perlxstut.pod

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Run "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?

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.

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.

=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.

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.

=head2 THE XSUBPP COMPILER

The compiler xsubpp takes the XS code in the .xs file and converts 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

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
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.

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 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 L<perlguts>; we'll talk more later about what that "ST(0)" means in the
section on the argument stack.

=head2 WARNING

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.

=head2 EXAMPLE 4

In this example, we'll now begin to write XSUBs that will interact with
predefined 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 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 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 following key-value
pair 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 indents
the line C<cd mylib && $(MAKE) $(PASTHRU)>, 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/Makefile.PL
	mylib/mylib.c
	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
.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't
currently support the const char * type.  Create a file called typemap 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
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 &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 check
for equality, but rather the difference being below a certain epsilon factor,
0.01 in this case)

Run "make test" and all should be well.

=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.

=over 4

=item *

In the .xs file, there's now a #include declaration with the full path to
the mylib.h header file.

=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
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.

=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.

=back

We've also told Perl about the library that we built in the mylib
subdirectory.  That required the addition of only 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
specified simply that the library to be created here was a static
archive (as opposed to a dynamically loadable library) and provided the
commands to build it.

=head2 SPECIFYING ARGUMENTS TO XSUBPP

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 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.

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 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
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:

	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 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".

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

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.

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.

=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.

=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 manpage 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 L<perlpod> for more information about the pod format.

=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
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

For more information, consult L<perlguts>, L<perlxs>, L<perlmod>,
and L<perlpod>.

=head2 Author

Jeff Okamoto <F<[e-mail]>>

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

=head2 Last Changed

1996/7/10