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1<html> 2<head> 3<title>SWIG:Examples:tcl:variables</title> 4</head> 5 6<body bgcolor="#ffffff"> 7 8<tt>SWIG/Examples/tcl/variables/</tt> 9<hr> 10 11<H2>Wrapping C Global Variables</H2> 12 13<tt>$Header$</tt><br> 14 15<p> 16When a C global variable appears in an interface file, SWIG tries to wrap it using a technique 17known as "variable linking." The idea is pretty simple---we try to create a Tcl 18variable that works exactly like you would expect in a Tcl script, but which magically 19retrieves or updates the value of the underlying C variable. 20Click <a href="example.i">here</a> to see a SWIG interface with some variable declarations in it. 21 22<h2>Manipulating Variables from Tcl</h2> 23 24Click <a href="runme.tcl">here</a> to see a script that updates and prints out the values of 25the variables defined in the above file. Notice how the C global variables work just 26like normal Tcl variables. 27 28<h2>Key points</h2> 29 30<ul> 31<li>The <tt>set</tt> statement changes the value of the corresponding C global variable. 32<li>Whenever you access the value of a variable such as <tt>$ivar</tt>, the value 33of the C global variable is read. 34<li>If a C program changes a global variable independently of Tcl, this change is 35automatically reflected in the Tcl variable (i.e., reads will always return the 36most up to date value of the variable). 37<li>When a global variable has the type "<tt>char *</tt>", SWIG manages it as a character 38string. However, whenever the value of such a variable is set from Tcl, the old 39value is destroyed using <tt>free()</tt> or <tt>delete</tt> (the choice of which depends 40on whether or not SWIG was run with the -c++ option). 41<li><tt>signed char</tt> and <tt>unsigned char</tt> are handled as small 8-bit integers. 42<li>String array variables such as '<tt>char name[256]</tt>' are managed as Tcl strings, but 43when setting the value, the result is truncated to the maximum length of the array. Furthermore, the string is assumed to be null-terminated. 44<li>When structures and classes are used as global variables, they are mapped into pointers. 45Getting 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. 46</ul> 47 48<h2>Creating read-only variables</h2> 49 50The <tt>%immutable</tt> and <tt>%mutable</tt> directives can be used to 51specify a collection of read-only variables. For example: 52 53<blockquote> 54<pre> 55%immutable; 56int status; 57double blah; 58... 59%mutable; 60</pre> 61</blockquote> 62 63The <tt>%immutable</tt> directive remains in effect until it is explicitly disabled 64using the <tt>%mutable</tt> directive. 65 66<h2>Comments</h2> 67<ul> 68<li>Management of global variables is one of the most problematic aspects 69of C/C++ wrapping because the scripting interface and resulting memory management 70is much trickier than simply creating a wrapper function. 71<p> 72<li>You may be better off hiding global variables behind a function based 73interface. 74</ul> 75 76</body> 77</html> 78<hr>