
 553      }
 555  Make sure you use a tab and not spaces on the lines beginning with "$(AR)"
 556  and "$(RANLIB)".  Make will not function properly if you use spaces.
 557  It has also been reported that the "cr" argument to $(AR) is unnecessary
 558  on Win32 systems.
 560  We will now create the main top-level Mytest2 files.  Change to the directory
 561  above Mytest2 and run the following command:
 563      % h2xs -O -n Mytest2 ./Mytest2/mylib/mylib.h
 565  This will print out a warning about overwriting Mytest2, but that's okay.
 566  Our files are stored in Mytest2/mylib, and will be untouched.
 568  The normal Makefile.PL that h2xs generates doesn't know about the mylib
 569  directory.  We need to tell it that there is a subdirectory and that we
 570  will be generating a library in it.  Let's add the argument MYEXTLIB to
 571  the WriteMakefile call so that it looks like this:
 573      WriteMakefile(
 574          'NAME'      => 'Mytest2',
 575          'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
 576          'LIBS'      => [''],   # e.g., '-lm'
 577          'DEFINE'    => '',     # e.g., '-DHAVE_SOMETHING'
 578          'INC'       => '',     # e.g., '-I/usr/include/other'
 579          'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
 580      );
 582  and then at the end add a subroutine (which will override the pre-existing
 583  subroutine).  Remember to use a tab character to indent the line beginning
 584  with "cd"!
 586      sub MY::postamble {
 587      '
 588      $(MYEXTLIB): mylib/Makefile
 589          cd mylib && $(MAKE) $(PASSTHRU)
 590      ';
 591      }
 593  Let's also fix the MANIFEST file so that it accurately reflects the contents
 594  of our extension.  The single line that says "mylib" should be replaced by
 595  the following three lines:
 597      mylib/Makefile.PL
 598      mylib/mylib.c
 599      mylib/mylib.h
 601  To keep our namespace nice and unpolluted, edit the .pm file and change
 602  the variable C<@EXPORT> to C<@EXPORT_OK>.  Finally, in the
 603  .xs file, edit the #include line to read:
 605      #include "mylib/mylib.h"
 607  And also add the following function definition to the end of the .xs file:
 609      double
 610      foo(a,b,c)
 611          int             a
 612          long            b
 613          const char *    c
 614          OUTPUT:
 615          RETVAL
 617  Now we also need to create a typemap file because the default Perl doesn't
 618  currently support the const char * type.  Create a file called typemap in
 619  the Mytest2 directory and place the following in it:
 621      const char *    T_PV
 623  Now run perl on the top-level Makefile.PL.  Notice that it also created a
 624  Makefile in the mylib directory.  Run make and watch that it does cd into
 625  the mylib directory and run make in there as well.
 627  Now edit the Mytest2.t script and change the number of tests to "4",
 628  and add the following lines to the end of the script:
 630      is( &Mytest2::foo(1, 2, "Hello, world!"), 7 );
 631      is( &Mytest2::foo(1, 2, "0.0"), 7 );
 632      ok( abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 );
 634  (When dealing with floating-point comparisons, it is best to not check for
 635  equality, but rather that the difference between the expected and actual
 636  result is below a certain amount (called epsilon) which is 0.01 in this case)
 638  Run "C<make test>" and all should be well. There are some warnings on missing tests
 639  for the Mytest2::mylib extension, but you can ignore them.
 641  =head2 What has happened here?
 643  Unlike previous examples, we've now run h2xs on a real include file.  This
 644  has caused some extra goodies to appear in both the .pm and .xs files.
 646  =over 4
 648  =item *
 650  In the .xs file, there's now a #include directive with the absolute path to
 651  the mylib.h header file.  We changed this to a relative path so that we
 652  could move the extension directory if we wanted to.
