How to know the output in the code with fork()?

Question

#include <unistd.h>
#include <sys/wait.h>
int main ()
{
    W(A);
    int child=fork();
    W(B);
    if (child)
      wait(NULL);
    W(C);
}

I think that ABBCC will be one of the outputs, but also one more of the next ones: ACCBB, ABB, ABCBC, ABCAA should be correct. W means write and W(A), W(B), W(C) mean to write A, B, C. Please help.

Answer 1

ACCBB is not possible.

I'll add a subscript _p for letters printed by the parent, _c for the child.

The parent process prints A_pB_pC_p in that order, and the child process prints B_cC_c in that order.

A_p will be printed first, because it's printed before anything is forked. The parent waits for the child to exit before printing C_p, so this has to come last. So this means the first and last characters are A and C.

In between this, there are no constraints on the order of execution between the two processes. So the following are possible:

B_pB_cC_c
B_cB_pC_c
B_cC_cB_p

The first two look the same in output, since we can't tell the difference between B_p and B_c

So the possible results are:

ABBCC
ABCBC

This all assumes that there's no buffering that might delay the output. I.e. this is just an exercise to examine the order that statements can be executed in concurrent processes, not really about I/O.