perlxstut(1)
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 <<EOD;
############
### This module uses frobnication framework which is not available
### before version 5.007 of Perl. Upgrade your Perl before
### installing Kara::Mba.
############
EOD
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"" 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.
Threads and PERL_NO_GET_CONTEXT
For threaded builds, perl requires the context pointer for the current
thread, without "PERL_NO_GET_CONTEXT", perl will call a function to
retrieve the context.
For improved performance, include:
#define PERL_NO_GET_CONTEXT
as shown below.
For more details, see perlguts.
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"". This creates a directory named Mytest,
possibly under ext/ if that directory exists in the current working
directory. Several files will be created under the Mytest dir,
including MANIFEST, Makefile.PL, lib/Mytest.pm, Mytest.xs, t/Mytest.t,
and Changes.
The MANIFEST file contains the names of all the files just created in
the Mytest directory.
The file Makefile.PL should look something like this:
use ExtUtils::MakeMaker;
# See lib/ExtUtils/MakeMaker.pm for details of how to influence
# the contents of the Makefile that is written.
WriteMakefile(
NAME => 'Mytest',
VERSION_FROM => 'Mytest.pm', # finds $VERSION
LIBS => [''], # e.g., '-lm'
DEFINE => '', # e.g., '-DHAVE_SOMETHING'
INC => '', # e.g., '-I/usr/include/other'
);
The file Mytest.pm should start with something like this:
package Mytest;
use 5.008008;
use strict;
use warnings;
require Exporter;
our @ISA = qw(Exporter);
our %EXPORT_TAGS = ( 'all' => [ qw(
) ] );
our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
our @EXPORT = qw(
);
our $VERSION = '0.01';
require XSLoader;
XSLoader::load('Mytest', $VERSION);
# Preloaded methods go here.
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:
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"
MODULE = Mytest PACKAGE = Mytest
Let's edit the .xs file by adding this to the end of the file:
void
hello()
CODE:
printf("Hello, world!\n");
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
cp lib/Mytest.pm blib/lib/Mytest.pm
perl xsubpp -typemap typemap Mytest.xs > Mytest.xsc && \
mv Mytest.xsc Mytest.c
Please specify prototyping behavior for Mytest.xs (see perlxs manual)
cc -c Mytest.c
Running Mkbootstrap for Mytest ()
chmod 644 Mytest.bs
rm -f blib/arch/auto/Mytest/Mytest.so
cc -shared -L/usr/local/lib Mytest.o -o blib/arch/auto/Mytest/Mytest.so
chmod 755 blib/arch/auto/Mytest/Mytest.so
cp Mytest.bs blib/arch/auto/Mytest/Mytest.bs
chmod 644 blib/arch/auto/Mytest/Mytest.bs
Manifying blib/man3/Mytest.3pm
%
You can safely ignore the line about "prototyping behavior" - it is
explained in "The PROTOTYPES: Keyword" in perlxs.
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 1 if the number is even or 0 if
the number is odd.
Add the following to the end of Mytest.xs:
int
is_even(input)
int input
CODE:
RETVAL = (input % 2 == 0);
OUTPUT:
RETVAL
There does not need to be 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 Mytest.t. This file is set up to imitate the same kind of testing
structure that Perl itself has. Within the test script, you perform a
number of tests to confirm the behavior of the extension, printing "ok"
when the test is correct, "not ok" when it is not.
use Test::More tests => 4;
BEGIN { use_ok('Mytest') };
#########################
# Insert your test code below, the Test::More module is use()ed here
# so read its man page ( perldoc Test::More ) for help writing this
# test script.
is(&Mytest::is_even(0), 1);
is(&Mytest::is_even(1), 0);
is(&Mytest::is_even(2), 1);
We will be calling the test script through the command ""make test"".
You should see output that looks something like this:
%make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e"
"test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/Mytest....ok
All tests successful.
