=head1 NAME
-perlXStut - Tutorial for XSUBs
+perlXStut - Tutorial for writing XSUBs
=head1 DESCRIPTION
This tutorial will educate the reader on the steps involved in creating
-a Perl extension. The reader is assumed to have access to L<perlguts> and
-L<perlxs>.
+a Perl extension. The reader is assumed to have access to L<perlguts>,
+L<perlapi> and L<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 to ease the
-reader slowly into building extensions.
+completely explained until later in the tutorial in order to slowly ease
+the reader into building extensions.
-=head2 VERSION CAVEAT
+This tutorial was written from a Unix point of view. Where I know them
+to be otherwise different for other platforms (e.g. Win32), I will list
+them. If you find something that was missed, please let me know.
-This tutorial tries hard to keep up with the latest development versions
-of Perl. This often means that it is sometimes in advance of the latest
-released version of Perl, and that certain features described here might
-not work on earlier versions. This section will keep track of when various
-features were added to Perl 5.
+=head1 SPECIAL NOTES
-=over 4
+=head2 make
-=item *
+This tutorial assumes that the make program that Perl is configured to
+use is called C<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 B<perl -V:make> should tell you what it is.
-In versions of Perl 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:
+=head2 Version caveat
- use lib './blib';
+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.
-=item *
+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.
-In versions of Perl 5.002 prior to version beta 3, the line in the .xs file
-about "PROTOTYPES: DISABLE" will cause a compiler error. Simply remove that
-line from the file.
+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 F<README> file, but nowadays
+installation of extensions may be performed automatically, guided by F<CPAN.pm>
+module or other tools.
-=item *
+In MakeMaker-based installations, F<Makefile.PL> provides the earliest
+opportunity to perform version checks. One can put something like this
+in F<Makefile.PL> for this purpose:
-In versions of Perl 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:
+ 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.
+ ############
+ EOD
- use lib './blib';
-
-=item *
-
-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") }
-
-=item *
-
-This document assumes that the executable named "perl" is Perl version 5.
-Some systems may have installed Perl version 5 as "perl5".
-
-=back
-
-=head2 DYNAMIC VERSUS STATIC
+=head2 Dynamic Loading versus Static Loading
It is commonly thought that if a system does not have the capability to
-load a library dynamically, you cannot build XSUBs. This is incorrect.
-You I<can> build them, but you must link the XSUB's subroutines with the
+dynamically load a library, you cannot build XSUBs. This is incorrect.
+You I<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.
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.
+where the command "C<make>" with no arguments is executed, run the command
+"C<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.
+instead of saying "C<make test>", you should say "C<make test_static>".
+On systems that cannot build dynamically-loadable libraries at all, simply
+saying "C<make test>" is sufficient.
+
+=head1 TUTORIAL
+
+Now let's go on with the show!
=head2 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 C<h2xs -A -n Mytest>. This creates a directory named Mytest, possibly under
-ext/ if that directory exists in the current working directory. Several files
-will be created in the Mytest dir, including MANIFEST, Makefile.PL, Mytest.pm,
-Mytest.xs, test.pl, and Changes.
+Run "C<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 created.
+The MANIFEST file contains the names of all the files just created in the
+Mytest directory.
The file Makefile.PL should look something like this:
- use ExtUtils::MakeMaker;
- # See lib/ExtUtils/MakeMaker.pm for details of how to influence
- # the contents of the Makefile that is written.
- WriteMakefile(
- 'NAME' => 'Mytest',
- 'VERSION_FROM' => 'Mytest.pm', # finds $VERSION
- 'LIBS' => [''], # e.g., '-lm'
- 'DEFINE' => '', # e.g., '-DHAVE_SOMETHING'
- 'INC' => '', # e.g., '-I/usr/include/other'
- );
+ use ExtUtils::MakeMaker;
+ # See lib/ExtUtils/MakeMaker.pm for details of how to influence
+ # the contents of the Makefile that is written.
+ WriteMakefile(
+ NAME => 'Mytest',
+ VERSION_FROM => 'Mytest.pm', # finds $VERSION
+ LIBS => [''], # e.g., '-lm'
+ DEFINE => '', # e.g., '-DHAVE_SOMETHING'
+ INC => '', # e.g., '-I/usr/include/other'
+ );
The file Mytest.pm should start with something like this:
- package Mytest;
+ package Mytest;
- require Exporter;
- require DynaLoader;
+ use 5.008008;
+ use strict;
+ use warnings;
- @ISA = qw(Exporter DynaLoader);
- # Items to export into callers namespace by default. Note: do not export
- # names by default without a very good reason. Use EXPORT_OK instead.
