MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLXSTUT

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

NAME

perlXStut - Tutorial for XSUBs

DESCRIPTION

This tutorial will educate the reader on the steps involved in creatinga Perl extension. The reader is assumed to have access to the perlguts manpage andthe perlxs manpage.

This tutorial starts with very simple examples and becomes more complex,with each new example adding new features. Certain concepts may not becompletely explained until later in the tutorial to ease thereader slowly into building extensions.

VERSION CAVEAT

This tutorial tries hard to keep up with the latest development versionsof Perl. This often means that it is sometimes in advance of the latestreleased version of Perl, and that certain features described here mightnot work on earlier versions. This section will keep track of when variousfeatures were added to Perl 5.

*

In versions of Perl 5.002 prior to the gamma version, the test scriptin Example 1 will not function properly. You need to change the ``uselib'' line to read:

        use lib './blib';

*

In versions of Perl 5.002 prior to version beta 3, the line in the .xs fileabout ``PROTOTYPES: DISABLE'' will cause a compiler error. Simply remove thatline from the file.

*

In versions of Perl 5.002 prior to version 5.002b1h, the test.pl file was notautomatically 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 needto 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''.

DYNAMIC VERSUS STATIC

It is commonly thought that if a system does not have the capability toload a library dynamically, you cannot build XSUBs. This is incorrect.You can build them, but you must link the XSUB's subroutines with therest of Perl, creating a new executable. This situation is similar toPerl 4.

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

Should you wish to build a statically-linked executable on a system whichcan 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, theninstead of saying ``make test'', you should say ``make test_static''. On systemsthat cannot build dynamically-loadable libraries at all, simply saying ``maketest'' is sufficient.

EXAMPLE 1

Our first extension will be very simple. When we call the routine in theextension, it will print out a well-known message and return.

Run h2xs -A -n Mytest. This creates a directory named Mytest, possibly underext/ if that directory exists in the current working directory. Several fileswill be created in the Mytest dir, including MANIFEST, Makefile.PL, Mytest.pm,Mytest.xs, test.pl, and Changes.

The MANIFEST file contains the names of all the files created.

The file Makefile.PL should look something like this:

        use ExtUtils::MakeMaker;        # See lib/ExtUtils/MakeMaker.pm for details of how to influence        # the contents of the Makefile that is written.        WriteMakefile(            'NAME'      => 'Mytest',            'VERSION_FROM' => 'Mytest.pm', # finds $VERSION            'LIBS'      => [''],   # e.g., '-lm'            'DEFINE'    => '',     # e.g., '-DHAVE_SOMETHING'            'INC'       => '',     # e.g., '-I/usr/include/other'        );

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

        package Mytest;

        require Exporter;        require DynaLoader;

        @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(

        );        $VERSION = '0.01';

        bootstrap Mytest $VERSION;

        # Preloaded methods go here.

        # Autoload methods go after __END__, and are processed by the autosplit program.

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

And the Mytest.xs file should look something like this:

        #ifdef __cplusplus        extern "C" {        #endif        #include "EXTERN.h"        #include "perl.h"        #include "XSUB.h"        #ifdef __cplusplus        }        #endif

        PROTOTYPES: DISABLE

        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");

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 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 thisexample only, we'll create a special test script. Create a file called hellothat looks like this:

        #! /opt/perl5/bin/perl

        use ExtUtils::testlib;

        use Mytest;

        Mytest::hello();

Now we run the script and we should see the following output:

        % perl hello        Hello, world!        %

EXAMPLE 2

Now let's add to our extension a subroutine that will take a single argumentand return 1 if the argument is even, 0 if the argument 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 ofthat line is also optional.

Any white space may be between the ``int'' and ``input''. It is also okay forthe 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).

Now rerun make to rebuild our new shared library.

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

To test that our extension works, we now need to look at thefile test.pl. This file is set up to imitate the same kind of testingstructure that Perl itself has. Within the test script, you perform anumber of tests to confirm the behavior of the extension, printing ``ok''when the test is correct, ``not ok'' when it is not. Change the printstatement in the BEGIN block to print ``1..4'', and add the following codeto the end of the file:

        print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok 2", "\n";        print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok 3", "\n";        print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok 4", "\n";

We will be calling the test script through the command ``make test''. Youshould 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        %

WHAT HAS GONE ON?

