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git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk/SWIG@727 626c5289-ae23-0410-ae9c-e8d60b6d4f22
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<title>SWIG:Examples:tcl:variables</title>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/tcl/variables/</tt>
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<hr>
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<H2>Wrapping C Global Variables</H2>
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<tt>$Header$</tt><br>
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<p>
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When a C global variable appears in an interface file, SWIG tries to wrap it using a technique
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known as "variable linking." The idea is pretty simple---we try to create a Tcl
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variable that works exactly like you would expect in a Tcl script, but which magically
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retrieves or updates the value of the underlying C variable.
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Click <a href="example.i">here</a> to see a SWIG interface with some variable declarations in it.
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<h2>Manipulating Variables from Tcl</h2>
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Click <a href="example.tcl">here</a> to see a script that updates and prints out the values of
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the variables defined in the above file. Notice how the C global variables work just
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like normal Tcl variables.
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<h2>Key points</h2>
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<ul>
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<li>The <tt>set</tt> statement changes the value of the corresponding C global variable.
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<li>Whenever you access the value of a variable such as <tt>$ivar</tt>, the value
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of the C global variable is read.
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<li>If a C program changes a global variable independently of Tcl, this change is
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automatically reflected in the Tcl variable (i.e., reads will always return the
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most up to date value of the variable).
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<li>When a global variable has the type "<tt>char *</tt>", SWIG manages it as a character
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string. However, whenever the value of such a variable is set from Tcl, the old
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value is destroyed using <tt>free()</tt> or <tt>delete</tt> (the choice of which depends
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on whether or not SWIG was run with the -c++ option).
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<li><tt>signed char</tt> and <tt>unsigned char</tt> are handled as small 8-bit integers.
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<li>Array variables such as '<tt>char name[256]</tt>' are read-only variables because
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SWIG doesn't really know how to change the "value" of an array. You can work
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around this by writing some kind of helper function in the SWIG interface.
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For example:
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<blockquote>
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<pre>
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%inline %{
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void set_name(char *newname) {
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strncpy(name,newname,256);
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}
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%}
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</pre>
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</blockquote>
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</ul>
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<h2>Creating read-only variables</h2>
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The <tt>%readonly</tt> and <tt>%readwrite</tt> directives can be used to
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specify a collection of read-only variables. For example:
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<blockquote>
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<pre>
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%readonly
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int status;
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double blah;
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...
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%readwrite
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</pre>
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</blockquote>
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The <tt>%readonly</tt> directive remains in effect until it is explicitly disabled
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using the <tt>%readwrite</tt> directive.
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<h2>Comments</h2>
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<ul>
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<li>Management of global variables is one of the most problematic aspects
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of C/C++ wrapping because the scripting interface and resulting memory management
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is much trickier than simply creating a wrapper function.
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<p>
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<li>You may be better off hiding global variables behind a function based
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interface.
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</ul>
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</body>
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</html>
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