XS (Perl)
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 "eXtendable Subroutine".
XS also refers to a glue language for specifying calling interfaces supporting such interfaces.
Background
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.
Perl interpreter
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 and highly technical, and using it requires some understanding of the interpreter. The earliest reference on the subject was the POD.Wrappers
It is possible to write XS modules that wrap C++ code. Doing so is mostly a matter of configuring the module build system.Example code
The following shows an XS module that exposes a functionconcat to concatenate two strings.- define PERL_NO_GET_CONTEXT
- include "EXTERN.h"
- include "perl.h"
- include "XSUB.h"
MODULE = Demo::XSModule PACKAGE = Demo::XSModule
SV*
concat
CODE:
SV* to_return = _do_sv_catsv;
RETVAL = to_return;
OUTPUT:
RETVAL
The first four lines are standard boilerplate.
After then follow any number of plain C functions that are callable locally.
The section that starts with
MODULE = Demo::XSModule defines the Perl interface to this code using the actual XS macro language. Note that the C code under the CODE: section calls the _do_sv_catsv pure-C function that was defined in the prior section.Perl’s documentation explains the meaning and purpose of all of the “special” symbols shown above.
To make this module available to Perl it must be compiled. Build tools like can do this automatically. Perl code then uses a module like to load the compiled XS module. At this point Perl can call
Demo::XSModule::concat and receive back a string foobar, as if concat were itself written in Perl.Note that, for building Perl interfaces to preexisting C libraries, the h2xs can automate much of the creation of the XS file itself.