`SWIG/Examples/ruby/variables/`

## Wrapping C Global Variables

`/cvsroot/SWIG/Examples/ruby/variables/index.html,v 1.3 2002/11/30 22:10:05 beazley Exp`

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 here to see a SWIG interface with some variable declarations in it.

## Manipulating Variables from Ruby

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 '`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 Ruby, 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 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 `Example.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 Ruby, the old value is destroyed using `free()`.
- `signed char` and `unsigned char` are handled as small 8-bit integers.
- String array variables such as '`char name[256]`' 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.
- 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.