# man > perlxstut(1)

[PERLXSTUT(1)](https://www.chedong.com/phpMan.php/man/PERLXSTUT/1/markdown)                      Perl Programmers Reference Guide                      [PERLXSTUT(1)](https://www.chedong.com/phpMan.php/man/PERLXSTUT/1/markdown)



## NAME
       perlxstut - Tutorial for writing XSUBs

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

       This tutorial starts with very simple examples and becomes more 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 substitute whatever
       make program Perl has been configured to use.  Running **perl** **-V:make** should tell you what it
       is.

### Version caveat
       When writing a Perl extension for general consumption, one should expect that the extension
       will be used with versions of Perl different from the version available on your machine.
       Since you are reading this document, the version of Perl on your machine is probably 5.005 or
       later, but the users of your extension may have more ancient versions.

       To understand what kinds of incompatibilities one may expect, and in the rare case that the
       version of Perl on your machine is older than this document, see the section on
       "Troubleshooting these Examples" for more information.

       If your extension uses some features of Perl which are not available on older releases of
       Perl, your users would appreciate an early meaningful warning.  You would probably put this
       information into the _README_ file, but nowadays installation of extensions may be performed
       automatically, guided by _CPAN.pm_ module or other tools.

       In MakeMaker-based installations, _Makefile.PL_ provides the earliest opportunity to perform
       version checks.  One can put something like this in _Makefile.PL_ for this purpose:

           eval { require 5.007 }
               or die <<EOD;
           ############
           ### This module uses frobnication framework which is not available
           ### before version 5.007 of Perl.  Upgrade your Perl before
           ### installing [Kara::Mba](https://www.chedong.com/phpMan.php/perldoc/Kara%3A%3AMba/markdown).
           ############
           EOD

### Dynamic Loading versus Static Loading
       It is commonly thought that if a system does not have the capability to dynamically load a
       library, you cannot build XSUBs.  This is incorrect.  You _can_ build them, but you must link
       the XSUBs subroutines with the rest of Perl, creating a new executable.  This situation is
       similar to Perl 4.

       This tutorial can still be used on such a system.  The XSUB build mechanism will check the
       system and build a dynamically-loadable library if possible, or else a static library and
       then, optionally, a new statically-linked executable with that static library linked in.

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

   **Threads** **and** **PERL**___**NO**___**GET**___**CONTEXT**
       For threaded builds, perl requires the context pointer for the current thread, without
       "PERL_NO_GET_CONTEXT", perl will call a function to retrieve the context.

       For improved performance, include:

         #define PERL_NO_GET_CONTEXT

       as shown below.

       For more details, see perlguts.

## 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 created under
       the Mytest dir, including MANIFEST, Makefile.PL, lib/Mytest.pm, Mytest.xs, t/Mytest.t, 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](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AMakeMaker/markdown);

           # 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          => '-I',        # e.g., '-I. -I/usr/include/other'
           );

       The file Mytest.pm should start with something like this:

           package Mytest;

           use 5.008008;
           use strict;
           use warnings;

           require Exporter;

           our @ISA = qw(Exporter);
           our %EXPORT_TAGS = ( 'all' => [ qw(

           ) ] );

           our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );

           our @EXPORT = qw(

           );

           our $VERSION = '0.01';

           require XSLoader;
           [XSLoader::load](https://www.chedong.com/phpMan.php/perldoc/XSLoader%3A%3Aload/markdown)('Mytest', $VERSION);

           # Preloaded methods go here.

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

       The rest of the .pm file contains sample code for providing documentation for the extension.

       Finally, the Mytest.xs file should look something like this:

           #define PERL_NO_GET_CONTEXT
           #include "EXTERN.h"
           #include "perl.h"
           #include "XSUB.h"

           #include "ppport.h"