The program h2xs is the starting point for creating extensions. In laterexamples we'll see how we can use h2xs to read header files and generatetemplates to connect to C routines.

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

The files <extension>.pm and <extension>.xs contain the meatof the extension.The .xs file holds the C routines that make up the extension. The .pm filecontains routines that tell Perl how to load your extension.

Generating and invoking the Makefile created a directory blib (which standsfor ``build library") in the current working directory. This directory willcontain the shared library that we will build. Once we have tested it, wecan install it into its final location.

Invoking the test script via ``make test'' did something very important. Itinvoked perl with all those -I arguments so that it could find the variousfiles that are part of the extension.

It is very important that while you are still testing extensions thatyou use ``make test''. If you try to run the test script all by itself, youwill get a fatal error.

Another reason it is important to use ``make test'' to run your test scriptis 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-existingversion.

When Perl sees a use extension;, it searches for a file with the same nameas 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 DynamicLoader extensions. It then sets the @ISA and @EXPORT arrays and the $VERSIONscalar; finally it tells perl to bootstrap the module. Perl will call itsdynamic 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 @ISAarray contains a list of other packages in which to search for methods (orsubroutines) that do not exist in the current package. The @EXPORT arraytells Perl which of the extension's routines should be placed into thecalling package's namespace.

It's important to select what to export carefully. Do NOT export method namesand do NOT export anything else by default without a good reason.

As a general rule, if the module is trying to be object-oriented then don'texport anything. If it's just a collection of functions then you can exportany of the functions via another array, called @EXPORT_OK.

See the perlmod manpage for more information.

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

WRITING GOOD TEST SCRIPTS

The importance of writing good test scripts cannot be overemphasized. Youshould closely follow the ``ok/not ok'' style that Perl itself uses, so thatit 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 usesthe correct version of your extension. If you have many test cases, youmight want to copy Perl's test style. Create a directory named ``t'', andensure all your test files end with the suffix ``.t''. The Makefile willproperly run all these test files.

EXAMPLE 3

Our third extension will take one argument as its input, round off thatvalue, and set the argument to the rounded value.

Add the following to the end of Mytest.xs:

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

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

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

Generate the Makefile and run make. Change the BEGIN block to print out``1..9'' and add the following to test.pl:

        $i = -1.5; &Mytest::round($i); print $i == -2.0 ? "ok 5" : "not ok 5", "\n";        $i = -1.1; &Mytest::round($i); print $i == -1.0 ? "ok 6" : "not ok 6", "\n";        $i = 0.0; &Mytest::round($i); print $i == 0.0 ? "ok 7" : "not ok 7", "\n";        $i = 0.5; &Mytest::round($i); print $i == 1.0 ? "ok 8" : "not ok 8", "\n";        $i = 1.2; &Mytest::round($i); print $i == 1.0 ? "ok 9" : "not ok 9", "\n";

Running ``make test'' should now print out that all nine tests are okay.

You might be wondering if you can round a constant. To see what happens, addthe following line to test.pl temporarily:

        &Mytest::round(3);

Run ``make test'' and notice that Perl dies with a fatal error. Perl won't letyou change the value of constants!

WHAT'S NEW HERE?

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 librarylibm. We'll talk later about how to write XSUBs that can call every routinein a library.

Second, the value of the function is being passed back not as the function'sreturn value, but through the same variable that was passed into the function.

INPUT AND OUTPUT PARAMETERS

You specify the parameters that will be passed into the XSUB just after youdeclare the function return value and name. Each parameter line starts withoptional white space, and may have an optional terminating semicolon.

The list of output parameters occurs after the OUTPUT: directive. The useof RETVAL tells Perl that you wish to send this value back as the returnvalue of the XSUB function. In Example 3, the value we wanted returned wascontained in the same variable we passed in, so we listed it (and not RETVAL)in the OUTPUT: section.

THE XSUBPP COMPILER

The compiler xsubpp takes the XS code in the .xs file and converts it intoC code, placing it in a file whose suffix is .c. The C code created makesheavy use of the C functions within Perl.

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 storedin the typemap file ($PERLLIB/ExtUtils/typemap). This file is split intothree parts.

The first part attempts to map various C data types to a coded flag, whichhas some correspondence with the various Perl types. The second part containsC code which xsubpp uses for input parameters. The third part contains Ccode which xsubpp uses for output parameters. We'll talk more about theC code later.