- # Do not simply export all your public functions/methods/constants.
- @EXPORT = qw(
+ require Exporter;
- );
- $VERSION = '0.01';
+ our @ISA = qw(Exporter);
+ our %EXPORT_TAGS = ( 'all' => [ qw(
- bootstrap Mytest $VERSION;
+ ) ] );
- # Preloaded methods go here.
+ our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
- # Autoload methods go after __END__, and are processed by the autosplit program.
+ our @EXPORT = qw(
- 1;
- __END__
- # Below is the stub of documentation for your module. You better edit it!
+ );
-And the Mytest.xs file should look something like this:
+ our $VERSION = '0.01';
- #ifdef __cplusplus
- extern "C" {
- #endif
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
- #ifdef __cplusplus
- }
- #endif
-
- PROTOTYPES: DISABLE
+ require XSLoader;
+ XSLoader::load('Mytest', $VERSION);
+
+ # Preloaded methods go here.
- MODULE = Mytest PACKAGE = Mytest
+ 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:
+
+ #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");
+ 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,
+Now we'll run "C<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 shortened for clarity):
-
- % make
- umask 0 && cp Mytest.pm ./blib/Mytest.pm
- perl xsubpp -typemap typemap Mytest.xs >Mytest.tc && mv Mytest.tc Mytest.c
- cc -c Mytest.c
- Running Mkbootstrap for Mytest ()
- chmod 644 Mytest.bs
- LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl -b Mytest.o
- chmod 755 ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl
- cp Mytest.bs ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
- chmod 644 ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
-
-Now, although there is already a test.pl template ready for us, for this
-example only, we'll create a special test script. Create a file called hello
+ % 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 L<perlxs/"The PROTOTYPES: Keyword">.
+
+Perl has its own special way of easily writing test scripts, but for this
+example only, we'll create our own test script. Create a file called hello
that looks like this:
- #! /opt/perl5/bin/perl
-
- use ExtUtils::testlib;
-
- use Mytest;
-
- Mytest::hello();
+ #! /opt/perl5/bin/perl
+
+ use ExtUtils::testlib;
+
+ use Mytest;
-Now we run the script and we should see the following output:
+ Mytest::hello();
- % perl hello
- Hello, world!
- %
+Now we make the script executable (C<chmod +x hello>), run the script
+and we should see the following output:
+
+ % ./hello
+ Hello, world!
+ %
=head2 EXAMPLE 2
-Now let's add to our extension a subroutine that will take a single argument
-and return 1 if the argument is even, 0 if the argument is odd.
+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 white space at the start of the "int input" line,
-but it is useful for improving readability. The semi-colon at the end of
-that line is also optional.
+ int
+ is_even(input)
+ int input
+ CODE:
+ RETVAL = (input % 2 == 0);
+ OUTPUT:
+ RETVAL
-Any white space may be between the "int" and "input". It is also okay for
-the four lines starting at the "CODE:" line to not be indented. However,
-for readability purposes, it is suggested that you indent them 8 spaces
-(or one normal tab stop).
+There does not need to be whitespace at the start of the "C<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 "C<int>" and "C<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.
-To test that our extension works, we now need to look at the
-file test.pl. This file is set up to imitate the same kind of testing
+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. Change the print
-statement in the BEGIN block to print "1..4", and add the following code
-to the end of the file:
+when the test is correct, "not ok" when it is not.
+
+ use Test::More tests => 4;
+ BEGIN { use_ok('Mytest') };
+
+ #########################
- print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok 2", "\n";
- print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok 3", "\n";
- print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok 4", "\n";
+ # Insert your test code below, the Test::More module is use()ed here so read
+ # its man page ( perldoc Test::More ) for help writing this test script.
-We will be calling the test script through the command "make test". You
+ is(&Mytest::is_even(0), 1);
+ is(&Mytest::is_even(1), 0);
+ is(&Mytest::is_even(2), 1);
+
+We will be calling the test script through the command "C<make test>". You
should see output that looks something like this:
- % make test
- PERL_DL_NONLAZY=1 /opt/perl5.002b2/bin/perl (lots of -I arguments) test.pl
- 1..4
- ok 1
- ok 2
- ok 3
- ok 4
- %
+ %make test
+ PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-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)
+ %
-=head2 WHAT HAS GONE ON?
