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perlXStut - Tutorial for writing XSUBs
This tutorial will educate the reader on the steps involved in creating
a Perl extension. The reader is assumed to have access to the perlguts manpage,
the perlapi manpage and the perlxs manpage.
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.
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.
When writing a Perl extension for general consumption, one should expect that
the extension will be used with versions of Perl different from the
version available on your machine. Since you are reading this document,
the version of Perl on your machine is probably 5.005 or later, but the users
of your extension may have more ancient versions.
To understand what kinds of incompatibilities one may expect, and in the rare
case that the version of Perl on your machine is older than this document,
see the section on "Troubleshooting these Examples" for more information.
If your extension uses some features of Perl which are not available on older
releases of Perl, your users would appreciate an early meaningful warning.
You would probably put this information into the README file, but nowadays
installation of extensions may be performed automatically, guided by CPAN.pm
module or other tools.
In MakeMaker-based installations, Makefile.PL provides the earliest
opportunity to perform version checks. One can put something like this
in Makefile.PL for this purpose:
eval { require 5.007 }
or die <<EOD;
############
### This module uses frobnication framework which is not available before
### version 5.007 of Perl. Upgrade your Perl before installing Kara::Mba.
############
EOD
It is commonly thought that if a system does not have the capability to
dynamically load a library, you cannot build XSUBs. This is incorrect.
You can build them, but you must link the XSUBs subroutines with the
rest of Perl, creating a new executable. This situation is similar to
Perl 4.
This tutorial can still be used on such a system. The XSUB build mechanism
will check the system and build a dynamically-loadable library if possible,
or else a static library and then, optionally, a new statically-linked
executable with that static library linked in.
Should you wish to build a statically-linked executable on a system which
can dynamically load libraries, you may, in all the following examples,
where the command "make" with no arguments is executed, run the command
"make perl" instead.
If you have generated such a statically-linked executable by choice, then
instead of saying "make test", you should say "make test_static".
On systems that cannot build dynamically-loadable libraries at all, simply
saying "make test" is sufficient.
Now let's go on with the show!
Our first extension will be very simple. When we call the routine in the
extension, it will print out a well-known message and return.
Run "h2xs -A -n Mytest". This creates a directory named Mytest,
possibly under ext/ if that directory exists in the current working
directory. Several files will be created 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 just created in the
Mytest directory.
The file Makefile.PL should look something like this:
use ExtUtils::MakeMaker;
WriteMakefile(
NAME => 'Mytest',
VERSION_FROM => 'Mytest.pm',
LIBS => [''],
DEFINE => '',
INC => '',
);
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);
our @EXPORT = qw(
);
our $VERSION = '0.01';
bootstrap Mytest $VERSION;
1;
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:
MODULE = Mytest PACKAGE = Mytest
Let's edit the .xs file by adding this to the end of the file:
void
hello()
CODE:
printf("Hello, world!\n");
It is okay for the lines starting at the "CODE:" line to not be indented.
However, for readability purposes, it is suggested that you indent CODE:
one level and the lines following one more level.
Now we'll run "perl Makefile.PL". This will create a real Makefile,
which make needs. Its output looks something like:
% perl Makefile.PL
Checking if your kit is complete...
Looks good
Writing Makefile for Mytest
%
Now, running make will produce output that looks something like this (some
long lines have been shortened for clarity and some extraneous lines have
been deleted):
% make
umask 0 && cp Mytest.pm ./blib/Mytest.pm
perl xsubpp -typemap typemap Mytest.xs >Mytest.tc && mv Mytest.tc Mytest.c
Please specify prototyping behavior for Mytest.xs (see perlxs manual)
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
Manifying ./blib/man3/Mytest.3
%
You can safely ignore the line about "prototyping behavior" - it is
explained in the section "The PROTOTYPES: Keyword" in the perlxs manpage.
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:
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!
%
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 whitespace at the start of the "int input"
line, but it is useful for improving readability. Placing a semi-colon at
the end of that line is also optional. Any amount and kind of whitespace
may be placed between the "int" and "input".
Now re-run make to rebuild our new shared library.
Now perform the same steps as before, generating a Makefile from the
Makefile.PL file, and running make.
In order to test that our extension works, we now need to look at the
file 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 &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". You
should see output that looks something like this:
% 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
%
The program h2xs is the starting point for creating extensions. In later
examples we'll see how we can use h2xs to read header files and generate
templates to connect to C routines.
h2xs creates a number of files in the extension directory. The file
Makefile.PL is a perl script which will generate a true Makefile to build
the extension. We'll take a closer look at it later.
