Wrapping C Global Variables

$Header: /cvs/projects/SWIG/Examples/perl5/variables/index.html,v 2002/04/08 03:55:34 beazley Exp $

When a C global variable appears in an interface file, SWIG tries to wrap it using a technique known as "variable linking." The idea is pretty simple---we try to create a Perl variable that magically retrieves or updates the value of the underlying C variable when it is accessed. Click here to see a SWIG interface with some variable declarations in it.

Manipulating Variables from Perl

Accessing a C global variable from Perl is easy---just reference it like a normal Perl variable. Click here to see a script that updates and prints some global variables.

Creating read-only variables

The %immutable and %mutable directives can be used to specify a collection of read-only variables. For example:
int    status;
double blah;
The %immutable directive remains in effect until it is explicitly disabled using the %mutable directive.


  • When a global variable has the type "char *", SWIG manages it as a character string. However, whenever the value of such a variable is set from Perl, the old value is destroyed using free() or delete (the choice of which depends on whether or not SWIG was run with the -c++ option).
  • signed char and unsigned char are handled as small 8-bit integers.
  • String array variables such as 'char name[256]' are managed as Perl strings, but when setting the value, the result is truncated to the maximum length of the array. Furthermore, the string is assumed to be null-terminated.
  • When structures and classes are used as global variables, they are mapped into pointers. Getting the "value" returns a pointer to the global variable. Setting the value of a structure results in a memory copy from a pointer to the global.
  • Variables are linked using Perl's magic mechanism. Take a look at the Advanced Perl Programming book to find out more about this feature.