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PERLXSTUT
!!!PERLXSTUT
NAME
DESCRIPTION
SPECIAL NOTES
TUTORIAL
See also
Author
----
!!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


__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


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
!!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
MANIFEST , Makefile.PL, Mytest.pm, Mytest.xs,
test.pl, and Changes.


The MANIFEST file contains the names of all
the files just created in the Mytest directory.


The file Makefile.PL should look something like
this:


        use !ExtUtils::!MakeMaker;
# See lib/!ExtUtils/!MakeMaker.pm for details of how to influence
# the contents of the Makefile that is written.
!WriteMakefile(
NAME         =
The file Mytest.pm should start with something like this:


        package Mytest;
        use strict;
use warnings;
        require Exporter;
require !DynaLoader;
        our @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.
our @EXPORT = qw(
        );
our $VERSION = '0.01';
        bootstrap Mytest $VERSION;
        # Preloaded methods go here.
        # Autoload methods go after __END__, and are processed by the autosplit program.
        1;
__END__
# Below is the stub of documentation for your module. You better edit it!
The rest of the .pm file contains sample code for providing documentation for the extension.


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


        #include
        MODULE = Mytest         PACKAGE = Mytest
Let's edit the .xs file by adding this to the end of the file:


        void
hello()
CODE:
printf(
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


        % 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
umask 0
You can safely ignore the line about ``prototyping behavior'' - it is explained in the section ``The PROTOTYPES: Keyword'' in perlxs.


If you are on a Win32 system, and the build process fails
with linker errors for functions in the C library, check if
your Perl is configured to use PerlCRT (running __perl
-V:libc__ should show you if this is the case). If Perl is
configured to use PerlCRT, you have to make sure PerlCRT.lib
is copied to the same location that msvcrt.lib lives in, so
that the compiler can find it on its own. msvcrt.lib is
usually found in the Visual C compiler's lib directory (e.g.
C:/!DevStudio/VC/lib).


Perl has its own special way of easily writing test scripts,
but for this example only, we'll create our own test script.
Create a file called hello that looks like
this:


        #! /opt/perl5/bin/perl
        use !ExtUtils::testlib;
        use Mytest;
        Mytest::hello();
Now we make the script executable (chmod -x hello), run the script and we should see the following output:


        % ./hello
Hello, world!
%


__EXAMPLE 2__


Now let's add to our extension a subroutine that will take a
single numeric argument as input and return 0 if the number
is even or 1 if the number is odd.


Add the following to the end of Mytest.xs:


        int
is_even(input)
int     input
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. Placing a semi-colon at the end of that line is also optional. Any amount and kind of white space 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 test.pl. This file is set up to imitate the
same kind of testing structure that Perl itself has. Within
the test script, you perform a number of tests to confirm
the behavior of the extension, printing ``ok'' when the test
is correct, ``not ok'' when it is not. Change the print
statement in the BEGIN block to print
``1..4'', and add the following code to the end of the
file:


        print
We will be calling the test script through the command make test


        % make test
PERL_DL_NONLAZY=1 /opt/perl5.004/bin/perl (lots of -I arguments) test.pl
1..4
ok 1
ok 2
ok 3
ok 4
%


__What has gone on?__


The program h2xs is the starting point for creating
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
-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


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
overemphasized. You should closely follow the ``ok/not ok''
style that Perl itself uses, so that it is very easy and
unambiguous to determine the outcome of each test case. When
you find and fix a bug, make sure you add a test case for
it.


By running make test``, you ensure that your
test.pl script runs and uses the correct version of your
extension. If you have many test cases, you might want to
copy Perl's test style. Create a directory named ''t`` in
the extension's directory and append the suffix ''.t`` to
the names of your test files. When you run ''make
test


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


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


        $i = -1.5;
Running make test


Notice that in these new test cases, the argument passed to
round was a scalar variable. You might be wondering if you
can round a constant or literal. To see what happens,
temporarily add the following line to test.pl:



Run make test


__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 white space, and may have an optional
terminating semicolon.


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


__The XSUBPP Program__


The __xsubpp__ program takes the XS code
in the .xs file and translates it into C code, placing it in
a file whose 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). This file is split into three
parts.


