Storage Classes

C has a concept of 'Storage classes' which are used to define the scope (visability) and life time of variables and/or functions.

auto - storage class
auto is the default storage class for local variables.
{
int Count;
auto int Month;
}

The example above defines two variables with the same storage class. auto can only be used within functions, i.e. local variables.

register - Storage Class

register is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and cant have the unary '&' operator applied to it (as it does not have a memory location).

{
register int Miles;
}

Register should only be used for variables that require quick access - such as counters. It should also be noted that defining 'register' goes not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register - depending on hardware and implimentation restrictions.

static - Storage Class

static is the default storage class for global variables. The two variables below (count and road) both have a static storage class.

'static' can also be defined within a function. If this is done, the variable is initalised at compilation time and retains its value between calls. Because it is initialsed at compilation time, the initalistation value must be a constant. This is serious stuff - tread with care.

extern - storage Class

extern defines a global variable that is visable to ALL object modules. When you use 'extern' the variable cannot be initalized as all it does is point the variable name at a storage location that has been previously defined.