XS is a Perl foreign function interface through which a program can call a C or C++ subroutine. XS or xsub is an abbreviation of "eXternal Subroutine", where external refers to programming languages external to Perl.
XS also refers to a glue language for specifying calling interfaces supporting such interfaces (see below).
Subroutine libraries in Perl are called modules, and modules that contain xsubs are called XS modules. Perl provides a framework for developing, packaging, distributing, and installing modules.
It may be desirable for a Perl program to invoke a C subroutine in order to handle very CPU or memory intensive tasks, to interface with hardware or low-level system facilities, or to make use of existing C subroutine libraries.
The Perl interpreter is a C program, so in principle there is no obstacle to calling from Perl to C. However, the XS interface is complex[why?] and highly technical, and using it requires some understanding of the interpreter. The earliest reference on the subject was the perlguts POD.
XS module creation
To create an XS module, h2xs[further explanation needed] is first used[why?] to create an empty module. Next, C subroutines are added, and the calling interfaces to the C subroutines are specified with a specialized glue language. Finally, the module is compiled, tested, and installed with make.
Once an XS module is installed, it can be loaded into a Perl program with a use statement, like an ordinary Perl module. Once the module is loaded, the C subroutines can be called from Perl code, as if they were written in Perl.
XS modules are difficult[why?] to write and maintain, and they may only be installed if a C compiler and the header files that the Perl interpreter was compiled against are available. Also, new versions of Perl may break binary compatibility requiring XS modules to be recompiled.
- SWIG, an alternative to XS which also supports calling C and C++ functions from several other languages.
- "Gluing C++ And Perl Together". johnkeiser.com. August 27, 2001.
- Jenness, Tim & Cozens, Simon (2002). "Extending and Embedding Perl". Greenwich: Manning Publications Co. ISBN 1-930110-82-0
- perlxs Perl XS application programming interface
- perlxstut Perl XS tutorial
- perlguts Perl internal functions for those doing extensions
- perlapi Perl API listing (autogenerated)
- XS Mechanics tutorial
- Perl and C++ building XS modules for C++
- xs-fun XS is fun: a simple and easy tutorial on writing Perl XS