Understanding stdout output when used with fork()

Question

Here is my C program :

int main() {
    fork();
    printf("one ");
    fork();
    printf("two ");
}

The output is :

one two one two one two one two

However, If I change the code and add a \n to the print statement :

int main() {
    fork();
    printf("one\n");
    fork();
    printf("two\n");
}

The output is :

one
one
two
two
two
two

Or sometimes (the order can change) :

one
two
one
two
two
two

Why is the output 8 words in the first program and 6 in the second?

stdout is line buffered. But it doesn't matter because process execution order is arbitrary anyway, so it could happen in any order. — Colonel Thirty Two, Aug 06 '16 at 14:34
This looks a lot like a homework problem. What explanation have YOU considered? — Peter, Aug 06 '16 at 14:34
This isn't a homework problem :) I want to understand buffer overwriting if happening here. — Elison Niven, Aug 06 '16 at 14:40

score 1 · Answer 1 · answered Aug 06 '16 at 14:35

2 processes execute the first printf(), and then 4 processes execute the second printf(), so there should be 6 output.

In the first code, it seems fork() is executed before what is in the buffer is flushed, then the contents of buffer is duplicated and then excess output appeared.

In the second code, it seems \n had it flush the buffer and therefore no excess output appeared.

Understanding stdout output when used with fork()

1 Answers1