External Variable - Scope, Lifetime and The static Keyword

Scope, Lifetime and The static Keyword

An external variable can be accessed by all the functions in all the modules of a program. It is a global variable. For a function to be able to use the variable, a declaration or the definition of the external variable must lie before the function definition in the source code. Or there must be a declaration of the variable, with the keyword extern, inside the function.

The static keyword (static and extern are mutually exclusive), applied to the definition of an external variable, changes this a bit: the variable can only be accessed by the functions in the same module where it was defined. But it is possible for a function in the same module to pass a reference (pointer) of the variable to another function in another module. In this case, even though the function is in another module, it can read and modify the contents of the variable—it just cannot refer to it by name.

It is also possible to use the static keyword on the definition of a local variable. Without the static keyword, the variable is automatically allocated when the function is called and released when the function exits (thus the name "automatic variable"). Its value is not retained between function calls. With the static keyword, the variable is allocated when the program starts and released when the program ends. Its value is not lost between function calls. The variable is still local, since it can only be accessed by name inside the function that defined it. But a reference (pointer) to it can be passed to another function, allowing it to read and modify the contents of the variable (again without referring to it by name).

External variables are allocated and initialized when the program starts, and the memory is only released when the program ends. Their lifetime is the same as the program's.

If the initialization is not done explicitly, external (static or not) and local static variables are initialized to zero. Local automatic variables are uninitialized, i.e. contain "trash" values.

The static keyword applied to a function definition prevents the function from being called by name from outside its module (it remains possible to pass a function pointer out of the module and use that to invoke the function).