Files=1, Tests=4, 0 wallclock secs ( 0.03 cusr + 0.00 csys = 0.03 CPU)
%
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"" 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 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 over-emphasized.
You should closely follow the "ok/not ok" style that Perl itself uses,
so that it is very easy and unambiguous to determine the outcome of
each test case. When you find and fix a bug, make sure you add a test
case for it.
By running ""make test"", you ensure that your Mytest.t script runs and
uses the correct version of your extension. If you have many test
cases, save your test files in the "t" directory and use the suffix
".t". When you run ""make test"", all of these test files will be
executed.
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 > 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 test number in Mytest.t
to "9" and add the following tests:
$i = -1.5; &Mytest::round($i); is( $i, -2.0 );
$i = -1.1; &Mytest::round($i); is( $i, -1.0 );
$i = 0.0; &Mytest::round($i); is( $i, 0.0 );
$i = 0.5; &Mytest::round($i); is( $i, 1.0 );
$i = 1.2; &Mytest::round($i); is( $i, 1.0 );
Running ""make test"" should now print out that all nine tests are
okay.
Notice that in these new test cases, the argument passed to round was a
scalar variable. You might be wondering if you can round a constant or
literal. To see what happens, temporarily add the following line to
Mytest.t:
&Mytest::round(3);
Run ""make test"" and notice that Perl dies with a fatal error. Perl
won't let you change the value of constants!
What's new here?
* We've made some changes to Makefile.PL. In this case, we've
specified an extra library to be linked into the extension's shared
library, the math library libm in this case. We'll talk later
about how to write XSUBs that can call every routine in a library.
* The value of the function is not being passed back as the
function's return value, but by changing the value of the variable
that was passed into the function. You might have guessed that
when you saw that the return value of round is of type "void".
Input and Output Parameters
You specify the parameters that will be passed into the XSUB on the
line(s) after you declare the function's return value and name. Each
input parameter line starts with optional whitespace, and may have an
optional terminating semicolon.
The list of output parameters occurs at the very end of the function,
just after the OUTPUT: directive. The use of RETVAL tells Perl that
you wish to send this value back as the return value of the XSUB
function. In Example 3, we wanted the "return value" placed in the
original variable which we passed in, so we listed it (and not RETVAL)
in the OUTPUT: section.
The XSUBPP Program
The xsubpp program takes the XS code in the .xs file and translates it
into C code, placing it in a file whose suffix is .c. The C code
created makes heavy use of the C functions within Perl.
The TYPEMAP file
The xsubpp program uses rules to convert from Perl's data types
(scalar, array, etc.) to C's data types (int, char, etc.). These rules
are stored in the typemap file ($PERLLIB/ExtUtils/typemap). There's a
brief discussion below, but all the nitty-gritty details can be found
in perlxstypemap. If you have a new-enough version of perl (5.16 and
up) or an upgraded XS compiler ("ExtUtils::ParseXS" 3.13_01 or better),
then you can inline typemaps in your XS instead of writing separate
files. Either way, this typemap thing 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)
Perl_croak(aTHX_ "Usage: Mytest::round(arg)");
PERL_UNUSED_VAR(cv); /* -W */
{
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 */
SvSETMAGIC(ST(0));
}
XSRETURN_EMPTY;
}
Notice the two lines commented with "XXXXX". If you check the first
part of the typemap file (or section), you'll see that doubles are of
type T_DOUBLE. In the INPUT part of the typemap, 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 <stdlib.h>
#include "./mylib.h"
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, 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' => 'Mytest2',
'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
'LIBS' => [''], # e.g., '-lm'
'DEFINE' => '', # e.g., '-DHAVE_SOMETHING'
'INC' => '', # e.g., '-I/usr/include/other'
'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
);
and then at the end add a subroutine (which will override the pre-
existing subroutine). Remember to use a tab character to indent the
line beginning with "cd"!