           MODULE = Mytest             PACKAGE = Mytest

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

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

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

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

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

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

        % make
        cp lib/Mytest.pm blib/lib/Mytest.pm
        perl xsubpp  -typemap typemap  Mytest.xs > Mytest.xsc && \
        mv Mytest.xsc Mytest.c
        Please specify prototyping behavior for Mytest.xs (see perlxs manual)
        cc -c     Mytest.c
        Running Mkbootstrap for Mytest ()
        chmod 644 Mytest.bs
        rm -f blib/arch/auto/Mytest/Mytest.so
        cc -shared -L/usr/local/lib Mytest.o -o blib/arch/auto/Mytest/Mytest.so

        chmod 755 blib/arch/auto/Mytest/Mytest.so
        cp Mytest.bs blib/arch/auto/Mytest/Mytest.bs
        chmod 644 blib/arch/auto/Mytest/Mytest.bs
        Manifying blib/man3/Mytest.3pm
        %

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

       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](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3Atestlib/markdown);

           use Mytest;

           [Mytest::hello](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Ahello/markdown)();

       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 1 if the number is even or 0 if the number is odd.

       Add the following to the end of Mytest.xs:

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

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

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

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

       In order to test that our extension works, we now need to look at the file Mytest.t.  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.

           use [Test::More](https://www.chedong.com/phpMan.php/perldoc/Test%3A%3AMore/markdown) tests => 4;
           BEGIN { use_ok('Mytest') };

           #########################

           # Insert your test code below, the [Test::More](https://www.chedong.com/phpMan.php/perldoc/Test%3A%3AMore/markdown) module is use()ed here
           # so read its man page ( perldoc [Test::More](https://www.chedong.com/phpMan.php/perldoc/Test%3A%3AMore/markdown) ) for help writing this
           # test script.

           is( Mytest::[is_even(0)](https://www.chedong.com/phpMan.php/man/iseven/0/markdown), 1 );
           is( Mytest::[is_even(1)](https://www.chedong.com/phpMan.php/man/iseven/1/markdown), 0 );
           is( Mytest::[is_even(2)](https://www.chedong.com/phpMan.php/man/iseven/2/markdown), 1 );

       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 /usr/bin/perl "-[MExtUtils::Command::MM](https://www.chedong.com/phpMan.php/perldoc/MExtUtils%3A%3ACommand%3A%3AMM/markdown)" "-e"
        "test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
        t/Mytest....ok
        All tests successful.
        Files=1, Tests=4, 0 wallclock secs ( 0.03 cusr + 0.00 csys = 0.03 CPU)
        %

### What has gone on?
       The program h2xs is the starting point for creating extensions.  In later examples we'll see
       how we can use h2xs to read header files and generate templates to connect to C routines.

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

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

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

       Invoking the test script via ""make test"" did something very important.  It invoked perl
       with all those "-I" arguments so that it could find the various files that are part of the
       extension.  It is _very_ important that while you are still testing extensions that you use
       ""make test"".  If you try to run the test script all by itself, you will get a fatal error.
       Another reason it is important to use ""make test"" to run your test script is that if you
       are testing an upgrade to an already-existing version, using ""make test"" ensures that you
       will test your new extension, not the already-existing version.

       When Perl sees a "use extension;", it searches for a file with the same name as the "use"'d
       extension that has a .pm suffix.  If that file cannot be found, Perl dies with a fatal error.
       The default search path is contained in the @INC array.

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

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

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

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

       See perlmod for more information.

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

### Writing good test scripts
       The importance of writing good test scripts cannot be over-emphasized.  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 Mytest.t script runs and uses the correct
       version of your extension.  If you have many test cases, save your test files in the "t"
       directory and use the suffix ".t".  When you run ""make test"", all of these test files will
       be executed.

   **EXAMPLE** **3**
       Our third extension will take one argument as its input, round off that value, and set the
       _argument_ to the rounded value.

       Add the following to the end of Mytest.xs:

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

       Edit the Makefile.PL file so that the corresponding line looks like this:

               LIBS      => ['-lm'],   # e.g., '-lm'

       Generate the Makefile and run make.  Change the test number in Mytest.t to "9" and add the
       following tests:

               my $i;

               $i = -1.5;
               [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)($i);
               is( $i, -2.0, 'Rounding -1.5 to -2.0' );

               $i = -1.1;
               [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)($i);
               is( $i, -1.0, 'Rounding -1.1 to -1.0' );

               $i = 0.0;
               [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)($i);
               is( $i, 0.0, 'Rounding 0.0 to 0.0' );

               $i = 0.5;
               [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)($i);
               is( $i, 1.0, 'Rounding 0.5 to 1.0' );

               $i = 1.2;
               [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)($i);
               is( $i, 1.0, 'Rounding 1.2 to 1.0' );

       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 Mytest.t:

               Mytest::[round(3)](https://www.chedong.com/phpMan.php/man/round/3/markdown);

       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.

