XSBuilder − Autogenerating XS-glue Code

O’Reilly OpenSource Convention 2002

Gerald Richter

ecos gmbh


XSBuilder − What is it?

It’s purpose is to automaticly generate a Perl interface to C Code
Solves the problem, that the Perl interface is not always uptodate with the C interface
Saves a lot of Copy&Paste work
Systematical changes have to be done only once

For example changes in the memory management of strings.

Is part of mod_perl 2.0 build system
Most code is developed by Doug MacEachern. Additionaly I have
abstracted the code from mod_perl so it’s useable for any C code
added new features like callbacks and the ability to parse comments
Replaced C::Scan with a Parse::RecDescent to be platform and compiler

Goal is to replace the current mod_perl XS generation code with XSBuilder

Inline versus XSBuilder
Inline: embed C−Code into Perl
XSBuilder: Create interface for existing C−libraries/applicationen

XSBuilder − What does it do?

Create Perl functions/methods for every C function
The function can be assigned to different packages, also automaticly by inspecting the first parameter

Create a Perl class for every C structure
Every element of structure becomes a Perl method to get/set it’s value. The object can be either a scalar reference (used by mod_perl) or a reference to a hash (use by Embperl), which allows to store extra data by the Perl code into this hash.

Create glue code to handle callbacks
There several sorts of callback, not all are implemented right now

Create Perl constant subs
Coming soon...

XSBuilder − How does it work?

Parse the C header files
Functions, their arguments and return types
Structures and it’s members

and create four tables which contains the results

Create the XS code
Input is
The source tables
Mapfiles which contains the mapping from C to Perl
Addtional C and Perl code that can be used to customize the interface

Output is
The XS files (one form every generated class)
Makefile.PL for every class
pm files

Parse the source

Create your own ParseSource class and override methods...
package Apache::DAV::ParseSource;
use strict;
use vars qw{@ISA $VERSION} ;
use ExtUtils::XSBuilder::ParseSource ;
@ISA = ('ExtUtils::XSBuilder::ParseSource') ;
$VERSION = '0.01';
my $dav_dir = 'C:\perl\msrc\cvs\mod_dav' ;
my $ap_dir = 'c:\programme\apache group\apache' ;
# ============================================================================
sub find_includes {
my $self = shift;
return $self−>{includes} if $self−>{includes};
my @includes = ("$ap_dir/include/ap_alloc.h", "$dav_dir/mod_dav.h") ;
return $self−>{includes} = $self −> sort_includes (\@includes) ;
# ============================================================================
sub package { 'Apache::DAV' }
# ============================================================================
sub preprocess {
my $self = shift ;
$_[0] =~ s/(?:API_EXPORT)(?:_NONSTD)?\s*\(\s*(.*?)\s*\)/$1/g ;

...run it
use FindBin ;
use lib ($FindBin::Bin) ;
require ParseSource ;
Apache::DAV::ParseSource −> run ;

...and you get
C:\perl\msrc\davint>perl xsbuilder\source_scan.pl
Will use mod_dav in C:\perl\msrc\cvs\mod_dav
Will use Apache in c:\programme\apache group\apache
Initialize parser
scan c:\programme\apache group\apache/include/ap_alloc.h ...
constant: APACHE_ALLOC_H
func: ap_init_alloc
func: ap_cleanup_alloc
func: ap_make_sub_pool
func: ap_destroy_pool
constant: ap_pool_join
func: ap_pool_join
func: ap_find_pool
func: ap_pool_is_ancestor
func: ap_clear_pool
func: ap_cleanup_for_exec
func: ap_palloc
func: ap_pcalloc
func: ap_pstrdup
func: ap_pstrndup
func: ap_pstrcat
func: ap_pvsprintf
valuefield: ap_pool * : pool
valuefield: int : elt_size
valuefield: int : nelts
valuefield: int : nalloc
valuefield: char * : elts
struct: (type=array_header)

The result is stored in four tables

Contains all function, it arguments and comments


Contains all constants


Contains all structures, it’s members and their comments


Contains all callback function definitions

Create the map files

Mapfiles are used to tell XSBuilder how C datatypes, structures and function aruments should be mapped into Perl ones.
Create your own WrapXS class and override methods

package Apache::DAV::WrapXS ;
use strict;
use vars qw{@ISA $VERSION} ;
use ExtUtils::XSBuilder::WrapXS ;
@ISA = ('ExtUtils::XSBuilder::WrapXS') ;
$VERSION = '0.01';
# ============================================================================
sub new_parsesource { [ Apache::DAV::ParseSource−>new ] }
# ============================================================================
sub my_xs_prefix { 'davxs_' }
# ============================================================================
sub h_filename_prefix { 'moddav_xs_' }
# ============================================================================
sub xs_includes {
my $self = shift ;
my $i = $self −> SUPER::xs_includes ;
my @i = grep (!/ap_alloc/, @$i) ;
return \@i ;

XSBuilder can create/update initial maps for you
use FindBin ;
use lib ($FindBin::Bin) ;
require ParseSource ;
require WrapXS ;
Apache::DAV::WrapXS−>checkmaps (' ');

run it
C:\perl\msrc\davint>perl xsbuilder\xs_check.pl
Will use mod_dav in C:\perl\msrc\cvs\mod_dav
Will use Apache in c:\programme\apache group\apache
Parse xsbuilder\maps/_types.map...
WARNING: No *_function.map file found in xsbuilder\maps
WARNING: No *_callback.map file found in xsbuilder\maps
WARNING: No *_structure.map file found in xsbuilder\maps
Write xsbuilder\maps/new_function.map...
Write xsbuilder\maps/new_callback.map...
Write xsbuilder\maps/new_structure.map...
Write xsbuilder\maps/new_type.map...

