Rewriting or breaking function linkage for C unit testing at compile time
void a_work(void) {
  b_work();
}

void b_work(void) {
  do_some_work();
}

/* for test, when a_work() calls b_work() I want this to be run */
void b_work(void) {
  if (fake_b_enabled)
    do_fake_b_work();
  else
    // call the real b_work() somehow
}