+=head2 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
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 files E<lt>extensionE<gt>.pm and E<lt>extensionE<gt>.xs 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 and invoking the Makefile created a directory 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 C<-I> arguments so that it could find the various
-files that are part of the extension.
-
-It is I<very> important that while you are still testing extensions that
-you use "make test". If you try to run the test script all by itself, you
-will get a fatal error.
-
-Another reason it is important to use "make test" to run your test script
-is that if you are testing an upgrade to an already-existing version, using
-"make test" insures that you use your new extension, not the already-existing
-version.
+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 C<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 "C<make test>" did something very important.
+It invoked perl with all those C<-I> arguments so that it could find the
+various files that are part of the extension. It is I<very> important that
+while you are still testing extensions that you use "C<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 "C<make test>" to run your test
+script is that if you are testing an upgrade to an already-existing version,
+using "C<make test>" ensures that you will test your new extension, not the
+already-existing version.
When Perl sees a C<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,
+as the C<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.
+C<@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 $VERSION
-scalar; finally it tells perl to bootstrap the module. Perl will call its
-dynamic loader routine (if there is one) and load the shared library.
+Loader extensions. It then sets the C<@ISA> and C<@EXPORT> arrays and the
+C<$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 that are set in the .pm file are very important. The @ISA
+The two arrays C<@ISA> and C<@EXPORT> are very important. The C<@ISA>
array contains a list of other packages in which to search for methods (or
-subroutines) that do not exist in the current package. The @EXPORT array
-tells Perl which of the extension's routines should be placed into the
-calling package's namespace.
+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.
-It's important to select what to export carefully. Do NOT export method names
-and do NOT export anything else I<by default> without a good reason.
+The C<@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 I<not>
+export method or variable names I<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 then you can export
-any of the functions via another array, called @EXPORT_OK.
+export anything. If it's just a collection of functions and variables, then
+you can export them via another array, called C<@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 L<perlmod> for more information.
-The $VERSION variable is used to ensure that the .pm file and the shared
+The C<$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.
-=head2 WRITING GOOD TEST SCRIPTS
+=head2 Writing good test scripts
-The importance of writing good test scripts cannot be overemphasized. You
+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 test.pl script runs and uses
-the correct version of your extension. If you have many test cases, you
-might want to copy Perl's test style. Create a directory named "t", and
-ensure all your test files end with the suffix ".t". The Makefile will
-properly run all these test files.
-
+By running "C<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 "C<make test>", all of these test files will be executed.
=head2 EXAMPLE 3
void
round(arg)
double arg
- CODE:
+ CODE:
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
} else {
arg = 0.0;
}
- OUTPUT:
+ OUTPUT:
arg
Edit the Makefile.PL file so that the corresponding line looks like this:
'LIBS' => ['-lm'], # e.g., '-lm'
-Generate the Makefile and run make. Change the BEGIN block to print out
-"1..9" and add the following to test.pl:
+Generate the Makefile and run make. Change the test number in Mytest.t to
+"9" and add the following tests:
- $i = -1.5; &Mytest::round($i); print $i == -2.0 ? "ok 5" : "not ok 5", "\n";
- $i = -1.1; &Mytest::round($i); print $i == -1.0 ? "ok 6" : "not ok 6", "\n";
- $i = 0.0; &Mytest::round($i); print $i == 0.0 ? "ok 7" : "not ok 7", "\n";
- $i = 0.5; &Mytest::round($i); print $i == 1.0 ? "ok 8" : "not ok 8", "\n";
- $i = 1.2; &Mytest::round($i); print $i == 1.0 ? "ok 9" : "not ok 9", "\n";
+ $i = -1.5; &Mytest::round($i); is( $i, -2.0 );
+ $i = -1.1; &Mytest::round($i); is( $i, -1.0 );
+ $i = 0.0; &Mytest::round($i); is( $i, 0.0 );
+ $i = 0.5; &Mytest::round($i); is( $i, 1.0 );
+ $i = 1.2; &Mytest::round($i); is( $i, 1.0 );
-Running "make test" should now print out that all nine tests are okay.
+Running "C<make test>" should now print out that all nine tests are okay.
