XSLoader - Dynamically load C libraries into Perl code


Version 1.03c


    package YourPackage;

    XSLoader::load("module", $VERSION);
    XSLoader::load_file("module", $sharedlib_path);


This module defines a standard simplified interface to the dynamic linking mechanisms available on many platforms. Its primary purpose is to implement cheap automatic dynamic loading of Perl modules.

This is now a cperl core builtin package, so in pure cperl code you don't need to require XSLoader.

For a more complicated interface, see DynaLoader. Many (most) features of DynaLoader are not implemented in XSLoader, like for example the dl_load_flags, not honored by XSLoader.




    load( [$module [, $VERSION]] )

With optional 1-2 arguments. A shorter DynaLoader::bootstrap variant which takes the package name name from caller(). It tries to call DynaLoader::bootstrap or XSLoader::bootstrap_inherit, tries to find the shared library in the auto subdir relative to the found pure perl loader, and finally calls DynaLoader::dl_load_file with the path of the shared libray and the package name.



    load_file("module", "sharedlib_path")

load_file is mainly used with embedded programs, which already know the path to the shared library. It simply calls DynaLoader::dl_load_file with the path of the shared libray and the package name.



    bootstrap_inherit($module [, $VERSION])

Forwards to DynaLoader::bootstrap_inherit.

Migration from DynaLoader

A typical module using DynaLoader starts like this:

    package YourPackage;
    require DynaLoader;

    our @ISA = qw( OnePackage OtherPackage DynaLoader );
    our $VERSION = '0.01';
    bootstrap YourPackage $VERSION;

Change this to

    package YourPackage;

    our @ISA = qw( OnePackage OtherPackage );
    our $VERSION = '0.01';
    XSLoader::load 'YourPackage', $VERSION;

In other words: replace require DynaLoader by use XSLoader, remove DynaLoader from @ISA, change bootstrap by XSLoader::load. Do not forget to quote the name of your package on the XSLoader::load line, and add comma (,) before the arguments ($VERSION above).

Of course, if @ISA contained only DynaLoader, there is no need to have the @ISA assignment at all; moreover, if instead of our one uses the more backward-compatible

    use vars qw($VERSION @ISA);

one can remove this reference to @ISA together with the @ISA assignment.

If no $VERSION was specified on the bootstrap line, the last line becomes

    XSLoader::load 'YourPackage';

If the call to load is from YourPackage, then that can be further simplified to


as load will use caller to determine the package.

Backward compatible boilerplate

If you want to have your cake and eat it too, you need a more complicated boilerplate. Add require XSLoader; to be backwards compatible with perl5.

    package YourPackage;
    use vars qw($VERSION @ISA);

    @ISA = qw( OnePackage OtherPackage );
    $VERSION = '0.01';
    eval {
       require XSLoader;
       XSLoader::load('YourPackage', $VERSION);
    } or do {
       require DynaLoader;
       push @ISA, 'DynaLoader';
       bootstrap YourPackage $VERSION;

The parentheses about XSLoader::load() arguments are needed in perl5 only since we replaced use XSLoader by require, so the compiler does not know that a function XSLoader::load() is present.

This boilerplate uses the low-overhead XSLoader if present; if used with an antique Perl which has no XSLoader, it falls back to using DynaLoader.

Order of initialization: early load()

Skip this section if the XSUB functions are supposed to be called from other modules only; read it only if you call your XSUBs from the code in your module, or have a BOOT: section in your XS file (see "The BOOT: Keyword" in perlxs). What is described here is equally applicable to the DynaLoader interface.

A sufficiently complicated module using XS would have both Perl code (defined in and XS code (defined in YourPackage.xs). If this Perl code makes calls into this XS code, and/or this XS code makes calls to the Perl code, one should be careful with the order of initialization.

The call to XSLoader::load() (or bootstrap()) calls the module's bootstrap code. For modules build by xsubpp (nearly all modules) this has three side effects:

Consequently, if the code in the .pm file makes calls to these XSUBs, it is convenient to have XSUBs installed before the Perl code is defined; for example, this makes prototypes for XSUBs visible to this Perl code. Alternatively, if the BOOT: section makes calls to Perl functions (or uses Perl variables) defined in the .pm file, they must be defined prior to the call to XSLoader::load() (or bootstrap()).

The first situation being much more frequent, it makes sense to rewrite the boilerplate as

    package YourPackage;
    use vars qw($VERSION @ISA);

    BEGIN {
       @ISA = qw( OnePackage OtherPackage );
       $VERSION = '0.01';

       # Put Perl code used in the BOOT: section here

       XSLoader::load 'YourPackage', $VERSION;

    # Put Perl code making calls into XSUBs here

The most hairy case

If the interdependence of your BOOT: section and Perl code is more complicated than this (e.g., the BOOT: section makes calls to Perl functions which make calls to XSUBs with prototypes), get rid of the BOOT: section altogether. Replace it with a function onBOOT(), and call it like this:

    package YourPackage;
    use vars qw($VERSION @ISA);

    BEGIN {
       @ISA = qw( OnePackage OtherPackage );
       $VERSION = '0.01';
       XSLoader::load 'YourPackage', $VERSION;

    # Put Perl code used in onBOOT() function here; calls to XSUBs are
    # prototype-checked.


    # Put Perl initialization code assuming that XS is initialized here


Can't find '%s' symbol in %s

(F) The bootstrap symbol could not be found in the extension module.

Can't load '%s' for module %s: %s

(F) The loading or initialisation of the extension module failed. The detailed error follows.

Undefined symbols present after loading %s: %s

(W) As the message says, some symbols stay undefined although the extension module was correctly loaded and initialised. The list of undefined symbols follows.


To reduce the overhead as much as possible, only one possible location is checked to find the extension DLL (this location is where make install would put the DLL). If not found, the search for the DLL is transparently delegated to DynaLoader, which looks for the DLL along the @INC list.

In particular, this is applicable to the structure of @INC used for testing not-yet-installed extensions. This means that running uninstalled extensions may have much more overhead than running the same extensions after make install.


The new simpler way to call XSLoader::load() with no arguments at all does not work on Perl 5.8.4 and 5.8.5.


This cperl variant of XSLoader is entirely written in C and available immediately without the need to require XSLoader;. It is builtin.

.bs bootstrap files are not supported anymore
More PERL_DL_DEBUG=3 diagnostics


Please report any bugs or feature requests via the perlbug(1) utility.




Ilya Zakharevich originally extracted XSLoader from DynaLoader.

CPAN version is currently maintained by Sébastien Aperghis-Tramoni <>.

Previous maintainer was Michael G Schwern <>.

Conversion to C and merged with DynaLoader by Reini Urban, 2015. This is now a cperl core builtin package.


Copyright (C) 1990-2011 by Larry Wall and others. Copyright (C) 2015 cPanel Inc

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.