 654  =item *
 656  There's now some new C code that's been added to the .xs file.  The purpose
 657  of the C<constant> routine is to make the values that are #define'd in the
 658  header file accessible by the Perl script (by calling either C<TESTVAL> or
 659  C<&Mytest2::TESTVAL>).  There's also some XS code to allow calls to the
 660  C<constant> routine.
 662  =item *
 664  The .pm file originally exported the name C<TESTVAL> in the C<@EXPORT> array.
 665  This could lead to name clashes.  A good rule of thumb is that if the #define
 666  is only going to be used by the C routines themselves, and not by the user,
 667  they should be removed from the C<@EXPORT> array.  Alternately, if you don't
 668  mind using the "fully qualified name" of a variable, you could move most
 669  or all of the items from the C<@EXPORT> array into the C<@EXPORT_OK> array.
 671  =item *
 673  If our include file had contained #include directives, these would not have
 674  been processed by h2xs.  There is no good solution to this right now.
 676  =item *
 678  We've also told Perl about the library that we built in the mylib
 679  subdirectory.  That required only the addition of the C<MYEXTLIB> variable
 680  to the WriteMakefile call and the replacement of the postamble subroutine
 681  to cd into the subdirectory and run make.  The Makefile.PL for the
 682  library is a bit more complicated, but not excessively so.  Again we
 683  replaced the postamble subroutine to insert our own code.  This code
 684  simply specified that the library to be created here was a static archive
 685  library (as opposed to a dynamically loadable library) and provided the
 686  commands to build it.
 688  =back
 690  =head2 Anatomy of .xs file
 692  The .xs file of L<"EXAMPLE 4"> contained some new elements.  To understand
 693  the meaning of these elements, pay attention to the line which reads
 695      MODULE = Mytest2        PACKAGE = Mytest2
 697  Anything before this line is plain C code which describes which headers
 698  to include, and defines some convenience functions.  No translations are
 699  performed on this part, apart from having embedded POD documentation
 700  skipped over (see L<perlpod>) it goes into the generated output C file as is.
 702  Anything after this line is the description of XSUB functions.
 703  These descriptions are translated by B<xsubpp> into C code which
 704  implements these functions using Perl calling conventions, and which
 705  makes these functions visible from Perl interpreter.
 707  Pay a special attention to the function C<constant>.  This name appears
 708  twice in the generated .xs file: once in the first part, as a static C
 709  function, then another time in the second part, when an XSUB interface to
 710  this static C function is defined.
 712  This is quite typical for .xs files: usually the .xs file provides
 713  an interface to an existing C function.  Then this C function is defined
 714  somewhere (either in an external library, or in the first part of .xs file),
 715  and a Perl interface to this function (i.e. "Perl glue") is described in the
 716  second part of .xs file.  The situation in L<"EXAMPLE 1">, L<"EXAMPLE 2">,
 717  and L<"EXAMPLE 3">, when all the work is done inside the "Perl glue", is
 718  somewhat of an exception rather than the rule.
