static function in C

pmg is spot on about encapsulation; beyond hiding the function from other translation units (or rather, because of it), making functions static can also confer performance benefits in the presence of compiler optimizations.

Because a static function cannot be called from anywhere outside of the current translation unit (unless the code takes a pointer to its address), the compiler controls all the call points into it.

This means that it is free to use a non-standard ABI, inline it entirely, or perform any number of other optimizations that might not be possible for a function with external linkage.

The static keyword in C is used in a compiled file (.c as opposed to .h) so that the function exists only in that file.

Normally, when you create a function, the compiler generates cruft the linker can use to, well, link a function call to that function. If you use the static keyword, other functions within the same file can call this function (because it can be done without resorting to the linker), while the linker has no information letting other files access the function.

Making a function static hides it from other translation units, which helps provide encapsulation.

helper_file.c

int f1(int);        /* prototype */
static int f2(int); /* prototype */

int f1(int foo) {
    return f2(foo); /* ok, f2 is in the same translation unit */
                    /* (basically same .c file) as f1         */
}

int f2(int foo) {
    return 42 + foo;
}

main.c:

int f1(int); /* prototype */
int f2(int); /* prototype */

int main(void) {
    f1(10); /* ok, f1 is visible to the linker */
    f2(12); /* nope, f2 is not visible to the linker */
    return 0;
}