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


Let's take a look at a portion of the .c file created for
our extension. The file name is Mytest.c:


        XS(XS_Mytest_round)
{
dXSARGS;
if (items != 1)
croak(
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, 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)'' 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
        double
foo(int a, long b, const char *c)
{
return (a + b + atof(c) + TESTVAL);
}
And finally create a file Makefile.PL that looks like this:


        use !ExtUtils::!MakeMaker;
$Verbose = 1;
!WriteMakefile(
NAME   =
        sub MY::top_targets {
'
all :: static
        pure_all :: static
        static ::       libmylib$(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'      =
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
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/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
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't 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 watch that it does cd into the mylib directory and run make in there as well.


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


        print
(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


__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 ). 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, the 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 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
For example, consider:


        int
foo(a,b)
char
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(
__xsubpp__ will parse the following function argument lists identically:


        char
However, to help ease understanding, it is suggested that you place a ``


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


__The Argument Stack__


If we look at any of the C code generated by any of the
examples except example 1, you will notice a number of
references to ST (n), where n is usually 0.
`` ST '' is 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__ which argument
corresponds to which of the argument stack (i.e., the first
one listed is the first argument, and so on). You invite
disaster if you do not list them in the same order as the
function expects them.


The actual values on the argument stack are pointers to the
values passed in. When an argument is listed as being an
OUTPUT value, its corresponding value on the
stack (i.e., ''ST'' (0) if it was the
first argument) is changed. You can verify this by looking
at the C code generated for Example 3. The code for the
''round()'' XSUB routine contains lines
that look like this:


        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 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 man page format, then placed in the blib
directory. It will be copied to Perl's man page directory
when the extension is installed.


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


See perlpod for more information about the pod
format.


__Installing your Extension__


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


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,
You'll also need to add the following code to the top of the .xs file, just after the include of `` XSUB .h'':


        #include
Also add the following code segment to test.pl while incrementing the ``1..9'' string in the BEGIN block to ``1..11'':


        @a =


__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
9-element array is returned. Since only one argument is
passed into this function, we need room on the stack to hold
the 9 values which may be returned.


We do this by using the PPCODE: directive,
rather than the CODE: directive. This tells
__xsubpp__ that we will be managing the return values
that will be put on the argument stack by
ourselves.


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


The XPUSH* macros will automatically extend the return stack
to prevent it from being overrun. You push 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, 9);
The tradeoff is that one needs to calculate the number of return values in advance (though overextending the stack will not typically hurt anything but memory 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;
I32 numpaths = 0;
int i, n;
struct statfs buf;
                if ((!SvROK(paths))
(SvTYPE(SvRV(paths)) != SVt_PVAV)
((numpaths = av_len((AV *)SvRV(paths)))
                    i = statfs(fn,
                    rh = (HV *)sv_2mortal((SV *)newHV());
                    hv_store(rh,
                    av_push(results, newRV((SV *)rh));
}
RETVAL = newRV((SV *)results);
OUTPUT:
RETVAL
And add the following code to test.pl, while incrementing the ``1..11'' string in the BEGIN block to ``1..13'':


        $results = Mytest::multi_statfs([[ '/', '/blech' ]);
print ((ref $results-


__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 checks that
SvROK returns true, which indicates that paths is a
valid reference. It then verifies that the object referenced
by paths is an array, using SvRV to dereference
paths, and SvTYPE to discover its type. As an added
test, it checks that the array referenced by paths is
non-empty, using the av_len function (which returns
-1 if the array is empty). The XSRETURN_UNDEF
macro is used to abort the XSUB and return
the undefined value whenever all three of these conditions
are not met.


We manipulate several arrays in this XSUB .
Note that an array is represented internally by an AV*
pointer. The functions and macros for manipulating arrays
are similar to the functions in Perl: av_len
returns the highest index in an AV*, much like $#array;
av_fetch fetches a single scalar value from an
array, given its index; av_push pushes a scalar
value onto the end of the array, automatically extending the
array as 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 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 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 (Coming Soon)__


Getting fd's from filehandles


__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,


This document assumes that the executable named ``perl'' is
Perl version 5. Some systems may have installed Perl version
5 as ``perl5''.
!!See also


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


Jeff Okamoto okamoto@corp.hp.com''''


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


__Last Changed__


1999/11/30
----
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Last edited on Monday, June 3, 2002 6:50:54 pm by "perry"
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