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.
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:
#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( $i, 1.0 );
Running ""make test"" should now print out that all nine
tests are okay.
Notice that in these new test cases, the argument passed to round was a scalar
variable. You might be wondering if you can round a constant or literal. To
see what happens, temporarily add the following line to Mytest.t:
&Mytest::round(3);
Run ""make test"" and notice that Perl dies with a fatal
error. Perl won't let you change the value of constants!
What's new here?¶
- •
- We've made some changes to Makefile.PL. In this case, we've
specified an extra library to be linked into the extension's shared
library, the math library libm in this case. We'll talk later about how to
write XSUBs that can call every routine in a library.
- •
- The value of the function is not being passed back as the
function's return value, but by changing the value of the variable that
was passed into the function. You might have guessed that when you saw
that the return value of round is of type "void".
You specify the parameters that will be passed into the XSUB on the line(s)
after you declare the function's return value and name. Each input parameter
line starts with optional whitespace, and may have an optional terminating
semicolon.
The list of output parameters occurs at the very end of the function, just after
the OUTPUT: directive. The use of RETVAL tells Perl that you wish to send this
value back as the return value of the XSUB function. In Example 3, we wanted
the "return value" placed in the original variable which we passed
in, so we listed it (and not RETVAL) in the OUTPUT: section.
The XSUBPP Program¶
The
xsubpp program takes the XS code in the .xs file and translates it
into C code, placing it in a file whose suffix is .c. The C code created makes
heavy use of the C functions within Perl.
The TYPEMAP file¶
The
xsubpp program uses rules to convert from Perl's data types (scalar,
array, etc.) to C's data types (int, char, etc.). These rules are stored in
the typemap file ($PERLLIB/ExtUtils/typemap). 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)
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
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 <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 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 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;
I32 numpaths = 0;
int i, n;
struct statfs buf;
if ((!SvROK(paths))
|| (SvTYPE(SvRV(paths)) != SVt_PVAV)
|| ((numpaths = av_len((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 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 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
#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 loose 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¶
2007/10/11
