Wrapping C Global Variables
$Header: /cvs/projects/SWIG/Examples/tcl/variables/index.html,v 18.104.22.168 2002/04/08 03:55:16 beazley Exp $
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 here to see a SWIG interface with some variable declarations in it.
Manipulating Variables from Tcl
Click here 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.
- The set statement changes the value of the corresponding C global variable.
- Whenever you access the value of a variable such as $ivar, the value
of the C global variable is read.
- 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).
- When a global variable has the type "char *", 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 free() or delete (the choice of which depends
on whether or not SWIG was run with the -c++ option).
- signed char and unsigned char are handled as small 8-bit integers.
- String array variables such as 'char name' 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.
- 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.
Creating read-only variables
The %immutable and %mutable directives can be used to
specify a collection of read-only variables. For example:
The %immutable directive remains in effect until it is explicitly disabled
using the %mutable directive.
- 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.
- You may be better off hiding global variables behind a function based