 720  =head2 Getting the fat out of XSUBs
 722  In L<"EXAMPLE 4"> the second part of .xs file contained the following
 723  description of an XSUB:
 725      double
 726      foo(a,b,c)
 727          int             a
 728          long            b
 729          const char *    c
 730          OUTPUT:
 731          RETVAL
 733  Note that in contrast with L<"EXAMPLE 1">, L<"EXAMPLE 2"> and L<"EXAMPLE 3">,
 734  this description does not contain the actual I<code> for what is done
 735  is done during a call to Perl function foo().  To understand what is going
 736  on here, one can add a CODE section to this XSUB:
 738      double
 739      foo(a,b,c)
 740          int             a
 741          long            b
 742          const char *    c
 743          CODE:
 744          RETVAL = foo(a,b,c);
 745          OUTPUT:
 746          RETVAL
 748  However, these two XSUBs provide almost identical generated C code: B<xsubpp>
 749  compiler is smart enough to figure out the C<CODE:> section from the first
 750  two lines of the description of XSUB.  What about C<OUTPUT:> section?  In
 751  fact, that is absolutely the same!  The C<OUTPUT:> section can be removed
 752  as well, I<as far as C<CODE:> section or C<PPCODE:> section> is not
 753  specified: B<xsubpp> can see that it needs to generate a function call
 754  section, and will autogenerate the OUTPUT section too.  Thus one can
 755  shortcut the XSUB to become:
 757      double
 758      foo(a,b,c)
 759          int             a
 760          long            b
 761          const char *    c
 763  Can we do the same with an XSUB
 765      int
 766      is_even(input)
 767          int    input
 768          CODE:
 769          RETVAL = (input % 2 == 0);
 770          OUTPUT:
 771          RETVAL
 773  of L<"EXAMPLE 2">?  To do this, one needs to define a C function C<int
 774  is_even(int input)>.  As we saw in L<Anatomy of .xs file>, a proper place
 775  for this definition is in the first part of .xs file.  In fact a C function
 777      int
 778      is_even(int arg)
 779      {
 780          return (arg % 2 == 0);
 781      }
 783  is probably overkill for this.  Something as simple as a C<#define> will
 784  do too:
 786      #define is_even(arg)    ((arg) % 2 == 0)
 788  After having this in the first part of .xs file, the "Perl glue" part becomes
 789  as simple as
 791      int
 792      is_even(input)
 793          int    input
 795  This technique of separation of the glue part from the workhorse part has
 796  obvious tradeoffs: if you want to change a Perl interface, you need to
 797  change two places in your code.  However, it removes a lot of clutter,
 798  and makes the workhorse part independent from idiosyncrasies of Perl calling
 799  convention.  (In fact, there is nothing Perl-specific in the above description,
 800  a different version of B<xsubpp> might have translated this to TCL glue or
 801  Python glue as well.)
 803  =head2 More about XSUB arguments
 805  With the completion of Example 4, we now have an easy way to simulate some
 806  real-life libraries whose interfaces may not be the cleanest in the world.
 807  We shall now continue with a discussion of the arguments passed to the
 808  B<xsubpp> compiler.
 810  When you specify arguments to routines in the .xs file, you are really
 811  passing three pieces of information for each argument listed.  The first
 812  piece is the order of that argument relative to the others (first, second,
 813  etc).  The second is the type of argument, and consists of the type
 814  declaration of the argument (e.g., int, char*, etc).  The third piece is
 815  the calling convention for the argument in the call to the library function.
 817  While Perl passes arguments to functions by reference,
 818  C passes arguments by value; to implement a C function which modifies data
 819  of one of the "arguments", the actual argument of this C function would be
 820  a pointer to the data.  Thus two C functions with declarations
 822      int string_length(char *s);
 823      int upper_case_char(char *cp);
 825  may have completely different semantics: the first one may inspect an array
 826  of chars pointed by s, and the second one may immediately dereference C<cp>
 827  and manipulate C<*cp> only (using the return value as, say, a success
 828  indicator).  From Perl one would use these functions in
 829  a completely different manner.
 831  One conveys this info to B<xsubpp> by replacing C<*> before the
 832  argument by C<&>.  C<&> means that the argument should be passed to a library
 833  function by its address.  The above two function may be XSUB-ified as
 835      int
 836      string_length(s)
 837          char *    s
 839      int
 840      upper_case_char(cp)
 841          char    &cp
 843  For example, consider:
 845      int
 846      foo(a,b)
 847          char    &a
 848          char *    b
 850  The first Perl argument to this function would be treated as a char and assigned
 851  to the variable a, and its address would be passed into the function foo.
 852  The second Perl argument would be treated as a string pointer and assigned to the
 853  variable b.  The I<value> of b would be passed into the function foo.  The
 854  actual call to the function foo that B<xsubpp> generates would look like this:
 856      foo(&a, b);
 858  B<xsubpp> will parse the following function argument lists identically:
 860      char    &a
 861      char&a
 862      char    & a
 864  However, to help ease understanding, it is suggested that you place a "&"
 865  next to the variable name and away from the variable type), and place a
 866  "*" near the variable type, but away from the variable name (as in the
 867  call to foo above).  By doing so, it is easy to understand exactly what
 868  will be passed to the C function -- it will be whatever is in the "last
 869  column".
