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The keyword "static" defines that the function will only be available to callers in the file where it is declared. If a function needs to be called from another file, the function cannot be static and will require a prototype -- see prototypes below. The static form
is convenient for utility functions which will only be used in the file where they are
declared. Next , the "int" in the function above is the type of its return value. Next comes name of the function and its list of parameters. When referring to a function by
name in documentation or other prose, it's a convention to keep the parenthesis () suffix,
so in this case I refer to the function as "Twice()". The parameters are listed with their types and names, just like variables.

Inside the function, the parameter num and the local variable result are "local" to the function -- they get their own memory and exist only so long as the function is executing. This independence of "local" memory is a standard feature of most languages (See CSLibrary/102 for the detailed discussion of local memory).

The "caller" code which calls Twice() looks like...

int num = 13;
int a = 1;
int b = 2;
a = Twice(a); // call Twice() passing the value of a
b = Twice(b + num); // call Twice() passing the value b+num
// a == 2
// b == 30
// num == 13 (this num is totally independent of the "num" local to Twice()

Things to notice...

(vocabulary) The expression passed to a function by its caller is called the "actual parameter" -- such as "a" and "b + num" above. The parameter storage local to the function is called the "formal parameter" such as the "num" in "static int Twice(int num)".

Parameters are passed "by value" that means there is a single copying assignment operation (=) from each actual parameter to set each formal parameter. The actual parameter is evaluated in the caller's context, and then the value is copied into the
function's formal parameter just before the function begins executing. The alternative parameter mechanism is "by reference" which C does not implement directly, but
which the programmer can implement manually when needed (see below). When a parameter is a struct, it is copied.

The variables local to Twice(), num and result, only exist temporarily while Twice() is executing. This is the standard definition for "local" storage for functions.

The return at the end of Twice() computes the return value and exits the function. Execution resumes with the caller. There can be multiple return statements within a function, but it's good style to at least have one at the end if a return value needs to be specified. Forgetting to account of a return somewhere in the middle of a function
is a traditional source of bugs.
 

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