-You might be wondering if you can round a constant. To see what happens, add
-the following line to test.pl temporarily:
+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);
-Run "make test" and notice that Perl dies with a fatal error. Perl won't let
-you change the value of constants!
+Run "C<make test>" and notice that Perl dies with a fatal error. Perl won't
+let you change the value of constants!
+
+=head2 What's new here?
+
+=over 4
+
+=item *
-=head2 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.
-Two things are new here. First, we've made some changes to Makefile.PL.
-In this case, we've specified an extra library to link in, the math library
-libm. We'll talk later about how to write XSUBs that can call every routine
-in a library.
+=item *
-Second, the value of the function is being passed back not as the function's
-return value, but through the same variable that was passed into the function.
+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".
+
+=back
-=head2 INPUT AND OUTPUT PARAMETERS
+=head2 Input and Output Parameters
-You specify the parameters that will be passed into the XSUB just after you
-declare the function return value and name. Each parameter line starts with
-optional white space, and may have an optional terminating semicolon.
+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 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, the value we wanted returned was
-contained in the same variable we passed in, so we listed it (and not RETVAL)
-in the OUTPUT: section.
+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.
-=head2 THE XSUBPP COMPILER
+=head2 The XSUBPP Program
-The compiler xsubpp takes the XS code in the .xs file and converts it into
+The B<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.
-=head2 THE TYPEMAP FILE
+=head2 The TYPEMAP file
-The xsubpp compiler uses rules to convert from Perl's data types (scalar,
-array, etc.) to C's data types (int, char *, etc.). These rules are stored
+The B<xsubpp> program uses rules to convert from Perl's data types (scalar,
+array, etc.) to C's data types (int, char, etc.). These rules are stored
in the typemap file ($PERLLIB/ExtUtils/typemap). This file is split into
three parts.
-The first part attempts to map various C data types to a coded flag, which
-has some correspondence with the various Perl types. The second part contains
-C code which xsubpp uses for input parameters. The third part contains C
-code which xsubpp uses for output parameters. We'll talk more about the
-C code later.
+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 B<xsubpp> uses to handle input parameters. The third section contains
+C code which B<xsubpp> uses to handle output parameters.
-Let's now take a look at a portion of the .c file created for our extension.
+Let's take a look at a portion of the .c file created for our extension.
+The file name is Mytest.c:
XS(XS_Mytest_round)
{
dXSARGS;
if (items != 1)
- croak("Usage: Mytest::round(arg)");
+ Perl_croak(aTHX_ "Usage: Mytest::round(arg)");
+ PERL_UNUSED_VAR(cv); /* -W */
{
double arg = (double)SvNV(ST(0)); /* XXXXX */
if (arg > 0.0) {
arg = 0.0;
}
sv_setnv(ST(0), (double)arg); /* XXXXX */
+ SvSETMAGIC(ST(0));
}
- XSRETURN(1);
+ XSRETURN_EMPTY;
}
-Notice the two lines marked with "XXXXX". If you check the first section of
-the typemap file, you'll see that doubles are of type T_DOUBLE. In the
+Notice the two lines commented with "XXXXX". If you check the first section
+of the typemap file, you'll see that doubles are of type T_DOUBLE. In the
INPUT section, an argument that is T_DOUBLE is assigned to the variable
arg by calling the routine SvNV on something, then casting it to double,
then assigned to the variable arg. Similarly, in the OUTPUT section,
in L<perlguts>; we'll talk more later about what that "ST(0)" means in the
section on the argument stack.
-=head2 WARNING
+=head2 Warning about Output Arguments
In general, it's not a good idea to write extensions that modify their input
-parameters, as in Example 3. However, to accommodate better calling
-pre-existing C routines, which often do modify their input parameters,
-this behavior is tolerated. The next example will show how to do this.
+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.
=head2 EXAMPLE 4
-In this example, we'll now begin to write XSUB's that will interact with
+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.