sub MY::postamble {
'
$(MYEXTLIB): mylib/Makefile
cd mylib && $(MAKE) $(PASSTHRU)
';
}
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 "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 because the default Perl doesn't
currently support the "const char *" type. Include a new TYPEMAP
section in your XS code before the above function:
TYPEMAP: <<END
const char * T_PV
END
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 Mytest2.t script and change the number of tests to "4",
and add the following lines to the end of the script:
is( &Mytest2::foo(1, 2, "Hello, world!"), 7 );
is( &Mytest2::foo(1, 2, "0.0"), 7 );
ok( abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 );
(When dealing with floating-point comparisons, it is best to not check
for equality, but rather that the difference between the expected and
actual result is below a certain amount (called epsilon) which is 0.01
in this case)
Run ""make test"" and all should be well. There are some warnings on
missing tests for the Mytest2::mylib extension, but you can ignore
them.
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 &Mytest2::TESTVAL). 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, then another time in the second part, when an XSUB interface
to this static C function is defined.
This is quite typical for .xs files: usually the .xs file provides an
interface to an existing C function. Then this C function is defined
somewhere (either in an external library, or in the first part of .xs
file), and a Perl interface to this function (i.e. "Perl glue") is
described in the second part of .xs file. The situation in "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
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 &cp
For example, consider:
int
foo(a,b)
char &a
char * b
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(&a, b);
xsubpp will parse the following function argument lists identically:
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 call to foo above). By doing so, it is easy to understand
exactly what will be passed to the C function; it will be whatever is
in the "last column".
You should take great pains to try to pass the function the type of
variable it wants, when possible. It will save you a lot of trouble in
the long run.
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
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 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, &buf);
if (i == 0) {
XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
} else {
XPUSHs(sv_2mortal(newSVnv(errno)));
}
You'll also need to add the following code to the top of the .xs file,
just after the include of "XSUB.h":
#include <sys/vfs.h>
Also add the following code segment to Mytest.t while incrementing the
"9" tests to "11":
@a = &Mytest::statfs("/blech");
ok( scalar(@a) == 1 && $a[0] == 2 );
@a = &Mytest::statfs("/");
is( scalar(@a), 7 );
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 7-element array is returned. Since
only one argument is passed into this function, we need room on the
stack to hold the 7 values which may be returned.
We do this by using the PPCODE: directive, rather than the CODE:
directive. This tells 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, 7);
The tradeoff is that one needs to calculate the number of return
values in advance (though overextending the stack will not
typically hurt anything but memory consumption).
Similarly, in the failure branch we could use "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;
SSize_t numpaths = 0, n;
int i;
struct statfs buf;
SvGETMAGIC(paths);
if ((!SvROK(paths))
|| (SvTYPE(SvRV(paths)) != SVt_PVAV)
|| ((numpaths = av_top_index((AV *)SvRV(paths))) < 0))
{
XSRETURN_UNDEF;
}
results = (AV *)sv_2mortal((SV *)newAV());
CODE:
for (n = 0; n <= numpaths; n++) {
HV * rh;
STRLEN l;
char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
i = statfs(fn, &buf);
if (i != 0) {
av_push(results, newSVnv(errno));
continue;
}
rh = (HV *)sv_2mortal((SV *)newHV());
hv_store(rh, "f_bavail", 8, newSVnv(buf.f_bavail), 0);
hv_store(rh, "f_bfree", 7, newSVnv(buf.f_bfree), 0);
hv_store(rh, "f_blocks", 8, newSVnv(buf.f_blocks), 0);
hv_store(rh, "f_bsize", 7, newSVnv(buf.f_bsize), 0);
hv_store(rh, "f_ffree", 7, newSVnv(buf.f_ffree), 0);
hv_store(rh, "f_files", 7, newSVnv(buf.f_files), 0);
hv_store(rh, "f_type", 6, newSVnv(buf.f_type), 0);
av_push(results, newRV((SV *)rh));
}
RETVAL = newRV((SV *)results);
OUTPUT:
RETVAL
And add the following code to Mytest.t, while incrementing the "11"
tests to "13":
$results = Mytest::multi_statfs([ '/', '/blech' ]);
ok( ref $results->[0] );
ok( ! ref $results->[1] );
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 calls SvGETMAGIC(paths), in case
paths is a tied variable. Then it checks that "SvROK" returns
true, which indicates that paths is a valid reference. (Simply
checking "SvROK" won't trigger FETCH on a tied variable.) 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_top_index" 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_top_index" 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 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 Passing open files to XSes
You would think passing files to an XS is difficult, with all the
typeglobs and stuff. Well, it isn't.
