Variable Attributes

Any variable may have one of the following attributes:


the variable may not be modified by the program, i.e. it is not allowed on the left-hand side of an assignment statement.


warns the compiler that the variable might be altered by some process external to the program.


the variable is external, declared as a global variable in another file.


(on a global variable) makes the variable local to this file, i.e. it cannot be referred to in another file using the extern attribute.


(on a local variable) allocates permanent storage for the variable, i.e. it is only initialised once, and it preserves its value between function calls.

auto  (the default for local variables) means that temporary storage is allocated for the variable on the stack, and its value is lost when the function returns.
register  if possible, the compiler will store the variable in a processor register rather than on the stack.