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<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>
<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="runme.pl">here</a> to see a script that updates and prints some global variables.
<h2>Creating read-only variables</h2>
The <tt>%immutable</tt> and <tt>%mutable</tt> directives can be used to
specify a collection of read-only variables. For example:
<blockquote>
<pre>
%immutable;
int status;
double blah;
...
%mutable;
</pre>
</blockquote>
The <tt>%immutable</tt> directive remains in effect until it is explicitly disabled
using the <tt>%mutable</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>
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