List ALL possible outputs of this program, assuming that all function and system calls succeed and return without error. You may also assume that no externally issued signals are sent to either process. int counter = 0; void handler (int sig) { counter++; } int main() { signal (SIGUSR1, handler); signal (SIGUSR2, handler); int parent = getpid(); int child = fork(); if (child == 0) { kill (parent, SIGUSR1); kill (parent, SIGUSR2); kill (parent, SIGUSR1); kill (parent, SIGUSR2); exit(0); } sleep (1); waitpid (child, NULL, 0); printf ("Rec %d USR 1,2 sigs ", counter); return 0; }