/trunk/Examples/tcl/variables/index.html
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- <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>
- <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="runme.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>%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>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>