2010/04/30

Perl - ExtUtils::H2PM

I've spent a lot of time lately writing modules that wrap Linux kernel features in some way or another. They all boil down to basically the same thing - export a bunch of constants, and structure packing/unpacking functions. Various bits of extra fancy interface code are sometimes nice, but most of the time these can be written in Pure Perl once the base bits are done.

It's always annoyed me that one has to write an XS module just to obtain these. It's a lot of extra work and bother, for something that ought to be so simple. So instead, I started thinking about it, how can I make this much simpler from Perl?

What I came up with is ExtUtils::H2PM; a module to make it trivially-easy to write this sort of boring module to wrap some constants and structures from OS's header files.

As a brief example, consider the following

use ExtUtils::H2PM;

module "Fcntl";

include "fcntl.h";

constant "O_RDONLY";

write_output $ARGV[0];

This is it. This is all the code you, as a module author, have to write. You store that in some file, lets call it Fcntl.pm.PL. You let the build system go about converting that into Fcntl.pm; which on my system now looks like this:

package Fcntl;
# This module was generated automatically by ExtUtils::H2PM from -e

push @EXPORT_OK, 'O_RDONLY';
use constant O_RDONLY => 0;

1;

This is a plain standard Perl module that can be installed and used, to provide that constant.

We can also create pack/unpack-style functions to wrap structure types too. Consider

use ExtUtils::H2PM;

module "Time";

include "time.h";

structure "struct timespec",
members => [
tv_sec => member_numeric,
tv_nsec => member_numeric,
];

write_output $ARGV[0];

For this we obtain:

package Time;
# This module was generated automatically by ExtUtils::H2PM from -e

use Carp;
push @EXPORT_OK, 'pack_timespec', 'unpack_timespec';

sub pack_timespec
{
@_ == 2 or croak "usage: pack_timespec(tv_sec, tv_nsec)";
my @v = @_;
pack "q q ", @v;
}

sub unpack_timespec
{
length $_[0] == 16 or croak "unpack_timespec: expected 16 bytes";
my @v = unpack "q q ", $_[0];
@v;
}

1;

This was done entirely automatically too.

In a real use-case, you'd be using this to wrap things like socket option constants/structures, and so on; cases where existing Perl functions wrap existing syscalls, you simply want to provide new option values or structures. I've now written several modules mostly or entirely relying on ExtUtils::H2PM to build them, freeing me of having to write all that XS code to implement them.

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