 871  You should take great pains to try to pass the function the type of variable
 872  it wants, when possible.  It will save you a lot of trouble in the long run.
 874  =head2 The Argument Stack
 876  If we look at any of the C code generated by any of the examples except
 877  example 1, you will notice a number of references to ST(n), where n is
 878  usually 0.  "ST" is actually a macro that points to the n'th argument
 879  on the argument stack.  ST(0) is thus the first argument on the stack and
 880  therefore the first argument passed to the XSUB, ST(1) is the second
 881  argument, and so on.
 883  When you list the arguments to the XSUB in the .xs file, that tells B<xsubpp>
 884  which argument corresponds to which of the argument stack (i.e., the first
 885  one listed is the first argument, and so on).  You invite disaster if you
 886  do not list them in the same order as the function expects them.
 888  The actual values on the argument stack are pointers to the values passed
 889  in.  When an argument is listed as being an OUTPUT value, its corresponding
 890  value on the stack (i.e., ST(0) if it was the first argument) is changed.
 891  You can verify this by looking at the C code generated for Example 3.
 892  The code for the round() XSUB routine contains lines that look like this:
 894      double  arg = (double)SvNV(ST(0));
 895      /* Round the contents of the variable arg */
 896      sv_setnv(ST(0), (double)arg);
 898  The arg variable is initially set by taking the value from ST(0), then is
 899  stored back into ST(0) at the end of the routine.
 901  XSUBs are also allowed to return lists, not just scalars.  This must be
 902  done by manipulating stack values ST(0), ST(1), etc, in a subtly
 903  different way.  See L<perlxs> for details.
 905  XSUBs are also allowed to avoid automatic conversion of Perl function arguments
 906  to C function arguments.  See L<perlxs> for details.  Some people prefer
 907  manual conversion by inspecting C<ST(i)> even in the cases when automatic
 908  conversion will do, arguing that this makes the logic of an XSUB call clearer.
 909  Compare with L<"Getting the fat out of XSUBs"> for a similar tradeoff of
 910  a complete separation of "Perl glue" and "workhorse" parts of an XSUB.
 912  While experts may argue about these idioms, a novice to Perl guts may
 913  prefer a way which is as little Perl-guts-specific as possible, meaning
 914  automatic conversion and automatic call generation, as in
 915  L<"Getting the fat out of XSUBs">.  This approach has the additional
 916  benefit of protecting the XSUB writer from future changes to the Perl API.
 918  =head2 Extending your Extension
 920  Sometimes you might want to provide some extra methods or subroutines
 921  to assist in making the interface between Perl and your extension simpler
 922  or easier to understand.  These routines should live in the .pm file.
 923  Whether they are automatically loaded when the extension itself is loaded
 924  or only loaded when called depends on where in the .pm file the subroutine
 925  definition is placed.  You can also consult L<AutoLoader> for an alternate
 926  way to store and load your extra subroutines.
 928  =head2 Documenting your Extension
 930  There is absolutely no excuse for not documenting your extension.
 931  Documentation belongs in the .pm file.  This file will be fed to pod2man,
 932  and the embedded documentation will be converted to the manpage format,
 933  then placed in the blib directory.  It will be copied to Perl's
 934  manpage directory when the extension is installed.
 936  You may intersperse documentation and Perl code within the .pm file.
 937  In fact, if you want to use method autoloading, you must do this,
 938  as the comment inside the .pm file explains.
 940  See L<perlpod> for more information about the pod format.
 942  =head2 Installing your Extension
 944  Once your extension is complete and passes all its tests, installing it
 945  is quite simple: you simply run "make install".  You will either need
 946  to have write permission into the directories where Perl is installed,
 947  or ask your system administrator to run the make for you.
