I try to draw out the process according to the code but I really need some explanation in why, here is the question:
B() {
pid_t pid;
if ((pid = fork()) != 0)
waitpid(pid,NULL,0);
printf("2");
if(fork() ==0)
{ printf("3"); exit(0); }
printf("5");
exit(0);
}
Which one are illegals output?
232553, 235325, 232355, 235253, 252533...
This is the process I draw out according to the code and my understanding of fork.
___3 (exit so no more here)
|
__2__|___5 (I guess 5 should be here)
| |
| |
____|____(wait)|(start again since printf 3 process end)
So I'm stuck right there... Any help is appreciated.
Okay, there are two forks. Here is control flow, from left to right, with parents on top and children on the bottom:
+-- B ---- waitpid() --+ +-- "5" -- E
| | |
A --+ fork() +-- C -- "2" -- D -+ fork()
| | |
+----------------------+ +-- "3" -- F
So, what do we know?
"2", "5", and "3" each appear twice (90 possibilities)
No prefix may contain more "3" than "2" (30 possibilities)
No prefix may contain more "5" than "2" (16 possibilities)
The second "2" must be preceded by the first "5" (7 possibilities)
The 7 possibilities are:
2,3,5,2,3,5 2,3,5,2,5,3 2,5,2,3,3,5 2,5,2,3,5,3 2,5,2,5,3,3 2,5,3,2,3,5 2,5,3,2,5,3