Bob Bob - 1 year ago 83
C Question

Forking in a for loop clarification

I've seen lots of examples of forking in for loops on here, but not much clarification on how it does what it does. Lets use this simple example from an answer of How to use Fork() to create only 2 child processes? as an example.

for (i = 0; i < n; ++i) {
pid = fork();
if (pid) {
} else if (pid == 0) {
} else {
printf("fork error\n");

Most of the examples I've seen follow this general format. But what I don't understand is, how does this prevent child processes from forking as well? From my understanding, every child that gets created has to go through this loop as well. But fork() is called at the very beginning of the for loop, and then the 3 comparisons happen. Could someone explain how, even though the children seem to call fork(), this for loop still ensures only the parent can create children?

Answer Source

The child starts at the line after fork. fork returns 0 for the child. In your example, the child would go into the pid == 0 block and break out of the for loop.

After a fork everything is exactly the same for the child and parent (including the next instruction to execute and variable values). The only difference is the return value from fork (0 for the child, and the child's pid for the parent).

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