Listing all external functions

[Guilhem BONNEFILLE]

Hello,

In order to check the norms (POSIX.1, POSIX.2, ANSI C, etc.) used on my
project, I would like to obtain the list of all external functions or
(generally) symbols used.

For example:
-=-=-=-
void func(void)
{
...
}
int main(void)
{
  printf("BEGIN\n");
...
  func()
...
  printf("END\n");
}
-=-=-=-

In this example,I would like to obtain (only):
func


Is it possible to do this with global (and perhaps sed/awk scripts)?

Regards,
-- 
 Guilhem BONNEFILLE