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 testlib directory, create a file mylib.h that looks like this:
+In the mylib directory, create a file mylib.h that looks like this:
#define TESTVAL 4
#include <stdlib.h>
#include "./mylib.h"
-
+
double
- foo(a, b, c)
- int a;
- long b;
- const char * c;
+ foo(int a, long b, const char *c)
{
return (a + b + atof(c) + TESTVAL);
}
use ExtUtils::MakeMaker;
$Verbose = 1;
WriteMakefile(
- NAME => 'Mytest2::mylib',
- SKIP => [qw(all static static_lib dynamic dynamic_lib)],
- clean => {'FILES' => 'libmylib$(LIB_EXT)'},
+ NAME => 'Mytest2::mylib',
+ SKIP => [qw(all static static_lib dynamic dynamic_lib)],
+ clean => {'FILES' => 'libmylib$(LIB_EXT)'},
);
'
all :: static
+ pure_all :: static
+
static :: libmylib$(LIB_EXT)
libmylib$(LIB_EXT): $(O_FILES)
';
}
+Make sure you use a tab and not spaces on the lines beginning with "$(AR)"
+and "$(RANLIB)". Make will not function properly if you use spaces.
+It has also been reported that the "cr" argument to $(AR) is unnecessary
+on Win32 systems.
+
We will now create the main top-level Mytest2 files. Change to the directory
above Mytest2 and run the following command:
The normal Makefile.PL that h2xs generates doesn't know about the mylib
directory. We need to tell it that there is a subdirectory and that we
-will be generating a library in it. Let's add the following key-value
-pair to the WriteMakefile call:
+will be generating a library in it. Let's add the argument MYEXTLIB to
+the WriteMakefile call so that it looks like this:
- 'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
+ 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 a new replacement subroutine too:
+and then at the end add a subroutine (which will override the pre-existing
+subroutine). Remember to use a tab character to indent the line beginning
+with "cd"!
sub MY::postamble {
'
$(MYEXTLIB): mylib/Makefile
- cd mylib && $(MAKE)
+ cd mylib && $(MAKE) $(PASSTHRU)
';
}
-(Note: Most makes will require that there be a tab character that indents
-the line "cd mylib && $(MAKE)", similarly for the Makefile in the
-subdirectory.)
-
Let's also fix the MANIFEST file so that it accurately reflects the contents
of our extension. The single line that says "mylib" should be replaced by
the following three lines:
mylib/mylib.h
To keep our namespace nice and unpolluted, edit the .pm file and change
-the lines setting @EXPORT to @EXPORT_OK (there are two: one in the line
-beginning "use vars" and one setting the array itself). Finally, in the
+the variable C<@EXPORT> to C<@EXPORT_OK>. Finally, in the
.xs file, edit the #include line to read:
#include "mylib/mylib.h"
int a
long b
const char * c
- OUTPUT:
+ OUTPUT:
RETVAL
Now we also need to create a typemap file because the default Perl doesn't
-currently support the const char * type. Create a file called typemap and
-place the following in it:
+currently support the const char * type. Create a file called typemap in
+the Mytest2 directory and place the following in it:
const char * T_PV
Now run perl on the top-level Makefile.PL. Notice that it also created a
-Makefile in the mylib directory. Run make and see that it does cd into
+Makefile in the mylib directory. Run make and watch that it does cd into
the mylib directory and run make in there as well.
-Now edit the test.pl script and change the BEGIN block to print "1..4",
+Now edit the Mytest2.t script and change the number of tests to "4",
and add the following lines to the end of the script:
- print &Mytest2::foo(1, 2, "Hello, world!") == 7 ? "ok 2\n" : "not ok 2\n";
- print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 3\n" : "not ok 3\n";
- print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 ? "ok 4\n" : "not ok 4\n";
+ is( &Mytest2::foo(1, 2, "Hello, world!"), 7 );
+ is( &Mytest2::foo(1, 2, "0.0"), 7 );
+ ok( abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 );
-(When dealing with floating-point comparisons, it is often useful not to check
-for equality, but rather the difference being below a certain epsilon factor,
-0.01 in this case)
+(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.
+Run "C<make test>" and all should be well. There are some warnings on missing tests
+for the Mytest2::mylib extension, but you can ignore them.
-=head2 WHAT HAS HAPPENED HERE?
+=head2 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.
=item *
-In the .xs file, there's now a #include declaration with the full path to
-the mylib.h header file.
+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.
=item *
There's now some new C code that's been added to the .xs file. The purpose
of the C<constant> routine is to make the values that are #define'd in the
-header file available to the Perl script (in this case, by calling
-C<&main::TESTVAL>). There's also some XS code to allow calls to the
+header file accessible by the Perl script (by calling either C<TESTVAL> or
+C<&Mytest2::TESTVAL>). There's also some XS code to allow calls to the
C<constant> routine.
=item *
-The .pm file has 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 going to be used by only 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 remove most
-or all of the items in the @EXPORT array.