Suppose that for some strange reason we need a wrapper around the
standard C library function "fputs()". This is all we need:
#define PERLIO_NOT_STDIO 0
#define PERL_NO_GET_CONTEXT
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <stdio.h>
int
fputs(s, stream)
char * s
FILE * stream
The real work is done in the standard typemap.
But you lose all the fine stuff done by the perlio layers. This calls
the stdio function "fputs()", which knows nothing about them.
The standard typemap offers three variants of PerlIO *: "InputStream"
(T_IN), "InOutStream" (T_INOUT) and "OutputStream" (T_OUT). A bare
"PerlIO *" is considered a T_INOUT. If it matters in your code (see
below for why it might) #define or typedef one of the specific names
and use that as the argument or result type in your XS file.
The standard typemap does not contain PerlIO * before perl 5.7, but it
has the three stream variants. Using a PerlIO * directly is not
backwards compatible unless you provide your own typemap.
For streams coming from perl the main difference is that "OutputStream"
will get the output PerlIO * - which may make a difference on a socket.
Like in our example...
For streams being handed to perl a new file handle is created (i.e. a
reference to a new glob) and associated with the PerlIO * provided. If
the read/write state of the PerlIO * is not correct then you may get
errors or warnings from when the file handle is used. So if you opened
the PerlIO * as "w" it should really be an "OutputStream" if open as
"r" it should be an "InputStream".
Now, suppose you want to use perlio layers in your XS. We'll use the
perlio "PerlIO_puts()" function as an example.
In the C part of the XS file (above the first MODULE line) you have
#define OutputStream PerlIO *
or
typedef PerlIO * OutputStream;
And this is the XS code:
int
perlioputs(s, stream)
char * s
OutputStream stream
CODE:
RETVAL = PerlIO_puts(stream, s);
OUTPUT:
RETVAL
We have to use a "CODE" section because "PerlIO_puts()" has the
arguments reversed compared to "fputs()", and we want to keep the
arguments the same.
Wanting to explore this thoroughly, we want to use the stdio "fputs()"
on a PerlIO *. This means we have to ask the perlio system for a stdio
"FILE *":
int
perliofputs(s, stream)
char * s
OutputStream stream
PREINIT:
FILE *fp = PerlIO_findFILE(stream);
CODE:
if (fp != (FILE*) 0) {
RETVAL = fputs(s, fp);
} else {
RETVAL = -1;
}
OUTPUT:
RETVAL
Note: "PerlIO_findFILE()" will search the layers for a stdio layer. If
it can't find one, it will call "PerlIO_exportFILE()" to generate a new
stdio "FILE". Please only call "PerlIO_exportFILE()" if you want a new
"FILE". It will generate one on each call and push a new stdio layer.
So don't call it repeatedly on the same file. "PerlIO_findFILE()" will
retrieve the stdio layer once it has been generated by
"PerlIO_exportFILE()".
This applies to the perlio system only. For versions before 5.7,
"PerlIO_exportFILE()" is equivalent to "PerlIO_findFILE()".
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, "./blib") }
* 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. PerlIO material contributed by Lupe Christoph, with some clarification by Nick Ing-Simmons. Changes for h2xs as of Perl 5.8.x by Renee Baecker Last Changed 2012-01-20
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