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<H2>Wrapping C Global Variables</H2>
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 Ruby variable (actually module method) 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 Ruby</h2>
Before going any further, it is important to understand some important
differences between C and Ruby 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 Ruby, 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 Ruby can refer
to a virtually unlimited number of different objects (memory locations)
over the lifetime of a program.
Because of Ruby's somewhat unusual variable assignment semantics, it is not
possible to directly link a C global variable into an equivalent Ruby variable.
Instead, all C global variables are accessed as attributes of the module.
For example, if you had a global variable
double foo;
it will be accessed in the Ruby module as <tt></tt>. Click
<a href="runme.rb">here</a> to see a script that updates and prints
out the values of the variables using this technique.
<h2>Key points</h2>
<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 Ruby, the old
value is destroyed using <tt>free()</tt>.
<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 Ruby 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.
<h2>Creating read-only variables</h2>
The <tt>%immutable</tt> and <tt>%mutable</tt> directives can be used to
specify a collection of read-only variables. For example:
int status;
double blah;
The <tt>%immutable</tt> directive remains in effect until it is explicitly disabled
using the <tt>%mutable</tt> directive.
<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.