+The .pm file originally exported the name C<TESTVAL> in the C<@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 C<@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 C<@EXPORT> array into the C<@EXPORT_OK> array.
=item *
-If our include file contained #include directives, these would not be
-processed at all by h2xs. There is no good solution to this right now.
+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.
-=back
+=item *
We've also told Perl about the library that we built in the mylib
-subdirectory. That required the addition of only the MYEXTLIB variable
+subdirectory. That required only the addition of the C<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
-specified simply that the library to be created here was a static
-archive (as opposed to a dynamically loadable library) and provided the
+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.
-=head2 SPECIFYING ARGUMENTS TO XSUBPP
+=back
+
+=head2 Anatomy of .xs file
+
+The .xs file of L<"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 L<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 B<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 C<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 L<"EXAMPLE 1">, L<"EXAMPLE 2">,
+and L<"EXAMPLE 3">, when all the work is done inside the "Perl glue", is
+somewhat of an exception rather than the rule.
+
+=head2 Getting the fat out of XSUBs
+
+In L<"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 L<"EXAMPLE 1">, L<"EXAMPLE 2"> and L<"EXAMPLE 3">,
+this description does not contain the actual I<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: B<xsubpp>
+compiler is smart enough to figure out the C<CODE:> section from the first
+two lines of the description of XSUB. What about C<OUTPUT:> section? In
+fact, that is absolutely the same! The C<OUTPUT:> section can be removed
+as well, I<as far as C<CODE:> section or C<PPCODE:> section> is not
+specified: B<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 L<"EXAMPLE 2">? To do this, one needs to define a C function C<int
+is_even(int input)>. As we saw in L<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 C<#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 B<xsubpp> might have translated this to TCL glue or
+Python glue as well.)
+
+=head2 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.
+B<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 C<cp>
+and manipulate C<*cp> only (using the return value as, say, a success
+indicator). From Perl one would use these functions in
+a completely different manner.
-When you specify arguments in the .xs file, you are really passing three
-pieces of information for each one 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 exact way in
-which the argument should be used in the call to the library function
-from this XSUB. This would mean whether or not to place a "&" before
-the argument or not, meaning the argument expects to be passed the address
-of the specified data type.
+One conveys this info to B<xsubpp> by replacing C<*> before the
+argument by C<&>. C<&> means that the argument should be passed to a library
+function by its address. The above two function may be XSUB-ified as
-There is a difference between the two arguments in this hypothetical function:
+ 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 argument to this function would be treated as a char and assigned
+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 argument would be treated as a string pointer and assigned to the
+The second Perl argument would be treated as a string pointer and assigned to the
variable b. The I<value> of b would be passed into the function foo. The
-actual call to the function foo that xsubpp generates would look like this:
+actual call to the function foo that B<xsubpp> generates would look like this:
foo(&a, b);
-Xsubpp will identically parse the following function argument lists:
+B<xsubpp> will parse the following function argument lists identically:
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
-complete example 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".
+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.
-=head2 THE ARGUMENT STACK
+=head2 The Argument Stack
If we look at any of the C code generated by any of the examples except
example 1, you will notice a number of references to ST(n), where n is
-usually 0. The "ST" is actually a macro that points to the n'th argument
-on the argument stack. ST(0) is thus the first argument passed to the
-XSUB, ST(1) is the second argument, and so on.
+usually 0. "ST" is actually a macro that points to the n'th argument
+on the argument stack. ST(0) is thus the first argument on the stack and
+therefore the first argument passed to the XSUB, ST(1) is the second
+argument, and so on.
-When you list the arguments to the XSUB in the .xs file, that tells xsubpp
+When you list the arguments to the XSUB in the .xs file, that tells B<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.
-=head2 EXTENDING YOUR EXTENSION
+The actual values on the argument stack are pointers to the values passed
+in. When an argument is listed as being an OUTPUT value, its corresponding
+value on the stack (i.e., ST(0) if it was the first argument) is changed.
+You can verify this by looking at the C code generated for Example 3.
+The code for the round() XSUB routine contains lines that look like this:
+
+ double arg = (double)SvNV(ST(0));
+ /* Round the contents of the variable arg */
+ sv_setnv(ST(0), (double)arg);
+
+The arg variable is initially set by taking the value from ST(0), then is
+stored back into ST(0) at the end of the routine.