 949  Alternately, you can specify the exact directory to place the extension's
 950  files by placing a "PREFIX=/destination/directory" after the make install.
 951  (or in between the make and install if you have a brain-dead version of make).
 952  This can be very useful if you are building an extension that will eventually
 953  be distributed to multiple systems.  You can then just archive the files in
 954  the destination directory and distribute them to your destination systems.
 956  =head2 EXAMPLE 5
 958  In this example, we'll do some more work with the argument stack.  The
 959  previous examples have all returned only a single value.  We'll now
 960  create an extension that returns an array.
 962  This extension is very Unix-oriented (struct statfs and the statfs system
 963  call).  If you are not running on a Unix system, you can substitute for
 964  statfs any other function that returns multiple values, you can hard-code
 965  values to be returned to the caller (although this will be a bit harder
 966  to test the error case), or you can simply not do this example.  If you
 967  change the XSUB, be sure to fix the test cases to match the changes.
 969  Return to the Mytest directory and add the following code to the end of
 970  Mytest.xs:
 972      void
 973      statfs(path)
 974          char *  path
 975          INIT:
 976          int i;
 977          struct statfs buf;
 979          PPCODE:
 980          i = statfs(path, &buf);
 981          if (i == 0) {
 982              XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
 983              XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
 984              XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
 985              XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
 986              XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
 987              XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
 988              XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
 989          } else {
 990              XPUSHs(sv_2mortal(newSVnv(errno)));
 991          }
 993  You'll also need to add the following code to the top of the .xs file, just
 994  after the include of "XSUB.h":
 996      #include <sys/vfs.h>
 998  Also add the following code segment to Mytest.t while incrementing the "9"
 999  tests to "11":
1001      @a = &Mytest::statfs("/blech");
1002      ok( scalar(@a) == 1 && $a[0] == 2 );
1003      @a = &Mytest::statfs("/");
1004      is( scalar(@a), 7 );
1006  =head2 New Things in this Example
1008  This example added quite a few new concepts.  We'll take them one at a time.
1010  =over 4
1012  =item *
1014  The INIT: directive contains code that will be placed immediately after
1015  the argument stack is decoded.  C does not allow variable declarations at
1016  arbitrary locations inside a function,
1017  so this is usually the best way to declare local variables needed by the XSUB.
1018  (Alternatively, one could put the whole C<PPCODE:> section into braces, and
1019  put these declarations on top.)
1021  =item *
1023  This routine also returns a different number of arguments depending on the
1024  success or failure of the call to statfs.  If there is an error, the error
1025  number is returned as a single-element array.  If the call is successful,
1026  then a 9-element array is returned.  Since only one argument is passed into
1027  this function, we need room on the stack to hold the 9 values which may be
1028  returned.
1030  We do this by using the PPCODE: directive, rather than the CODE: directive.
1031  This tells B<xsubpp> that we will be managing the return values that will be
1032  put on the argument stack by ourselves.
1034  =item *
1036  When we want to place values to be returned to the caller onto the stack,
1037  we use the series of macros that begin with "XPUSH".  There are five
1038  different versions, for placing integers, unsigned integers, doubles,
1039  strings, and Perl scalars on the stack.  In our example, we placed a
1040  Perl scalar onto the stack.  (In fact this is the only macro which
1041  can be used to return multiple values.)
1043  The XPUSH* macros will automatically extend the return stack to prevent
1044  it from being overrun.  You push values onto the stack in the order you
1045  want them seen by the calling program.
1047  =item *
1049  The values pushed onto the return stack of the XSUB are actually mortal SV's.
1050  They are made mortal so that once the values are copied by the calling
1051  program, the SV's that held the returned values can be deallocated.
1052  If they were not mortal, then they would continue to exist after the XSUB
1053  routine returned, but would not be accessible.  This is a memory leak.