       •   The value of the function is not being passed back as the function's return value, but by
           changing the value of the variable that was passed into the function.  You might have
           guessed that when you saw that the return value of round is of type "void".

### Input and Output Parameters
       You specify the parameters that will be passed into the XSUB on the line(s) after you declare
       the function's return value and name.  Each input parameter line starts with optional
       whitespace, and may have an optional terminating semicolon.

       The list of output parameters occurs at the very end of the function, just after the OUTPUT:
       directive.  The use of RETVAL tells Perl that you wish to send this value back as the return
       value of the XSUB function.  In Example 3, we wanted the "return value" placed in the
       original variable which we passed in, so we listed it (and not RETVAL) in the OUTPUT:
       section.

### The XSUBPP Program
       The **xsubpp** program takes the XS code in the .xs file and translates it into C code, placing
       it in a file whose suffix is .c.  The C code created makes heavy use of the C functions
       within Perl.

### The TYPEMAP file
       The **xsubpp** program uses rules to convert from Perl's data types (scalar, array, etc.) to C's
       data types (int, char, etc.).  These rules are stored in the typemap file
       ($PERLLIB/ExtUtils/typemap).  There's a brief discussion below, but all the nitty-gritty
       details can be found in perlxstypemap.  If you have a new-enough version of perl (5.16 and
       up) or an upgraded XS compiler ("[ExtUtils::ParseXS](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AParseXS/markdown)" 3.13_01 or better), then you can inline
       typemaps in your XS instead of writing separate files.  Either way, this typemap thing 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)
                       Perl_croak(aTHX_ "Usage: [Mytest::round](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Around/markdown)(arg)");
                   PERL_UNUSED_VAR(cv); /* -W */
                   {
                       double  arg = (double)SvNV([ST(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown));      /* 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)](https://www.chedong.com/phpMan.php/man/ST/0/markdown), (double)arg);   /* XXXXX */
                       SvSETMAGIC([ST(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown));
                   }
                   XSRETURN_EMPTY;
               }

       Notice the two lines commented with "XXXXX".  If you check the first part of the typemap file
       (or section), you'll see that doubles are of type T_DOUBLE.  In the INPUT part of the
       typemap, an argument that is T_DOUBLE is assigned to the variable arg by calling the routine
       SvNV on something, then casting it to double, then assigned to the variable arg.  Similarly,
       in the OUTPUT section, once arg has its final value, it is passed to the sv_setnv function to
       be passed back to the calling subroutine.  These two functions are explained in perlguts;
       we'll talk more later about what that "[**ST**(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown)" means in the section on the argument stack.

### Warning about Output Arguments
       In general, it's not a good idea to write extensions that modify their input parameters, as
       in Example 3.  Instead, you should probably return multiple values in an array and let the
       caller handle them (we'll do this in a later example).  However, in order to better
       accommodate calling pre-existing C routines, which often do modify their input parameters,
       this behavior is tolerated.

   **EXAMPLE** **4**
       In this example, we'll now begin to write XSUBs that will interact with pre-defined C
       libraries.  To begin with, we will build a small library of our own, then let h2xs write our
       .pm and .xs files for us.

       Create a new directory called Mytest2 at the same level as the directory Mytest.  In the
       Mytest2 directory, create another directory called mylib, and cd into that directory.

       Here we'll create some files that will generate a test library.  These will include a C
       source file and a header file.  We'll also create a Makefile.PL in this directory.  Then
       we'll make sure that running make at the Mytest2 level will automatically run this
       Makefile.PL file and the resulting Makefile.

