perlXStut - Tutorial for writing XSUBs
This tutorial will educate the reader on the steps
involved in creating a Perl extension. The reader is
assumed to have access to 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.
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 [Toc] [Back]
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 [Toc] [Back]
It is commonly thought that if a system does not have the
capability to dynamically load a library, you cannot build
XSUBs. This is incorrect. You can build them, but you
must link the XSUBs subroutines with the rest of Perl,
creating a new executable. This situation is similar to
Perl 4.
This tutorial can still be used on such a system. The
XSUB build mechanism will check the system and build a
dynamically-loadable library if possible, or else a static
library and then, optionally, a new statically-linked executable
with that static library linked in.
Should you wish to build a statically-linked executable on
a system which can dynamically load libraries, you may, in
all the following examples, where the command ""make""
with no arguments is executed, run the command ""make
perl"" instead.
If you have generated such a statically-linked executable
by choice, then instead of saying ""make test"", you
should say ""make test_static"". On systems that cannot
build dynamically-loadable libraries at all, simply saying
""make test"" is sufficient.
Now let's go on with the show!
EXAMPLE 1 [Toc] [Back]
Our first extension will be very simple. When we call the
routine in the extension, it will print out a well-known
message and return.
Run ""h2xs -A -n Mytest"". This creates a directory named
Mytest, possibly under ext/ if that directory exists in
the current working directory. Several files will be created
in the Mytest dir, including MANIFEST, Makefile.PL,
Mytest.pm, Mytest.xs, test.pl, and Changes.
The MANIFEST file contains the names of all the files just
created in the Mytest directory.
The file Makefile.PL should look something like this:
use ExtUtils::MakeMaker;
# 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 strict;
use warnings;
require Exporter;
require DynaLoader;
our @ISA = qw(Exporter DynaLoader);
# Items to export into callers namespace by default. Note: do not export
# names by default without a very good reason. Use
EXPORT_OK instead.
# Do not simply export all your public functions/methods/constants.
our @EXPORT = qw(
);
our $VERSION = '0.01';
bootstrap Mytest $VERSION;
# Preloaded methods go here.
# Autoload methods go after __END__, and are processed by the autosplit program.
1;
__END__
# Below is the stub of documentation for your module. You better edit it!
The rest of the .pm file contains sample code for providing
documentation for the extension.
Finally, the Mytest.xs file should look something like
this:
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.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!0);
It is okay for the lines starting at the "CODE:" line to
not be indented. However, for readability purposes, it is
suggested that you indent CODE: one level and the lines
following one more level.
Now we'll run ""perl Makefile.PL"". This will create a
real Makefile, which make needs. Its output looks something
like:
% perl Makefile.PL
Checking if your kit is complete...
Looks good
Writing Makefile for Mytest
%
Now, running make will produce output that looks something
like this (some long lines have been shortened for clarity
and some extraneous lines have been deleted):
% make
umask 0 && cp Mytest.pm ./blib/Mytest.pm
perl xsubpp -typemap typemap Mytest.xs >Mytest.tc
&& mv Mytest.tc Mytest.c
Please specify prototyping behavior for Mytest.xs
(see perlxs manual)
cc -c Mytest.c
Running Mkbootstrap for Mytest ()
chmod 644 Mytest.bs
LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl -b Mytest.o
chmod 755 ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl
cp Mytest.bs ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
chmod 644 ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
Manifying ./blib/man3/Mytest.3
%
You can safely ignore the line about "prototyping behavior"
- it is explained in the section "The PROTOTYPES:
Keyword" in perlxs.
If you are on a Win32 system, and the build process fails
with linker errors for functions in the C library, check
if your Perl is configured to use PerlCRT (running perl
-V:libc should show you if this is the case). If Perl is
configured to use PerlCRT, you have to make sure PerlCRT.lib
is copied to the same location that msvcrt.lib
lives in, so that the compiler can find it on its own.
msvcrt.lib is usually found in the Visual C compiler's lib
directory (e.g. C:/DevStudio/VC/lib).
Perl has its own special way of easily writing test
scripts, but for this example only, we'll create our own
test script. Create a file called hello that looks like
this:
#! /opt/perl5/bin/perl
use ExtUtils::testlib;
use Mytest;
Mytest::hello();
Now we make the script executable ("chmod +x hello"), run
the script and we should see the following output:
% ./hello
Hello, world!
%
EXAMPLE 2 [Toc] [Back]
Now let's add to our extension a subroutine that will take
a single numeric argument as input and return 0 if the
number is even or 1 if the number is odd.
Add the following to the end of Mytest.xs:
int
is_even(input)
int input
CODE:
RETVAL = (input % 2 == 0);
OUTPUT:
RETVAL
There does not need to be white space at the start of the
""int input"" line, but it is useful for improving readability.
Placing a semi-colon at the end of that line is
also optional. Any amount and kind of white space may be
placed between the ""int"" and ""input"".
Now re-run make to rebuild our new shared library.