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

        XS(XS_Mytest_round)        {            dXSARGS;            if (items != 1)                croak("Usage: Mytest::round(arg)");            {                double  arg = (double)SvNV(ST(0));      /* XXXXX */                if (arg > 0.0) {                        arg = floor(arg + 0.5);                } else if (arg < 0.0) {                        arg = ceil(arg - 0.5);                } else {                        arg = 0.0;                }                sv_setnv(ST(0), (double)arg);   /* XXXXX */            }            XSRETURN(1);        }

Notice the two lines marked with ``XXXXX''. If you check the first section ofthe typemap file, you'll see that doubles are of type T_DOUBLE. In theINPUT section, an argument that is T_DOUBLE is assigned to the variablearg 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 tobe passed back to the calling subroutine. These two functions are explainedin the perlguts manpage; we'll talk more later about what that ``ST(0)'' means in thesection on the argument stack.

WARNING

In general, it's not a good idea to write extensions that modify their inputparameters, as in Example 3. However, to accommodate better callingpre-existing C routines, which often do modify their input parameters,this behavior is tolerated. The next example will show how to do this.

EXAMPLE 4

In this example, we'll now begin to write XSUBs that will interact withpredefined C libraries. To begin with, we will build a small library ofour own, then let h2xs write our .pm and .xs files for us.

Create a new directory called Mytest2 at the same level as the directoryMytest. 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 willinclude a C source file and a header file. We'll also create a Makefile.PLin this directory. Then we'll make sure that running make at the Mytest2level will automatically run this Makefile.PL file and the resulting Makefile.

In the testlib 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(a, b, c)        int             a;        long            b;        const char *    c;        {                return (a + b + atof(c) + TESTVAL);        }

And finally create a file Makefile.PL that looks like this:

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

        sub MY::top_targets {                '        all :: static

        static ::       libmylib$(LIB_EXT)

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

        ';        }

We will now create the main top-level Mytest2 files. Change to the directoryabove 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 mylibdirectory. We need to tell it that there is a subdirectory and that wewill be generating a library in it. Let's add the following key-valuepair to the WriteMakefile call:

        'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',

and a new replacement subroutine too:

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

(Note: Most makes will require that there be a tab character that indentsthe line cd mylib && $(MAKE) $(PASTHRU), similarly for the Makefile in thesubdirectory.)

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

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

To keep our namespace nice and unpolluted, edit the .pm file and changethe lines setting @EXPORT to @EXPORT_OK (there are two: one in the linebeginning ``use vars'' and one setting the array itself). Finally, in the.xs file, edit the #include line to read:

        #include "mylib/mylib.h"

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

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

Now we also need to create a typemap file because the default Perl doesn'tcurrently support the const char * type. Create a file called typemap andplace the following in it:

        const char *    T_PV

Now run perl on the top-level Makefile.PL. Notice that it also created aMakefile in the mylib directory. Run make and see that it does cd intothe mylib directory and run make in there as well.

Now edit the test.pl script and change the BEGIN block to print ``1..4'',and add the following lines to the end of the script:

        print &Mytest2::foo(1, 2, "Hello, world!") == 7 ? "ok 2\n" : "not ok 2\n";        print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 3\n" : "not ok 3\n";        print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 ? "ok 4\n" : "not ok 4\n";

(When dealing with floating-point comparisons, it is often useful not to checkfor equality, but rather the difference being below a certain epsilon factor,0.01 in this case)

Run ``make test'' and all should be well.

WHAT HAS HAPPENED HERE?

Unlike previous examples, we've now run h2xs on a real include file. Thishas caused some extra goodies to appear in both the .pm and .xs files.

*: In the .xs file, there's now a #include declaration with the full path tothe mylib.h header file.
*: There's now some new C code that's been added to the .xs file. The purposeof the constant routine is to make the values that are #define'd in theheader file available to the Perl script (in this case, by calling&main::TESTVAL). There's also some XS code to allow calls to theconstant routine.
*: The .pm file has exported the name TESTVAL in the @EXPORT array. Thiscould lead to name clashes. A good rule of thumb is that if the #defineis 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'tmind using the ``fully qualified name'' of a variable, you could remove mostor all of the items in the @EXPORT array.
*: If our include file contained #include directives, these would not beprocessed at all by h2xs. There is no good solution to this right now.

