<html> <head> <title>SWIG:Examples:tcl:variables</title> </head> <body bgcolor="#ffffff"> <tt>SWIG/Examples/tcl/variables/</tt> <hr> <H2>Wrapping C Global Variables</H2> <tt>$Header: /cvs/projects/SWIG/Examples/tcl/variables/index.html,v 1.2 2000/08/30 20:31:30 beazley Exp $</tt><br> <p> When a C global variable appears in an interface file, SWIG tries to wrap it using a technique known as "variable linking." The idea is pretty simple---we try to create a Tcl variable that works exactly like you would expect in a Tcl script, but which magically retrieves or updates the value of the underlying C variable. Click <a href="example.i">here</a> to see a SWIG interface with some variable declarations in it. <h2>Manipulating Variables from Tcl</h2> Click <a href="example.tcl">here</a> to see a script that updates and prints out the values of the variables defined in the above file. Notice how the C global variables work just like normal Tcl variables. <h2>Key points</h2> <ul> <li>The <tt>set</tt> statement changes the value of the corresponding C global variable. <li>Whenever you access the value of a variable such as <tt>$ivar</tt>, the value of the C global variable is read. <li>If a C program changes a global variable independently of Tcl, this change is automatically reflected in the Tcl variable (i.e., reads will always return the most up to date value of the variable). <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 Tcl, 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 Tcl 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. </ul> <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>Comments</h2> <ul> <li>Management of global variables is one of the most problematic aspects of C/C++ wrapping because the scripting interface and resulting memory management is much trickier than simply creating a wrapper function. <p> <li>You may be better off hiding global variables behind a function based interface. </ul> </body> </html> <hr>
