Strange fork() behavior (previous printf is recalled)

Question

I'm trying to solve a school problem about simulating a store with one employee (parent process) and multiple customers (child processes). I've simplified the code below to highlight the problem I've encountered while testing:

#define N_CUSTOMERS 10

void employee() {
}

void customer(int id) {
    exit(0);
}

int main() {

    printf("working!");

    // create customers
    for(int i = 0; i < N_CUSTOMERS; i++)
        if(!fork())
            customer(i);

    // be employee
    employee();

    // end
    exit(0);
}

The output is (compiled using gcc -Wall -pthread store.c -o store):

working!working!working!working!working!working!working!working!working!working!

I expected the printf to only execute once by the parent process, however, there seems to be a print for each child process created.

What am I missing here?

printf does execute only once by the parent process. But each child flushes that buffer when it exits, so it calls `write` as well. — William Pursell, Oct 10 '19 at 16:02

score 5 · Accepted Answer · answered Oct 10 '19 at 16:01

5

printf uses line-buffered output. Since the string printed doesn't have '\n', it's not printed before fork(). On exit() the buffer is flushed, and this happens in parent and all the children.

answered Oct 10 '19 at 16:01

Ruslan

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You might add a note to print with `puts` instead. – S.S. Anne Oct 10 '19 at 16:03

Strange fork() behavior (previous printf is recalled)

1 Answers1