We've also told Perl about the library that we built in the mylibsubdirectory. That required the addition of only the MYEXTLIB variableto the WriteMakefile call and the replacement of the postamble subroutineto cd into the subdirectory and run make. The Makefile.PL for thelibrary is a bit more complicated, but not excessively so. Again wereplaced the postamble subroutine to insert our own code. This codespecified simply that the library to be created here was a staticarchive (as opposed to a dynamically loadable library) and provided thecommands to build it.

SPECIFYING ARGUMENTS TO XSUBPP

With the completion of Example 4, we now have an easy way to simulate somereal-life libraries whose interfaces may not be the cleanest in the world.We shall now continue with a discussion of the arguments passed to thexsubpp compiler.

When you specify arguments in the .xs file, you are really passing threepieces of information for each one listed. The first piece is the orderof that argument relative to the others (first, second, etc). The secondis the type of argument, and consists of the type declaration of theargument (e.g., int, char*, etc). The third piece is the exact way inwhich the argument should be used in the call to the library functionfrom this XSUB. This would mean whether or not to place a ``&'' beforethe argument or not, meaning the argument expects to be passed the addressof the specified data type.

There is a difference between the two arguments in this hypothetical function:

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

The first argument to this function would be treated as a char and assignedto 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 thevariable b. The value of b would be passed into the function foo. Theactual call to the function foo that xsubpp generates would look like this:

        foo(&a, b);

Xsubpp will identically parse the following function argument lists:

        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 thecomplete example above). By doing so, it is easy to understand exactlywhat 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 variableit 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 exceptexample 1, you will notice a number of references to ST(n), where n isusually 0. The ``ST'' is actually a macro that points to the n'th argumenton the argument stack. ST(0) is thus the first argument passed to theXSUB, ST(1) is the second argument, and so on.

When you list the arguments to the XSUB in the .xs file, that tells xsubppwhich argument corresponds to which of the argument stack (i.e., the firstone listed is the first argument, and so on). You invite disaster if youdo not list them in the same order as the function expects them.

EXTENDING YOUR EXTENSION

Sometimes you might want to provide some extra methods or subroutinesto assist in making the interface between Perl and your extension simpleror easier to understand. These routines should live in the .pm file.Whether they are automatically loaded when the extension itself is loadedor loaded only when called depends on where in the .pm file the subroutinedefinition is placed.

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 the perlpod manpage for more information about the pod format.

INSTALLING YOUR EXTENSION

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

Author

Jeff Okamoto <okamotoAATTcorp.hp.com>

Reviewed and assisted by Dean Roehrich, Ilya Zakharevich, Andreas Koenig,and Tim Bunce.

Last Changed

1996/7/10

Index

NAME
DESCRIPTION
VERSION CAVEAT
DYNAMIC VERSUS STATIC
EXAMPLE 1
EXAMPLE 2
WHAT HAS GONE ON?
WRITING GOOD TEST SCRIPTS
EXAMPLE 3
WHAT'S NEW HERE?
INPUT AND OUTPUT PARAMETERS
THE XSUBPP COMPILER
THE TYPEMAP FILE
WARNING
EXAMPLE 4
WHAT HAS HAPPENED HERE?
SPECIFYING ARGUMENTS TO XSUBPP
THE ARGUMENT STACK
EXTENDING YOUR EXTENSION
DOCUMENTING YOUR EXTENSION
INSTALLING YOUR EXTENSION
SEE ALSO
Author
Last Changed

This document was created byman2html,using the manual pages.

PERLXSTUT

NAME

DESCRIPTION

VERSION CAVEAT

DYNAMIC VERSUS STATIC

EXAMPLE 1

EXAMPLE 2

WHAT HAS GONE ON?

WRITING GOOD TEST SCRIPTS

EXAMPLE 3

WHAT'S NEW HERE?

INPUT AND OUTPUT PARAMETERS

THE XSUBPP COMPILER

THE TYPEMAP FILE

WARNING

EXAMPLE 4

WHAT HAS HAPPENED HERE?

SPECIFYING ARGUMENTS TO XSUBPP

THE ARGUMENT STACK

EXTENDING YOUR EXTENSION

DOCUMENTING YOUR EXTENSION

INSTALLING YOUR EXTENSION

SEE ALSO

Author

Last Changed

Index