1055  =item *
1057  If we were interested in performance, not in code compactness, in the success
1058  branch we would not use C<XPUSHs> macros, but C<PUSHs> macros, and would
1059  pre-extend the stack before pushing the return values:
1061      EXTEND(SP, 7);
1063  The tradeoff is that one needs to calculate the number of return values
1064  in advance (though overextending the stack will not typically hurt
1065  anything but memory consumption).
1067  Similarly, in the failure branch we could use C<PUSHs> I<without> extending
1068  the stack: the Perl function reference comes to an XSUB on the stack, thus
1069  the stack is I<always> large enough to take one return value.
1071  =back
1073  =head2 EXAMPLE 6
1075  In this example, we will accept a reference to an array as an input
1076  parameter, and return a reference to an array of hashes.  This will
1077  demonstrate manipulation of complex Perl data types from an XSUB.
1079  This extension is somewhat contrived.  It is based on the code in
1080  the previous example.  It calls the statfs function multiple times,
1081  accepting a reference to an array of filenames as input, and returning
1082  a reference to an array of hashes containing the data for each of the
1083  filesystems.
1085  Return to the Mytest directory and add the following code to the end of
1086  Mytest.xs:
1088      SV *
1089      multi_statfs(paths)
1090          SV * paths
1091      INIT:
1092          AV * results;
1093          I32 numpaths = 0;
1094          int i, n;
1095          struct statfs buf;
1097          if ((!SvROK(paths))
1098          || (SvTYPE(SvRV(paths)) != SVt_PVAV)
1099          || ((numpaths = av_len((AV *)SvRV(paths))) < 0))
1100          {
1101          XSRETURN_UNDEF;
1102          }
1103          results = (AV *)sv_2mortal((SV *)newAV());
1104      CODE:
1105          for (n = 0; n <= numpaths; n++) {
1106          HV * rh;
1107          STRLEN l;
1108          char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
1110          i = statfs(fn, &buf);
1111          if (i != 0) {
1112              av_push(results, newSVnv(errno));
1113              continue;
1114          }
1116          rh = (HV *)sv_2mortal((SV *)newHV());
1118          hv_store(rh, "f_bavail", 8, newSVnv(buf.f_bavail), 0);
1119          hv_store(rh, "f_bfree",  7, newSVnv(buf.f_bfree),  0);
1120          hv_store(rh, "f_blocks", 8, newSVnv(buf.f_blocks), 0);
1121          hv_store(rh, "f_bsize",  7, newSVnv(buf.f_bsize),  0);
1122          hv_store(rh, "f_ffree",  7, newSVnv(buf.f_ffree),  0);
1123          hv_store(rh, "f_files",  7, newSVnv(buf.f_files),  0);
1124          hv_store(rh, "f_type",   6, newSVnv(buf.f_type),   0);
1126          av_push(results, newRV((SV *)rh));
1127          }
1128          RETVAL = newRV((SV *)results);
1129      OUTPUT:
1130          RETVAL
1132  And add the following code to Mytest.t, while incrementing the "11"
1133  tests to "13":
1135      $results = Mytest::multi_statfs([ '/', '/blech' ]);
1136      ok( ref $results->[0]) );
1137      ok( ! ref $results->[1] );
1139  =head2 New Things in this Example
1141  There are a number of new concepts introduced here, described below:
1143  =over 4
1145  =item *
1147  This function does not use a typemap.  Instead, we declare it as accepting
1148  one SV* (scalar) parameter, and returning an SV* value, and we take care of
1149  populating these scalars within the code.  Because we are only returning
1150  one value, we don't need a C<PPCODE:> directive - instead, we use C<CODE:>
1151  and C<OUTPUT:> directives.
1153  =item *
1155  When dealing with references, it is important to handle them with caution.
1156  The C<INIT:> block first checks that
1157  C<SvROK> returns true, which indicates that paths is a valid reference.  It
1158  then verifies that the object referenced by paths is an array, using C<SvRV>
1159  to dereference paths, and C<SvTYPE> to discover its type.  As an added test,
1160  it checks that the array referenced by paths is non-empty, using the C<av_len>
1161  function (which returns -1 if the array is empty).  The XSRETURN_UNDEF macro
1162  is used to abort the XSUB and return the undefined value whenever all three of
1163  these conditions are not met.
