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Perl 5 version 12.0 documentation
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XSLoader - Dynamically load C libraries into Perl code


Version 0.10


  1. package YourPackage;
  2. use XSLoader;
  3. XSLoader::load 'YourPackage', $YourPackage::VERSION;


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.

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 .

Migration from DynaLoader

A typical module using DynaLoader starts like this:

  1. package YourPackage;
  2. require DynaLoader;
  3. our @ISA = qw( OnePackage OtherPackage DynaLoader );
  4. our $VERSION = '0.01';
  5. bootstrap YourPackage $VERSION;

Change this to

  1. package YourPackage;
  2. use XSLoader;
  3. our @ISA = qw( OnePackage OtherPackage );
  4. our $VERSION = '0.01';
  5. 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

  1. 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

  1. XSLoader::load 'YourPackage';

Backward compatible boilerplate

If you want to have your cake and eat it too, you need a more complicated boilerplate.

  1. package YourPackage;
  2. use vars qw($VERSION @ISA);
  3. @ISA = qw( OnePackage OtherPackage );
  4. $VERSION = '0.01';
  5. eval {
  6. require XSLoader;
  7. XSLoader::load('YourPackage', $VERSION);
  8. 1;
  9. } or do {
  10. require DynaLoader;
  11. push @ISA, 'DynaLoader';
  12. bootstrap YourPackage $VERSION;
  13. };

The parentheses about XSLoader::load() arguments are needed 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 antic 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() ) has three side effects:

  • if $VERSION was specified, a sanity check is done to ensure that the versions of the .pm and the (compiled) .xs parts are compatible;

  • the XSUBs are made accessible from Perl;

  • if a BOOT: section was present in the .xs file, the code there is called.

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

  1. package YourPackage;
  2. use XSLoader;
  3. use vars qw($VERSION @ISA);
  4. BEGIN {
  5. @ISA = qw( OnePackage OtherPackage );
  6. $VERSION = '0.01';
  7. # Put Perl code used in the BOOT: section here
  8. XSLoader::load 'YourPackage', $VERSION;
  9. }
  10. # 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:

  1. package YourPackage;
  2. use XSLoader;
  3. use vars qw($VERSION @ISA);
  4. BEGIN {
  5. @ISA = qw( OnePackage OtherPackage );
  6. $VERSION = '0.01';
  7. XSLoader::load 'YourPackage', $VERSION;
  8. }
  9. # Put Perl code used in onBOOT() function here; calls to XSUBs are
  10. # prototype-checked.
  11. onBOOT;
  12. # 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.

  • XSLoader::load('Your::Module', $Your::Module::VERSION)

    (F) You tried to invoke load() without any argument. You must supply a module name, and optionally its version.


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 .


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 <>.


Copyright (C) 1990-2007 by Larry Wall and others.

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