       In the mylib directory, create a file mylib.h that looks like this:

               #define TESTVAL 4

               extern double   foo(int, long, const char*);

       Also create a file mylib.c that looks like this:

               #include <stdlib.h>
               #include "mylib.h"

               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](https://www.chedong.com/phpMan.php/perldoc/ExtUtils%3A%3AMakeMaker/markdown);
               $Verbose = 1;
               WriteMakefile(
                   NAME  => '[Mytest2::mylib](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3Amylib/markdown)',
                   SKIP  => [qw(all static static_lib dynamic dynamic_lib)],
                   clean => {'FILES' => 'libmylib$(LIB_EXT)'},
               );


               sub [MY::top_targets](https://www.chedong.com/phpMan.php/perldoc/MY%3A%3Atoptargets/markdown) {
                       '
               all :: static

               pure_all :: static

               static ::       libmylib$(LIB_EXT)

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

               ';
               }

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

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

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

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

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

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

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

               sub [MY::postamble](https://www.chedong.com/phpMan.php/perldoc/MY%3A%3Apostamble/markdown) {
               '
               $(MYEXTLIB): mylib/Makefile
                       cd mylib && $(MAKE) $(PASSTHRU)
               ';
               }

       Let's also fix the MANIFEST file by appending the following three lines:

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

       To keep our namespace nice and unpolluted, edit the .pm file and change the variable @EXPORT
       to @EXPORT_OK.  Finally, in the .xs file, edit the #include line to read:

               #include "mylib/mylib.h"

       And also add the following function definition to the end of the .xs file:

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

       Now we also need to create a typemap because the default Perl doesn't currently support the
       "const char *" type.  Include a new TYPEMAP section in your XS code before the above
       function:

               TYPEMAP: <<END
               const char *    T_PV
               END

       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 Mytest2.t script and change the number of tests to "5", and add the following
       lines to the end of the script:

               is( [Mytest2::foo](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3Afoo/markdown)( 1, 2, "Hello, world!" ), 7 );
               is( [Mytest2::foo](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3Afoo/markdown)( 1, 2, "0.0" ),           7 );
               ok( abs( [Mytest2::foo](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3Afoo/markdown)( 0, 0, "-3.4" ) - 0.6 ) <= 0.01 );

       (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. There are some warnings on missing tests for the
       [Mytest2::mylib](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3Amylib/markdown) extension, but you can ignore them.

### 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 "TESTVAL" or &[Mytest2::TESTVAL](https://www.chedong.com/phpMan.php/perldoc/Mytest2%3A%3ATESTVAL/markdown)).  There's also some
           XS code to allow calls to the "constant" routine.

       •   The .pm file originally exported the name "TESTVAL" in the @EXPORT array.  This could
           lead to name clashes.  A good rule of thumb is that if the #define is only going to be
           used by the C routines themselves, and not by the user, they should be removed from the
           @EXPORT array.  Alternately, if you don't mind using the "fully qualified name" of a
           variable, you could move most or all of the items from the @EXPORT array into the
           @EXPORT_OK array.

       •   If our include file had contained #include directives, these would not have been
           processed by h2xs.  There is no good solution to this right now.

       •   We've also told Perl about the library that we built in the mylib subdirectory.  That
           required only the addition of the "MYEXTLIB" variable to the WriteMakefile call and the
           replacement of the postamble subroutine to cd into the subdirectory and run make.  The
           Makefile.PL for the library is a bit more complicated, but not excessively so.  Again we
           replaced the postamble subroutine to insert our own code.  This code simply specified
           that the library to be created here was a static archive library (as opposed to a
           dynamically loadable library) and provided the commands to build it.

### Anatomy of .xs file
       The .xs file of "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 functions.  These descriptions are
       translated by **xsubpp** into C code which implements these functions using Perl calling
       conventions, and which makes these functions visible from Perl interpreter.

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

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

### Getting the fat out of XSUBs
       In "EXAMPLE 4" the second part of .xs file contained the following description of an XSUB:

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

       Note that in contrast with "EXAMPLE 1", "EXAMPLE 2" and "EXAMPLE 3", this description does
       not contain the actual _code_ for what is done during a call to Perl function **foo()**.  To
       understand what is going on here, one can add a CODE section to this XSUB:

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

       However, these two XSUBs provide almost identical generated C code: **xsubpp** compiler is smart
       enough to figure out the "CODE:" section from the first two lines of the description of XSUB.
       What about "OUTPUT:" section?  In fact, that is absolutely the same!  The "OUTPUT:" section
       can be removed as well, _as_ _far_ _as_ _"CODE:"_ _section_ _or_ _"PPCODE:"_ _section_ is not specified:
       **xsubpp** can see that it needs to generate a function call section, and will autogenerate the
       OUTPUT section too.  Thus one can shortcut the XSUB to become:

