mirror of https://github.com/swig/swig
101 lines
3.9 KiB
HTML
101 lines
3.9 KiB
HTML
<html>
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<title>SWIG:Examples:python:variables</title>
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</head>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/python/variables/</tt>
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<hr>
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<H2>Wrapping C Global Variables</H2>
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<p>
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When a C global variable appears in an interface file, SWIG tries to
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wrap it using a technique known as "variable linking." The idea is
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pretty simple---we try to create a Python variable that magically
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retrieves or updates the value of the underlying C variable when it is
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accessed. Click <a href="example.i">here</a> to see a SWIG interface with some variable
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declarations in it.
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<h2>Manipulating Variables from Python</h2>
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Before going any further, it is important to understand some important
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differences between C and Python variables. In C, a variable is
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simply a name that refers to a specific location in memory. For
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example, when you declare a global variable '<tt>double a</tt>' you
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know that somewhere in memory, 8 bytes have been set aside to hold a
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<tt>double</tt> and that <tt>a</tt> is bound to this location for the
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life of the program. In Python, variable creation is nothing more
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than a naming operation. For example, when you say '<tt>a = 3</tt>',
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'a' becomes a name that refers to some object '3'. Later on, if you say
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'<tt>a = 7.5</tt>, the name 'a' is bound to an entirely different object
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containing the value '7.5' (the contents of the original object are not
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changed). The end result of this is that a variable in Python can refer
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to a virtually unlimited number of different objects (memory locations)
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over the lifetime of a program.
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<p>
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Because of Python's somewhat unusual variable assignment semantics, it is not
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possible to directly link a C global variable into an equivalent Python variable.
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Instead, all C global variables are accessed as attributes of a special object
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called 'cvar'. For example, if you had a global variable
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<blockquote>
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<pre>
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double foo;
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</pre>
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</blockquote>
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it will be accessed in the Python module as <tt>cvar.foo</tt>. Click
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<a href="runme.py">here</a> to see a script that updates and prints
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out the values of the variables using this technique.
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<h2>Key points</h2>
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<ul>
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<li>When a global variable has the type "<tt>char *</tt>", SWIG manages it as a character
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string. However, whenever the value of such a variable is set from Python, the old
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value is destroyed using <tt>free()</tt> or <tt>delete</tt> (the choice of which depends
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on whether or not SWIG was run with the -c++ option).
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<li><tt>signed char</tt> and <tt>unsigned char</tt> are handled as small 8-bit integers.
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<li>String array variables such as '<tt>char name[256]</tt>' are managed as Python strings, but
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when setting the value, the result is truncated to the maximum length of the array. Furthermore, the string is assumed to be null-terminated.
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<li>When structures and classes are used as global variables, they are mapped into pointers.
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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.
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</ul>
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<h2>Creating read-only variables</h2>
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The <tt>%immutable</tt> and <tt>%mutable</tt> directives can be used to
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specify a collection of read-only variables. For example:
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<blockquote>
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<pre>
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%immutable;
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int status;
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double blah;
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...
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%mutable;
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</pre>
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</blockquote>
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The <tt>%immutable</tt> directive remains in effect until it is explicitly disabled
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using the <tt>%mutable</tt> directive.
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<h2>Comments</h2>
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<ul>
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<li>Management of global variables is one of the most problematic aspects
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of C/C++ wrapping because the scripting interface and resulting memory management
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is much trickier than simply creating a wrapper function.
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<p>
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<li>Because of the potential for a namespace conflict, you should not use
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the <tt>from module import *</tt> statement for a SWIG module with global
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variables. Doing so will cause a collision on the 'cvar' object should
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more than one module be loaded in this manner.
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</ul>
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</body>
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</html>
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<hr>
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