<html> <head> <title>SWIG:Examples:python:variables</title> </head> <body bgcolor="#ffffff"> <tt>SWIG/Examples/python/variables/</tt> <hr> <H2>Wrapping C Global Variables</H2> <tt>$Header: /cvs/projects/SWIG/Examples/python/variables/index.html,v 1.1 2000/08/30 22:38:51 beazley Exp $</tt><br> <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 Python variable that magically retrieves or updates the value of the underlying C variable when it is accessed. Click <a href="example.i">here</a> to see a SWIG interface with some variable declarations in it. <h2>Manipulating Variables from Python</h2> 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 '<tt>double a</tt>' you know that somewhere in memory, 8 bytes have been set aside to hold a <tt>double</tt> and that <tt>a</tt> 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 '<tt>a = 3</tt>', 'a' becomes a name that refers to some object '3'. Later on, if you say '<tt>a = 7.5</tt>, 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. <p> 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 <blockquote> <pre> double foo; </pre> </blockquote> it will be accessed in the Python module as <tt>cvar.foo</tt>. Click <a href="example.py">here</a> to see a script that updates and prints out the values of the variables using this technique. <h2>Key points</h2> <ul> <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 Python, 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 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. <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>%readonly</tt> and <tt>%readwrite</tt> directives can be used to specify a collection of read-only variables. For example: <blockquote> <pre> %readonly int status; double blah; ... %readwrite </pre> </blockquote> The <tt>%readonly</tt> directive remains in effect until it is explicitly disabled using the <tt>%readwrite</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>Because of the potential for a namespace conflict, you should not use the <tt>from module import *</tt> 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. </ul> </body> </html> <hr>