+
+XSUBs are also allowed to return lists, not just scalars. This must be
+done by manipulating stack values ST(0), ST(1), etc, in a subtly
+different way. See L<perlxs> for details.
+
+XSUBs are also allowed to avoid automatic conversion of Perl function arguments
+to C function arguments. See L<perlxs> for details. Some people prefer
+manual conversion by inspecting C<ST(i)> even in the cases when automatic
+conversion will do, arguing that this makes the logic of an XSUB call clearer.
+Compare with L<"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
+L<"Getting the fat out of XSUBs">. This approach has the additional
+benefit of protecting the XSUB writer from future changes to the Perl API.
+
+=head2 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 loaded only when called depends on where in the .pm file the subroutine
-definition is placed.
+or only loaded when called depends on where in the .pm file the subroutine
+definition is placed. You can also consult L<AutoLoader> for an alternate
+way to store and load your extra subroutines.
-=head2 DOCUMENTING YOUR EXTENSION
+=head2 Documenting your Extension
There is absolutely no excuse for not documenting your extension.
Documentation belongs in the .pm file. This file will be fed to pod2man,
-and the embedded documentation will be converted to the man page format,
-then placed in the blib directory. It will be copied to Perl's man
-page directory when the extension is installed.
+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,
See L<perlpod> for more information about the pod format.
-=head2 INSTALLING YOUR EXTENSION
+=head2 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
+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.
-=head2 SEE ALSO
+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.
+
+=head2 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":
+
+ @a = &Mytest::statfs("/blech");
+ ok( scalar(@a) == 1 && $a[0] == 2 );
+ @a = &Mytest::statfs("/");
+ is( scalar(@a), 7 );
+
+=head2 New Things in this Example
+
+This example added quite a few new concepts. We'll take them one at a time.
+
+=over 4
+
+=item *
+
+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 C<PPCODE:> section into braces, and
+put these declarations on top.)
+
+=item *
+
+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 B<xsubpp> that we will be managing the return values that will be
+put on the argument stack by ourselves.
+
+=item *
+
+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.
+
+=item *
+
+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.
+
+=item *
+
+If we were interested in performance, not in code compactness, in the success
+branch we would not use C<XPUSHs> macros, but C<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 C<PUSHs> I<without> extending
+the stack: the Perl function reference comes to an XSUB on the stack, thus
+the stack is I<always> large enough to take one return value.
+
+=back
+
+=head2 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;
+ I32 numpaths = 0;
+ int i, n;
+ struct statfs buf;
+
+ if ((!SvROK(paths))
+ || (SvTYPE(SvRV(paths)) != SVt_PVAV)
+ || ((numpaths = av_len((AV *)SvRV(paths))) < 0))
+ {
+ XSRETURN_UNDEF;
+ }
+ results = (AV *)sv_2mortal((SV *)newAV());
+ CODE:
+ for (n = 0; n <= numpaths; n++) {
+ HV * rh;
+ STRLEN l;
+ char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
+
+ i = statfs(fn, &buf);
+ if (i != 0) {
+ av_push(results, newSVnv(errno));
+ continue;
+ }
+
+ rh = (HV *)sv_2mortal((SV *)newHV());
+
+ hv_store(rh, "f_bavail", 8, newSVnv(buf.f_bavail), 0);
+ hv_store(rh, "f_bfree", 7, newSVnv(buf.f_bfree), 0);
+ hv_store(rh, "f_blocks", 8, newSVnv(buf.f_blocks), 0);
+ hv_store(rh, "f_bsize", 7, newSVnv(buf.f_bsize), 0);
+ hv_store(rh, "f_ffree", 7, newSVnv(buf.f_ffree), 0);
+ hv_store(rh, "f_files", 7, newSVnv(buf.f_files), 0);
+ hv_store(rh, "f_type", 6, newSVnv(buf.f_type), 0);
+
+ av_push(results, newRV((SV *)rh));
+ }
+ RETVAL = newRV((SV *)results);
+ OUTPUT:
+ RETVAL
+
+And add the following code to Mytest.t, while incrementing the "11"
+tests to "13":
+
+ $results = Mytest::multi_statfs([ '/', '/blech' ]);
+ ok( ref $results->[0] );
+ ok( ! ref $results->[1] );
+
+=head2 New Things in this Example
+
+There are a number of new concepts introduced here, described below:
+
+=over 4
+
+=item *
+
+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 C<PPCODE:> directive - instead, we use C<CODE:>
+and C<OUTPUT:> directives.