Now we have four map files

Contains the mapping from C type to Perl classes


Contains the mapping form C functions to Perl functions. Can be used to reorder arguments, tell XSBuilder which arguments are actualy return values and in which Perl package the function will be created.


Contains the mapping from C structures to Perl classes and defines for which members a access methods should be created. You can also specify if you want a "new" method for the class.


Contains the mapping form C callback functions to Perl callback functions. Can be used to reorder arguments, tell XSBuilder which arguments are actualy return values and in which Perl package the function will be created.

It’s a good idea to rename the prefix from "new_" to something unique, here we use "dav"

Everytime you rerun checkmaps, XSBuilder will create new_* files with the items that are not already part of the other maps.

Next step is to customize the maps...

type map

autogenerated dav_type.map
array_header |
dav_buffer |
dav_dyn_context |
dav_dyn_hooks |
dav_dyn_module |
dav_dyn_provider |
dav_error |
dav_get_props_result |
dav_hooks_liveprop |
dav_hooks_locks |
dav_hooks_repository |
dav_if_header |
dav_if_state_type |

Add Perl classes
struct array_header | Apache::Array
struct dav_buffer |
struct dav_datum | Apache::DAV::Datum
struct dav_dyn_context | Apache::DAV::DynContext
struct dav_dyn_hooks | Apache::DAV::DynHooks
struct dav_dyn_module | Apache::DAV::DynModule
struct dav_dyn_provider | Apache::DAV::DynProvider
struct dav_error | Apache::DAV::Error
struct dav_get_props_result | Apache::DAV::PropsResult
struct dav_hooks_db | Apache::DAV::HooksDb
struct dav_hooks_liveprop | Apache::DAV::HooksLiveprop
struct dav_hooks_locks | Apache::DAV::HooksLocks
struct dav_hooks_repository | Apache::DAV::HooksRepository
struct dav_hooks_vsn |
struct dav_if_header | Apache::DAV::IfHeader
struct dav_if_state_list | Apache::DAV::StateList
struct pool | Apache::Pool
struct request_rec | Apache::
struct server_rec | Apache::Server

Defines the mapping from C datatypes to Perl datatypes and classes and tells XSBuilder which datatype are (like) structures

function map

Function map defines the mapping from C functions arguments to Perl arguments
Tell XSBuilder where to place functions and which prefix to strip


Simple entries in the function map will be mapped 1:1 from C to Perl


The following map file entry tells XSBuilder that the value of "res_p"
should be returned

dav_get_resource | | r, <res_p

The C function

int dav_get_resource(request_rec *r, dav_resource **res_p) ;

How to call it from Perl

# get a sub request of the actual document
$subr = $r −> lookup_uri($uri);
# get a mod_dav resource object
my ($err, $resource) = $subr−>get_resource;

You can let XSBuilder insert your custom code, for the interface

If you call "get_props" from Perl "glue_dav_get_props" will be called, which can adjust the arguments and return types as necessary. The actual code for "glue_dav_get_props" will be taken from separate include file.

dav_get_props | glue_
dav_get_allprops | glue_


dav_get_props_result * dav_glue_get_props(dav_propdb * db, dav_xml_doc *doc)
dav_get_props_result * result = (dav_get_props_result *)ap_palloc (db −> p, sizeof (dav_get_props_result)) ;
*result = dav_get_props(db, doc) ;
return result ;

Arguments can be replaced

MODULE=Apache::Array PACKAGE=Apache::Array PREFIX=ap_
ap_make_array | ap_make_array(r−>pool, nelts, elt_size) | request_rec *:r, nelts, elt_size

ap_make_array requires a pool a it’s first parameter, we pass the request_rec from Perl and XSBuilder will take the pool from the request_rec.

structure map

MALLOC=strdup:$dest = ($type)strdup($src)
MALLOC=strdup:$dest = ($type)ap_pstrdup(obj −> pool, $src)