1165  =item *
1167  We manipulate several arrays in this XSUB.  Note that an array is represented
1168  internally by an AV* pointer.  The functions and macros for manipulating
1169  arrays are similar to the functions in Perl: C<av_len> returns the highest
1170  index in an AV*, much like $#array; C<av_fetch> fetches a single scalar value
1171  from an array, given its index; C<av_push> pushes a scalar value onto the
1172  end of the array, automatically extending the array as necessary.
1174  Specifically, we read pathnames one at a time from the input array, and
1175  store the results in an output array (results) in the same order.  If
1176  statfs fails, the element pushed onto the return array is the value of
1177  errno after the failure.  If statfs succeeds, though, the value pushed
1178  onto the return array is a reference to a hash containing some of the
1179  information in the statfs structure.
1181  As with the return stack, it would be possible (and a small performance win)
1182  to pre-extend the return array before pushing data into it, since we know
1183  how many elements we will return:
1185      av_extend(results, numpaths);
1187  =item *
1189  We are performing only one hash operation in this function, which is storing
1190  a new scalar under a key using C<hv_store>.  A hash is represented by an HV*
1191  pointer.  Like arrays, the functions for manipulating hashes from an XSUB
1192  mirror the functionality available from Perl.  See L<perlguts> and L<perlapi>
1193  for details.
1195  =item *
1197  To create a reference, we use the C<newRV> function.  Note that you can
1198  cast an AV* or an HV* to type SV* in this case (and many others).  This
1199  allows you to take references to arrays, hashes and scalars with the same
1200  function.  Conversely, the C<SvRV> function always returns an SV*, which may
1201  need to be cast to the appropriate type if it is something other than a
1202  scalar (check with C<SvTYPE>).
1204  =item *
1206  At this point, xsubpp is doing very little work - the differences between
1207  Mytest.xs and Mytest.c are minimal.
1209  =back
1211  =head2 EXAMPLE 7 (Coming Soon)
1213  XPUSH args AND set RETVAL AND assign return value to array
1215  =head2 EXAMPLE 8 (Coming Soon)
1217  Setting $!
1219  =head2 EXAMPLE 9 Passing open files to XSes
1221  You would think passing files to an XS is difficult, with all the
1222  typeglobs and stuff. Well, it isn't.
1224  Suppose that for some strange reason we need a wrapper around the
1225  standard C library function C<fputs()>. This is all we need:
1227      #define PERLIO_NOT_STDIO 0
1228      #include "EXTERN.h"
1229      #include "perl.h"
1230      #include "XSUB.h"
1232      #include <stdio.h>
1234      int
1235      fputs(s, stream)
1236          char *          s
1237          FILE *            stream
1239  The real work is done in the standard typemap.
1241  B<But> you loose all the fine stuff done by the perlio layers. This
1242  calls the stdio function C<fputs()>, which knows nothing about them.
1244  The standard typemap offers three variants of PerlIO *:
1245  C<InputStream> (T_IN), C<InOutStream> (T_INOUT) and C<OutputStream>
1246  (T_OUT). A bare C<PerlIO *> is considered a T_INOUT. If it matters
1247  in your code (see below for why it might) #define or typedef
1248  one of the specific names and use that as the argument or result
1249  type in your XS file.
1251  The standard typemap does not contain PerlIO * before perl 5.7,
1252  but it has the three stream variants. Using a PerlIO * directly
1253  is not backwards compatible unless you provide your own typemap.
1255  For streams coming I<from> perl the main difference is that
1256  C<OutputStream> will get the output PerlIO * - which may make
1257  a difference on a socket. Like in our example...
1259  For streams being handed I<to> perl a new file handle is created
1260  (i.e. a reference to a new glob) and associated with the PerlIO *
1261  provided. If the read/write state of the PerlIO * is not correct then you
1262  may get errors or warnings from when the file handle is used.