+
+=item *
+
+When dealing with references, it is important to handle them with caution.
+The C<INIT:> block first checks that
+C<SvROK> returns true, which indicates that paths is a valid reference. It
+then verifies that the object referenced by paths is an array, using C<SvRV>
+to dereference paths, and C<SvTYPE> to discover its type. As an added test,
+it checks that the array referenced by paths is non-empty, using the C<av_len>
+function (which returns -1 if the array is empty). The XSRETURN_UNDEF macro
+is used to abort the XSUB and return the undefined value whenever all three of
+these conditions are not met.
+
+=item *
+
+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: C<av_len> returns the highest
+index in an AV*, much like $#array; C<av_fetch> fetches a single scalar value
+from an array, given its index; C<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);
+
+=item *
+
+We are performing only one hash operation in this function, which is storing
+a new scalar under a key using C<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 L<perlguts> and L<perlapi>
+for details.
+
+=item *
+
+To create a reference, we use the C<newRV> function. Note that you can
+cast an AV* or an HV* to type SV* in this case (and many others). This
+allows you to take references to arrays, hashes and scalars with the same
+function. Conversely, the C<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 C<SvTYPE>).
+
+=item *
+
+At this point, xsubpp is doing very little work - the differences between
+Mytest.xs and Mytest.c are minimal.
+
+=back
+
+=head2 EXAMPLE 7 (Coming Soon)
+
+XPUSH args AND set RETVAL AND assign return value to array
+
+=head2 EXAMPLE 8 (Coming Soon)
+
+Setting $!
+
+=head2 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 C<fputs()>. This is all we need:
+
+ #define PERLIO_NOT_STDIO 0
+ #include "EXTERN.h"
+ #include "perl.h"
+ #include "XSUB.h"
+
+ #include <stdio.h>
+
+ int
+ fputs(s, stream)
+ char * s
+ FILE * stream
+
+The real work is done in the standard typemap.
+
+B<But> you loose all the fine stuff done by the perlio layers. This
+calls the stdio function C<fputs()>, which knows nothing about them.
-For more information, consult L<perlguts>, L<perlxs>, L<perlmod>,
+The standard typemap offers three variants of PerlIO *:
+C<InputStream> (T_IN), C<InOutStream> (T_INOUT) and C<OutputStream>
+(T_OUT). A bare C<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 I<from> perl the main difference is that
+C<OutputStream> will get the output PerlIO * - which may make
+a difference on a socket. Like in our example...
+
+For streams being handed I<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
+C<OutputStream> if open as "r" it should be an C<InputStream>.
+
+Now, suppose you want to use perlio layers in your XS. We'll use the
+perlio C<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 C<CODE> section because C<PerlIO_puts()> has the arguments
+reversed compared to C<fputs()>, and we want to keep the arguments the same.
+
+Wanting to explore this thoroughly, we want to use the stdio C<fputs()>
+on a PerlIO *. This means we have to ask the perlio system for a stdio
+C<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: C<PerlIO_findFILE()> will search the layers for a stdio
+layer. If it can't find one, it will call C<PerlIO_exportFILE()> to
+generate a new stdio C<FILE>. Please only call C<PerlIO_exportFILE()> if
+you want a I<new> C<FILE>. It will generate one on each call and push a
+new stdio layer. So don't call it repeatedly on the same
+file. C<PerlIO_findFILE()> will retrieve the stdio layer once it has been
+generated by C<PerlIO_exportFILE()>.
+
+This applies to the perlio system only. For versions before 5.7,
+C<PerlIO_exportFILE()> is equivalent to C<PerlIO_findFILE()>.
+
+=head2 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.
+
+=over 4
+
+=item *
+
+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';
+
+=item *
+
+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';
+
+=item *
+
+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") }
+
+=item *
+
+This document assumes that the executable named "perl" is Perl version 5.
+Some systems may have installed Perl version 5 as "perl5".
+
+=back
+
+=head1 See also
+
+For more information, consult L<perlguts>, L<perlapi>, L<perlxs>, L<perlmod>,
and L<perlpod>.
-=head2 Author
+=head1 Author
-Jeff Okamoto E<lt>F<okamoto@corp.hp.com>E<gt>
+Jeff Okamoto <F<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
+
=head2 Last Changed
-1996/7/10
+2007/10/11
https://perl5.git.perl.org / perl5.git / blobdiff