SWIG:Examples:tcl:variables

`SWIG/Examples/tcl/variables/`

## Wrapping C Global Variables

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.