Create a accessor functions for every element and, if requested, a new method
$setprop = Apache::DAV::XMLElem −> new ({name => 'prop'}) ;
$elem = Apache::DAV::XMLElem −> new ({name => $name, ns => $namespaces}) ;
$setprop −> first_child($elem) ;
$first = $setprop −> first_child ;

some datatypes, like strings, requires dynamic allocated memory
From _types.map,which conatins a set of standard types

int | IV
unsigned int | UV
signed int | IV
long | IV
long int | IV
unsigned long | UV
unsigned | UV
char * | PV | | | strdup
const char * | PV | | | strdup
const char ** | UNDEFINED
char const * | PV | | | strdup
unsigned char * | PV | | | strdup
const unsigned char * | PV | | | strdup

callback maps

Callback maps have the same options a function maps

# dav_hooks_db −> open
dav_error *(*)(pool * p,const dav_resource * resource,int ro,dav_db * * pdb) | p, resource, ro=0, <pdb
# dav_hooks_lock −> has_locks
dav_error *(*)(dav_lockdb * lockdb,const dav_resource * resource,int * locks_present)

Generate the XS files

use FindBin ;
use lib ($FindBin::Bin) ;
require ParseSource ;
require WrapXS ;

...and run...
C:\perl\msrc\davint>perl xsbuilder\xs_generate.pl
Will use mod_dav in C:\perl\msrc\cvs\mod_dav
Will use Apache in c:\programme\apache group\apache
Parse xsbuilder\maps/_types.map...
Parse xsbuilder\maps/dav_type.map...
mkdir xs
Parse xsbuilder\maps/dav_functions.map...
WARNING: Cannot map type int(*)(void * ,const char * ,const char * ) for function ap_table_do
WARNING: Cannot map type dav_buffer * for function dav_buffer_append
WARNING: Cannot map type dav_buffer * for function dav_buffer_init
WARNING: Cannot map type dav_buffer * for function dav_buffer_place
WARNING: Cannot map type dav_buffer * for function dav_buffer_place_mem
WARNING: Cannot map type dav_buffer * for function dav_check_bufsize
WARNING: Cannot map return type int * for function dav_collect_liveprop_uris
WARNING: Cannot map type dav_resource * * for function dav_ensure_resource_writable
WARNING: Cannot map type dav_buffer * for function dav_lock_get_activelock
WARNING: Cannot map type dav_buffer * for function dav_set_bufsize
WARNING: Cannot map type int * for function dav_xml2text
struct array_header...
Parse xsbuilder\maps/dav_structure.map...
struct dav_buffer...
struct dav_datum...
struct dav_dyn_context...


We need create a top level Makefile.PL
use ExtUtils::MakeMaker ();
my $apdir = '/path/to/apache';
my $davdir = '/path/to/moddav';
'INC' => "−I\"$davdir\" −I\"$apdir/include\" −I\"$apdir/os/unix\" −I\"$dir/xs\" −I\"$dir/xsinclude\"",
) ;
open FH, ">xs/mmargs.pl" or die "Cannot open xs/mmargs.pl ($!)" ;
print FH Data::Dumper −> Dump ([\%MMARGS], ['MMARGS']) ;
close FH ;
'NAME' => 'Apache::DAV',
'VERSION' => '0.13',

Makefile.PL’s for all class are generated automaticly
C:\perl\msrc\davint>perl Makefile.PL
Will use Apache in c:\programme\apache group\apache
Will use mod_dav in C:\perl\msrc\cvs\mod_dav
Checking if your kit is complete...
Looks good
Writing Makefile for Apache::Array
Writing Makefile for Apache::DAV::Datum
Writing Makefile for Apache::DAV::DynContext
Writing Makefile for Apache::DAV::DynHooks
Writing Makefile for Apache::DAV::DynModule
Writing Makefile for Apache::DAV::DynProvider
Writing Makefile for Apache::DAV::Error
Writing Makefile for Apache::DAV::HooksDb
Writing Makefile for Apache::DAV::HooksLiveprop
Writing Makefile for Apache::DAV::HooksLocks
Writing Makefile for Apache::DAV::HooksRepository
Writing Makefile for Apache::DAV::IfHeader
Writing Makefile for Apache::DAV::Lock
Writing Makefile for Apache::DAV::LockDB
Writing Makefile for Apache::DAV::LockTokenList
Writing Makefile for Apache::DAV::LockupResult
Writing Makefile for Apache::DAV::PropCtx
Writing Makefile for Apache::DAV::PropsResult
Writing Makefile for Apache::DAV::Resource
Writing Makefile for Apache::DAV::Response
Writing Makefile for Apache::DAV::StateList
Writing Makefile for Apache::DAV::Text
Writing Makefile for Apache::DAV::TextHeader
Writing Makefile for Apache::DAV::WalkerCtx
Writing Makefile for Apache::DAV::XMLAttr
Writing Makefile for Apache::DAV::XMLDoc
Writing Makefile for Apache::DAV::XMLElem
Writing Makefile for Apache::DAV
Writing Makefile for Apache::TableEntry
Writing Makefile for Apache
Writing Makefile for WrapXS
Writing Makefile for Apache::DAV

and now compile...

How does it go on...

Generating documentation
XSBuilder already extracts source comments for functions and structures. It also parses doxygen comments, which are used in Apache 2.0. Lyle Brooks has started on automaticly createing POD files from this information.

Improving callbacks
Callbacks are the main area that needs improvement.

Bring it back to mod_perl 2.0
First version will be released just after the conference to CPAN
Any feedback and help appreciated