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SWIG/Examples/tcl/variables/
Wrapping C Global Variables
$Header: /cvs/projects/SWIG/Examples/tcl/variables/index.html,v 1.2.4.1 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.
Key points
- 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[256]' 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:
%immutable;
int status;
double blah;
...
%mutable;
The %immutable directive remains in effect until it is explicitly disabled
using the %mutable directive.
Comments
- 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
interface.
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