SWIG:Examples:python:variables

SWIG/Examples/python/variables/

Wrapping C Global Variables

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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 Python variable that magically retrieves or updates the value of the underlying C variable when it is accessed. Click here to see a SWIG interface with some variable declarations in it.

Manipulating Variables from Python

Before going any further, it is important to understand some important differences between C and Python variables. In C, a variable is simply a name that refers to a specific location in memory. For example, when you declare a global variable 'double a' you know that somewhere in memory, 8 bytes have been set aside to hold a double and that a is bound to this location for the life of the program. In Python, variable creation is nothing more than a naming operation. For example, when you say 'a = 3', 'a' becomes a name that refers to some object '3'. Later on, if you say 'a = 7.5, the name 'a' is bound to an entirely different object containing the value '7.5' (the contents of the original object are not changed). The end result of this is that a variable in Python can refer to a virtually unlimited number of different objects (memory locations) over the lifetime of a program.

Because of Python's somewhat unusual variable assignment semantics, it is not possible to directly link a C global variable into an equivalent Python variable. Instead, all C global variables are accessed as attributes of a special object called 'cvar'. For example, if you had a global variable

double foo;

it will be accessed in the Python module as cvar.foo. Click here to see a script that updates and prints out the values of the variables using this technique.

Key points

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 Python, 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 Python 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.
Because of the potential for a namespace conflict, you should not use the from module import * statement for a SWIG module with global variables. Doing so will cause a collision on the 'cvar' object should more than one module be loaded in this manner.