1263  So if you opened the PerlIO * as "w" it should really be an
1264  C<OutputStream> if open as "r" it should be an C<InputStream>.
1266  Now, suppose you want to use perlio layers in your XS. We'll use the
1267  perlio C<PerlIO_puts()> function as an example.
1269  In the C part of the XS file (above the first MODULE line) you
1270  have
1272      #define OutputStream    PerlIO *
1273      or
1274      typedef PerlIO *    OutputStream;
1277  And this is the XS code:
1279      int
1280      perlioputs(s, stream)
1281          char *          s
1282          OutputStream    stream
1283      CODE:
1284          RETVAL = PerlIO_puts(stream, s);
1285      OUTPUT:
1286          RETVAL
1288  We have to use a C<CODE> section because C<PerlIO_puts()> has the arguments
1289  reversed compared to C<fputs()>, and we want to keep the arguments the same.
1291  Wanting to explore this thoroughly, we want to use the stdio C<fputs()>
1292  on a PerlIO *. This means we have to ask the perlio system for a stdio
1293  C<FILE *>:
1295      int
1296      perliofputs(s, stream)
1297          char *          s
1298          OutputStream    stream
1299      PREINIT:
1300          FILE *fp = PerlIO_findFILE(stream);
1301      CODE:
1302          if (fp != (FILE*) 0) {
1303              RETVAL = fputs(s, fp);
1304          } else {
1305              RETVAL = -1;
1306          }
1307      OUTPUT:
1308          RETVAL
1310  Note: C<PerlIO_findFILE()> will search the layers for a stdio
1311  layer. If it can't find one, it will call C<PerlIO_exportFILE()> to
1312  generate a new stdio C<FILE>. Please only call C<PerlIO_exportFILE()> if
1313  you want a I<new> C<FILE>. It will generate one on each call and push a
1314  new stdio layer. So don't call it repeatedly on the same
1315  file. C<PerlIO()>_findFILE will retrieve the stdio layer once it has been
1316  generated by C<PerlIO_exportFILE()>.
1318  This applies to the perlio system only. For versions before 5.7,
1319  C<PerlIO_exportFILE()> is equivalent to C<PerlIO_findFILE()>.
1321  =head2 Troubleshooting these Examples
1323  As mentioned at the top of this document, if you are having problems with
1324  these example extensions, you might see if any of these help you.
1326  =over 4
1328  =item *
1330  In versions of 5.002 prior to the gamma version, the test script in Example
1331  1 will not function properly.  You need to change the "use lib" line to
1332  read:
1334      use lib './blib';
1336  =item *
1338  In versions of 5.002 prior to version 5.002b1h, the test.pl file was not
1339  automatically created by h2xs.  This means that you cannot say "make test"
1340  to run the test script.  You will need to add the following line before the
1341  "use extension" statement:
1343      use lib './blib';
1345  =item *
1347  In versions 5.000 and 5.001, instead of using the above line, you will need
1348  to use the following line:
1350      BEGIN { unshift(@INC, "./blib") }
1352  =item *
1354  This document assumes that the executable named "perl" is Perl version 5.
1355  Some systems may have installed Perl version 5 as "perl5".
1357  =back
1359  =head1 See also
1361  For more information, consult L<perlguts>, L<perlapi>, L<perlxs>, L<perlmod>,
1362  and L<perlpod>.
1364  =head1 Author
1366  Jeff Okamoto <F<okamoto@corp.hp.com>>
1368  Reviewed and assisted by Dean Roehrich, Ilya Zakharevich, Andreas Koenig,
1369  and Tim Bunce.
1371  PerlIO material contributed by Lupe Christoph, with some clarification
1372  by Nick Ing-Simmons.
1374  Changes for h2xs as of Perl 5.8.x by Renee Baecker
1376  =head2 Last Changed
1378  2007/10/11

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