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

       Can we do the same with an XSUB

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

       of "EXAMPLE 2"?  To do this, one needs to define a C function "int is_even(int input)".  As
       we saw in "Anatomy of .xs file", a proper place for this definition is in the first part of
       .xs file.  In fact a C function

               int
               is_even(int arg)
               {
                       return (arg % 2 == 0);
               }

       is probably overkill for this.  Something as simple as a "#define" will do too:

               #define is_even(arg)    ((arg) % 2 == 0)

       After having this in the first part of .xs file, the "Perl glue" part becomes as simple as

               int
               is_even(input)
                       int     input

       This technique of separation of the glue part from the workhorse part has obvious tradeoffs:
       if you want to change a Perl interface, you need to change two places in your code.  However,
       it removes a lot of clutter, and makes the workhorse part independent from idiosyncrasies of
       Perl calling convention.  (In fact, there is nothing Perl-specific in the above description,
       a different version of **xsubpp** might have translated this to TCL glue or Python glue as well.)

### More about XSUB arguments
       With the completion of Example 4, we now have an easy way to simulate some real-life
       libraries whose interfaces may not be the cleanest in the world.  We shall now continue with
       a discussion of the arguments passed to the **xsubpp** compiler.

       When you specify arguments to routines in the .xs file, you are really passing three pieces
       of information for each argument listed.  The first piece is the order of that argument
       relative to the others (first, second, etc).  The second is the type of argument, and
       consists of the type declaration of the argument (e.g., int, char*, etc).  The third piece is
       the calling convention for the argument in the call to the library function.

       While Perl passes arguments to functions by reference, C passes arguments by value; to
       implement a C function which modifies data of one of the "arguments", the actual argument of
       this C function would be a pointer to the data.  Thus two C functions with declarations

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

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

               int
               upper_case_char(cp)
                       char    &cp

       For example, consider:

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

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

               foo(&a, b);

       **xsubpp** will parse the following function argument lists identically:

               char    &a
               char&a
               char    & a

       However, to help ease understanding, it is suggested that you place a "&" next to the
       variable name and away from the variable type), and place a "*" near the variable type, but
       away from the variable name (as in the call to foo above).  By doing so, it is easy to
       understand exactly what will be passed to the C function; it will be whatever is in the "last
       column".

       You should take great pains to try to pass the function the type of variable it wants, when
       possible.  It will save you a lot of trouble in the long run.

### The Argument Stack
       If we look at any of the C code generated by any of the examples except example 1, you will
       notice a number of references to [ST(n)](https://www.chedong.com/phpMan.php/man/ST/n/markdown), where n is usually 0.  "ST" is actually a macro that
       points to the n'th argument on the argument stack.  [**ST**(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown) is thus the first argument on the
       stack and therefore the first argument passed to the XSUB, [**ST**(1)](https://www.chedong.com/phpMan.php/man/ST/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/ST/0/markdown) if it was the first argument) is changed.  You can verify this by looking at the C code
       generated for Example 3.  The code for the **round()** XSUB routine contains lines that look like
       this:

               double  arg = (double)SvNV([ST(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown));
               /* Round the contents of the variable arg */
               sv_setnv([ST(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown), (double)arg);

       The arg variable is initially set by taking the value from [**ST**(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown), then is stored back into
       [**ST**(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown) at the end of the routine.

       XSUBs are also allowed to return lists, not just scalars.  This must be done by manipulating
       stack values [**ST**(0)](https://www.chedong.com/phpMan.php/man/ST/0/markdown), [**ST**(1)](https://www.chedong.com/phpMan.php/man/ST/1/markdown), etc, in a subtly different way.  See perlxs for details.

       XSUBs are also allowed to avoid automatic conversion of Perl function arguments to C function
       arguments.  See perlxs for details.  Some people prefer manual conversion by inspecting ST(i)
       even in the cases when automatic conversion will do, arguing that this makes the logic of an
       XSUB call clearer.  Compare with "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 protecting 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 documentation will be
       converted to the manpage format, then placed in the blib directory.  It will be copied to
       Perl's manpage directory when the extension is installed.

