How to implement a “private/restricted” function in C?

Here’s one way:

int f_real_name(void)
{
    ...
}

#define f f_real_name
int g(void)
{
    // call f()
}
#undef f

// calling f() now won't work

Another way, if you can guarantee that f() and g() are the only functions in the file, is to declare f() as static.

EDIT: Another macro trick to cause compiler errors:

static int f(void) // static works for other files
{
    ...
}

int g(void)
{
    // call f()
}
#define f call function

// f() certainly produces compiler errors here