The .pm and .xs files contain the meat of the extension. The .xs file holds
the C routines that make up the extension. The .pm file contains routines
that tell Perl how to load your extension.
Generating the Makefile and running make created a directory called blib
(which stands for "build library") in the current working directory. This
directory will contain the shared library that we will build. Once we have
tested it, we can install it into its final location.
Invoking the test script via "make test" did something very important.
It invoked perl with all those -I arguments so that it could find the
various files that are part of the extension. It is very important that
while you are still testing extensions that you use "make test". If you
try to run the test script all by itself, you will get a fatal error.
Another reason it is important to use "make test" to run your test
script is that if you are testing an upgrade to an already-existing version,
using "make test" ensures that you will test your new extension, not the
already-existing version.
When Perl sees a use extension;, it searches for a file with the same name
as the use'd extension that has a .pm suffix. If that file cannot be found,
Perl dies with a fatal error. The default search path is contained in the
@INC array.
In our case, Mytest.pm tells perl that it will need the Exporter and Dynamic
Loader extensions. It then sets the @ISA and @EXPORT arrays and the
$VERSION scalar; finally it tells perl to bootstrap the module. Perl
will call its dynamic loader routine (if there is one) and load the shared
library.
The two arrays @ISA and @EXPORT are very important. The @ISA
array contains a list of other packages in which to search for methods (or
subroutines) that do not exist in the current package. This is usually
only important for object-oriented extensions (which we will talk about
much later), and so usually doesn't need to be modified.
The @EXPORT array tells Perl which of the extension's variables and
subroutines should be placed into the calling package's namespace. Because
you don't know if the user has already used your variable and subroutine
names, it's vitally important to carefully select what to export. Do not
export method or variable names by default without a good reason.
As a general rule, if the module is trying to be object-oriented then don't
export anything. If it's just a collection of functions and variables, then
you can export them via another array, called @EXPORT_OK. This array
does not automatically place its subroutine and variable names into the
namespace unless the user specifically requests that this be done.
See the perlmod manpage 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.
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", all of these test files will be executed.
Our third extension will take one argument as its input, round off that
value, and set the argument to the rounded value.
Add the following to the end of Mytest.xs:
void
round(arg)
double arg
CODE:
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
arg = ceil(arg - 0.5);
} else {
arg = 0.0;
}
OUTPUT:
arg
Edit the Makefile.PL file so that the corresponding line looks like this:
'LIBS' => ['-lm'], # e.g., '-lm'
Generate the Makefile and run make. Change the BEGIN block to print
"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 $i == 1.0 ? "ok 9" : "not ok 9", "\n";
Running "make test" should now print out that all nine tests are okay.
Notice that in these new test cases, the argument passed to round was a
scalar variable. You might be wondering if you can round a constant or
literal. To see what happens, temporarily add the following line to test.pl:
&Mytest::round(3);
Run "make test" and notice that Perl dies with a fatal error. Perl won't
let you change the value of constants!
-
We've made some changes to Makefile.PL. In this case, we've specified an
extra library to be linked into the extension's shared library, the math
library libm in this case. We'll talk later about how to write XSUBs that
can call every routine in a library.
-
The value of the function is not being passed back as the function's return
value, but by changing the value of the variable that was passed into the
function. You might have guessed that when you saw that the return value
of round is of type "void".
You specify the parameters that will be passed into the XSUB on the line(s)
after you declare the function's return value and name. Each input parameter
line starts with optional whitespace, and may have an optional terminating
semicolon.
The list of output parameters occurs at the very end of the function, just
before after the OUTPUT: directive. The use of RETVAL tells Perl that you
wish to send this value back as the return value of the XSUB function. In
Example 3, we wanted the "return value" placed in the original variable
which we passed in, so we listed it (and not RETVAL) in the OUTPUT: section.
The xsubpp program takes the XS code in the .xs file and translates it into
C code, placing it in a file whose suffix is .c. The C code created makes
heavy use of the C functions within Perl.
The xsubpp program uses rules to convert from Perl's data types (scalar,
array, etc.) to C's data types (int, char, etc.). These rules are stored
in the typemap file ($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("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 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 the perlguts manpage; we'll talk more later about what that "ST(0)" means in the
section on the argument stack.
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.
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:
extern double foo(int, long, const char*);
Also create a file mylib.c that looks like this:
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;
$Verbose = 1;
WriteMakefile(
NAME => 'Mytest2::mylib',
SKIP => [qw(all static static_lib dynamic dynamic_lib)],
clean => {'FILES' => 'libmylib$(LIB_EXT)'},
);
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, bu | 