Now perform the same steps as before, generating a Makefile
from the Makefile.PL file, and running make.
In order to test that our extension works, we now need to
look at the file test.pl. This file is set up to imitate
the same kind of testing structure that Perl itself has.
Within the test script, you perform a number of tests to
confirm the behavior of the extension, printing "ok" when
the test is correct, "not ok" when it is not. Change the
print statement in the BEGIN block to print "1..4", and
add the following code to the end of the file:
print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok
2", "0;
print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok
3", "0;
print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok
4", "0;
We will be calling the test script through the command
""make test"". You should see output that looks something
like this:
% make test
PERL_DL_NONLAZY=1 /opt/perl5.004/bin/perl (lots of
-I arguments) test.pl
1..4
ok 1
ok 2
ok 3
ok 4
%
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"" insures 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 objectoriented
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 [Toc] [Back]
The importance of writing good test scripts cannot be
overemphasized. You should closely follow the "ok/not ok"
style that Perl itself uses, so that it is very easy and
unambiguous to determine the outcome of each test case.
When you find and fix a bug, make sure you add a test case
for it.
By running ""make test"", you ensure that your test.pl
script runs and uses the correct version of your extension.
If you have many test cases, you might want to copy
Perl's test style. Create a directory named "t" in the
extension's directory and append the suffix ".t" to the
names of your test files. When you run ""make test"", all
of these test files will be executed.
EXAMPLE 3 [Toc] [Back]
Our third extension will take one argument as its input,
round off that value, and set the argument to the rounded
value.
Add the following to the end of Mytest.xs:
void
round(arg)
double arg
CODE:
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
arg = ceil(arg - 0.5);
} else {
arg = 0.0;
}
OUTPUT:
arg
Edit the Makefile.PL file so that the corresponding line
looks like this:
'LIBS' => ['-lm'], # e.g., '-lm'
Generate the Makefile and run make. Change the BEGIN
block to print "1..9" and add the following to test.pl:
$i = -1.5; &Mytest::round($i); print $i == -2.0 ?
"ok 5" : "not ok 5", "0;
$i = -1.1; &Mytest::round($i); print $i == -1.0 ?
"ok 6" : "not ok 6", "0;
$i = 0.0; &Mytest::round($i); print $i == 0.0 ?
"ok 7" : "not ok 7", "0;
$i = 0.5; &Mytest::round($i); print $i == 1.0 ?
"ok 8" : "not ok 8", "0;
$i = 1.2; &Mytest::round($i); print $i == 1.0 ?
"ok 9" : "not ok 9", "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 test.pl:
&Mytest::round(3);
Run ""make test"" and notice that Perl dies with a fatal
error. Perl won't let you change the value of constants!
What's new here?
o 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.
o 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 [Toc] [Back]
You specify the parameters that will be passed into the
XSUB on the line(s) after you declare the function's
return value and name. Each input parameter line starts
with optional white space, and may have an optional terminating
semicolon.
The list of output parameters occurs at the very end of
the function, just before after the OUTPUT: directive.
The use of RETVAL tells Perl that you wish to send this
value back as the return value of the XSUB function. In
Example 3, we wanted the "return value" placed in the
original variable which we passed in, so we listed it (and
not RETVAL) in the OUTPUT: section.
The XSUBPP Program [Toc] [Back]
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 [Toc] [Back]
The xsubpp program uses rules to convert from Perl's data
types (scalar, array, etc.) to C's data types (int, char,
etc.). These rules are stored in the typemap file ($PERLLIB/ExtUtils/typemap).
This file is split into three
parts.
The first section maps various C data types to a name,
which corresponds somewhat with the various Perl types.
The second section contains C code which xsubpp uses to
handle input parameters. The third section contains C
code which xsubpp uses to handle output parameters.
Let's take a look at a portion of the .c file created for
our extension. The file name is Mytest.c:
XS(XS_Mytest_round)
{
dXSARGS;
if (items != 1)
croak("Usage: Mytest::round(arg)");
{
double arg = (double)SvNV(ST(0)); /*
XXXXX */
if (arg > 0.0) {
arg = floor(arg + 0.5);
} else if (arg < 0.0) {
arg = ceil(arg - 0.5);
} else {
arg = 0.0;
}
sv_setnv(ST(0), (double)arg); /* XXXXX
*/
}
XSRETURN(1);
}
Notice the two lines commented with "XXXXX". If you check
the first section of the typemap file, you'll see that
doubles are of type T_DOUBLE. In the INPUT section, an
argument that is T_DOUBLE is assigned to the variable arg
by calling the routine SvNV on something, then casting it
to double, then assigned to the variable arg. Similarly,
in the OUTPUT section, once arg has its final value, it is
passed to the sv_setnv function to be passed back to the
calling subroutine. These two functions are explained in
perlguts; we'll talk more later about what that "ST(0)"
means in the section on the argument stack.
