<html> <head> <title>SWIG:Examples:perl5:variables</title> </head> <body bgcolor="#ffffff"> <tt>SWIG/Examples/perl5/variables/</tt> <hr> <H2>Wrapping C Global Variables</H2> <tt>$Header: /cvs/projects/SWIG/Examples/perl5/variables/index.html,v 1.1 2000/08/31 04:36:54 beazley Exp $</tt><br> <p> 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 <a href="example.i">here</a> to see a SWIG interface with some variable declarations in it. <h2>Manipulating Variables from Perl</h2> Accessing a C global variable from Perl is easy---just reference it like a normal Perl variable. Click <a href="example.pl">here</a> to see a script that updates and prints some global variables. <h2>Creating read-only variables</h2> The <tt>%readonly</tt> and <tt>%readwrite</tt> directives can be used to specify a collection of read-only variables. For example: <blockquote> <pre> %readonly int status; double blah; ... %readwrite </pre> </blockquote> The <tt>%readonly</tt> directive remains in effect until it is explicitly disabled using the <tt>%readwrite</tt> directive. <h2>Notes:</h2> <ul> <li>When a global variable has the type "<tt>char *</tt>", 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 <tt>free()</tt> or <tt>delete</tt> (the choice of which depends on whether or not SWIG was run with the -c++ option). <li><tt>signed char</tt> and <tt>unsigned char</tt> are handled as small 8-bit integers. <li>String array variables such as '<tt>char name[256]</tt>' 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. <li>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. <li>Variables are linked using Perl's magic mechanism. Take a look at the Advanced Perl Programming book to find out more about this feature. </ul> </body> </html> <hr>