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

       See perlpod for more information about the pod format.

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

       Alternately, you can specify the exact directory to place the extension's files by placing a
       "PREFIX=/destination/directory" after the make install (or in between the make and install if
       you have a brain-dead version of make).  This can be very useful if you are building an
       extension that will eventually be distributed to multiple systems.  You can then just archive
       the files in the destination directory and distribute them to your destination systems.

   **EXAMPLE** **5**
       In this example, we'll do some more work with the argument stack.  The previous examples have
       all returned only a single value.  We'll now create an extension that returns an 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)));
                       } 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 Mytest.t while incrementing the "9" tests to "11":

           my @a;

               @a = [Mytest::statfs](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Astatfs/markdown)("/blech");
               ok( scalar(@a) == 1 && $a[0] == 2 );

               @a = [Mytest::statfs](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Astatfs/markdown)("/");
               is( scalar(@a), 7 );

### New Things in this Example
       This example added quite a few new concepts.  We'll take them one at a time.

       •   The INIT: directive contains code that will be placed immediately after the argument
           stack is decoded.  C does not allow variable declarations at arbitrary locations inside a
           function, so this is usually the best way to declare local variables needed by the XSUB.
           (Alternatively, one could put the whole "PPCODE:" section into braces, and put these
           declarations on top.)

       •   This routine also returns a different number of arguments depending on the success or
           failure of the call to statfs.  If there is an error, the error number is returned as a
           single-element array.  If the call is successful, then a 7-element array is returned.
           Since only one argument is passed into this function, we need room on the stack to hold
           the 7 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 values 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 program, the SV's that held
           the returned values can be deallocated.  If they were not mortal, then they would
           continue to exist after the XSUB routine returned, but would not be accessible.  This is
           a memory leak.

       •   If we were interested in performance, not in code compactness, in the success branch we
           would not use "XPUSHs" macros, but "PUSHs" macros, and would pre-extend the stack before
           pushing the return values:

                   EXTEND(SP, 7);

           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 consumption).

           Similarly, in the failure branch we could use "PUSHs" _without_ extending the stack: the
           Perl function reference comes to an XSUB on the stack, thus the stack is _always_ large
           enough to take one return value.

   **EXAMPLE** **6**
       In this example, we will accept a reference to an array as an input parameter, and return a
       reference to an array of hashes.  This will demonstrate manipulation of complex Perl data
       types from an XSUB.

       This extension is somewhat contrived.  It is based on the code in the previous example.  It
       calls the statfs function multiple times, accepting a reference to an array of filenames as
       input, and returning a reference to an array of hashes containing the data for each of the
       filesystems.

       Return to the Mytest directory and add the following code to the end of Mytest.xs:

           SV *
           multi_statfs(paths)
                   SV * paths
               INIT:
                   AV * results;
                   SSize_t numpaths = 0, n;
                   int i;
                   struct statfs buf;

                   SvGETMAGIC(paths);
                   if ((!SvROK(paths))
                       || (SvTYPE(SvRV(paths)) != SVt_PVAV)
                       || ((numpaths = av_top_index((AV *)SvRV(paths))) < 0))
                   {
                       XSRETURN_UNDEF;
                   }
                   results = (AV *)sv_2mortal((SV *)newAV());
               CODE:
                   for (n = 0; n <= numpaths; n++) {
                       HV * rh;
                       STRLEN l;
                       SV * path = *av_fetch((AV *)SvRV(paths), n, 0);
                       char * fn = SvPVbyte(path, 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_inc((SV *)rh));
                   }
                   RETVAL = newRV_inc((SV *)results);
               OUTPUT:
                   RETVAL

       And add the following code to Mytest.t, while incrementing the "11" tests to "13":

               my $results = [Mytest::multi_statfs](https://www.chedong.com/phpMan.php/perldoc/Mytest%3A%3Amultistatfs/markdown)([ '/', '/blech' ]);
               ok( ref $results->[0] );
               ok( ! ref $results->[1] );

### New Things in this Example
       There are a number of new concepts introduced here, described below:

       •   This function does not use a typemap.  Instead, we declare it as accepting one SV*
           (scalar) parameter, and returning an SV* value, and we take care of populating these
           scalars within the code.  Because we are only returning one value, we don't need a
           "PPCODE:" directive - instead, we use "CODE:" and "OUTPUT:" directives.