Warning about Output Arguments [Toc] [Back]
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 [Toc] [Back]
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 test.pl script and change the BEGIN block to
print "1..4", and add the following lines to the end of
the script:
print &Mytest2::foo(1, 2, "Hello, world!") == 7 ?
"ok 20 : "not ok 20;
print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 30 :
"not ok 30;
print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <=
0.01 ? "ok 40 : "not ok 40;
(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.
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.
o 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.
o 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.
o 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.
o 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.
o 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 [Toc] [Back]
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 [Toc] [Back]
In "EXAMPLE 4" the second part of .xs file contained the
following description of an XSUB:
double
foo(a,b,c)
int a
long b
const char * c
OUTPUT:
RETVAL
Note that in contrast with "EXAMPLE 1", "EXAMPLE 2" and
"EXAMPLE 3", this description does not contain the actual
code for what is done is done during a call to Perl function
foo(). To understand what is going on here, one can
add a CODE section to this XSUB:
double
foo(a,b,c)
int a
long b
const char * c
CODE:
RETVAL = foo(a,b,c);
OUTPUT:
RETVAL
However, these two XSUBs provide almost identical generated
C code: xsubpp compiler is smart enough to figure out
the "CODE:" section from the first two lines of the
description of XSUB. What about "OUTPUT:" section? In
fact, that is absolutely the same! The "OUTPUT:" section
can be removed as well, as far as "CODE:" section or
"PPCODE:" section is not specified: xsubpp can see that it
needs to generate a function call section, and will autogenerate
the OUTPUT section too. Thus one can shortcut
the XSUB to become:
double
foo(a,b,c)
int a
long b
const char * c
Can we do the same with an XSUB
int
is_even(input)
int input
CODE:
RETVAL = (input % 2 == 0);
OUTPUT:
RETVAL
of "EXAMPLE 2"? To do this, one needs to define a C function
"int is_even(int input)". As we saw in "Anatomy of
.xs file", a proper place for this definition is in the
first part of .xs file. In fact a C function
int
is_even(int arg)
{
return (arg % 2 == 0);
}
is probably overkill for this. Something as simple as a
"#define" will do too:
#define is_even(arg) ((arg) % 2 == 0)
After having this in the first part of .xs file, the "Perl
glue" part becomes as simple as
int
is_even(input)
int input
This technique of separation of the glue part from the
workhorse part has obvious tradeoffs: if you want to
change a Perl interface, you need to change two places in
your code. However, it removes a lot of clutter, and
makes the workhorse part independent from idiosyncrasies
of Perl calling convention. (In fact, there is nothing
Perl-specific in the above description, a different version
of xsubpp might have translated this to TCL glue or
Python glue as well.)
More about XSUB arguments [Toc] [Back]
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 [Toc] [Back]
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-gutsspecific
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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)));
XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[0])));
XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[1])));
} 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 test.pl while
incrementing the "1..9" string in the BEGIN block to
"1..11":
@a = &Mytest::statfs("/blech");
print ((scalar(@a) == 1 && $a[0] == 2) ? "ok 100 :
"not ok 100);
@a = &Mytest::statfs("/");
print scalar(@a) == 9 ? "ok 110 : "not ok 110;
New Things in this Example [Toc] [Back]
This example added quite a few new concepts. We'll take
them one at a time.
o 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.)
o This routine also returns a different number of arguments
depending on the success or failure of the call
to statfs. If there is an error, the error number is
returned as a single-element array. If the call is
successful, then a 9-element array is returned. Since
only one argument is passed into this function, we
need room on the stack to hold the 9 values which may
be returned.
We do this by using the PPCODE: directive, rather than
the CODE: directive. This tells xsubpp that we will
be managing the return values that will be put on the
argument stack by ourselves.
o 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.
o 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.
o 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 preextend
the stack before pushing the return values:
EXTEND(SP, 9);
The tradeoff is that one needs to calculate the number
of return values in advance (though overextending the
stack will not typically hurt anything but memory consumption).
Similarly, in the failure branch we could use "PUSHs"
without extending the stack: the Perl function reference
comes to an XSUB on the stack, thus the stack is
always large enough to take one return value.
EXAMPLE 6 [Toc] [Back]
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 test.pl, while incrementing
the "1..11" string in the BEGIN block to "1..13":
$results = Mytest::multi_statfs([ '/', '/blech'
]);
print ((ref $results->[0]) ? "ok 120 : "not ok
120);
print ((! ref $results->[1]) ? "ok 130 : "not ok
130);
New Things in this Example [Toc] [Back]
There are a number of new concepts introduced here,
described below:
o 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.
o 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.
o 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);
o 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.
o 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").
o At this point, xsubpp is doing very little work - the
differences between Mytest.xs and Mytest.c are minimal.
EXAMPLE 7 (Coming Soon) [Toc] [Back]
XPUSH args AND set RETVAL AND assign return value to array
EXAMPLE 8 (Coming Soon) [Toc] [Back]
Setting $!
EXAMPLE 9 Passing open files to XSes [Toc] [Back]
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.
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