       •   When dealing with references, it is important to handle them with caution.  The "INIT:"
           block first calls SvGETMAGIC(paths), in case paths is a tied variable.  Then it checks
           that "SvROK" returns true, which indicates that paths is a valid reference.  (Simply
           checking "SvROK" won't trigger FETCH on a tied variable.)  It then verifies that the
           object referenced by paths is an array, using "SvRV" to dereference paths, and "SvTYPE"
           to discover its type.  As an added test, it checks that the array referenced by paths is
           non-empty, using the "av_top_index" 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_top_index" returns the highest index in an AV*, much like $#array;
           "av_fetch" fetches a single scalar value from an array, given its index; "av_push" pushes
           a scalar value onto the end of the array, automatically extending the array as necessary.

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

           As with the return stack, it would be possible (and a small 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_inc" 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 returns an SV*, which may need to be cast to the appropriate type if it is
           something other than a scalar (check with "SvTYPE").

       •   At this point, xsubpp is doing very little work - the differences between Mytest.xs and
           Mytest.c are minimal.

### EXAMPLE 7 (Coming Soon)
       XPUSH args AND set RETVAL AND assign return value to array

### EXAMPLE 8 (Coming Soon)
       Setting $!

### EXAMPLE 9 Passing open files to XSes
       You would think passing files to an XS is difficult, with all the 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  /* For co-existence with stdio only */
         #define PERL_NO_GET_CONTEXT /* This is more efficient */
         #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.

       For more details, see "Co-existence with stdio" in perlapio.

       **But** you lose all the fine stuff done by the perlio layers. This calls the stdio function
       "fputs()", which knows nothing about them.

       The standard typemap offers three variants of PerlIO *: "InputStream" (T_IN), "InOutStream"
       (T_INOUT) and "OutputStream" (T_OUT). A bare "PerlIO *" is considered a T_INOUT. If it
       matters in your code (see below for why it might) #define or typedef one of the specific
       names and use that as the argument or result type in your XS file.

       The standard typemap does not contain PerlIO * before perl 5.7, but it has the three stream
       variants. Using a PerlIO * directly is not 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 created (i.e. a reference to a new
       glob) and associated with the PerlIO * provided. If the read/write state of the PerlIO * is
       not correct then you may get errors or warnings from when the file handle is used.  So if you
       opened the PerlIO * as "w" it should really be an "OutputStream" if open as "r" it should be
       an "InputStream".

       Now, suppose you want to use perlio layers in your XS. We'll use the perlio "PerlIO_puts()"
       function as an example.

       In the C part of the XS file (above the first MODULE line) you have

               #define OutputStream    PerlIO *
           or
               typedef PerlIO *        OutputStream;

       And this is the XS code:

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

       We have to use a "CODE" section because "PerlIO_puts()" has the 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 "PerlIO_exportFILE()" to generate a new stdio "FILE". Please only call
       "PerlIO_exportFILE()" if you want a _new_ "FILE". It will generate one on each call and push a
       new stdio layer. So don't call it repeatedly on the same file. "PerlIO_findFILE()" will
       retrieve the stdio layer once it has been generated by "PerlIO_exportFILE()".

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

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

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

                   use lib './blib';

       •   In versions of 5.002 prior to version 5.002b1h, the test.pl file was not automatically
           created by h2xs.  This means that you cannot say "make test" to run the test script.  You
           will need to add the following line before the "use extension" statement:

                   use lib './blib';

       •   In versions 5.000 and 5.001, instead of using the above line, you will need to use the
           following line:

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

       •   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, perlapio, 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.

       Changes for h2xs as of Perl 5.8.x by Renee Baecker

       This document is now maintained as part of Perl itself.

### Last Changed
       2020-10-05



perl v5.34.0                                 2026-06-23                                 [PERLXSTUT(1)](https://www.chedong.com/phpMan.php/man/PERLXSTUT/1/markdown)