fork() call printing more times than expected

Question

The code looks something like this

int main() {
    printf("Starting process id is %d\t", getpid());
    if (fork() == 0) {
         fork();
     } else {
         fork();
         fork();
         exit(0);
     }
     printf("%d\t", getpid());
}

For the most part the outputs behave as expected where tracing. That is if we start with id n we get n+1 and n+5 being printed. However when I print the starting process id it gets printed 5 times and I am not sure why. Any help would be appreciated

Output from a sample run looks like this

Starting process id is 27252 pc@pc-VirtualBox:~/Desktop$ Starting porcess id is 27252 Starting porcess id is 27252 Starting porcess id is 27252 27253 Starting porcess id is 27252 Starting process id is 27252 27257

However I would have expected to see

Staring process id is 27252 27253 27257

Answer 1

Your initial printf doesn't end in a newline, and by default stdout connected to a terminal is line buffered. Thus, when you fork, the stdout buffer isn't empty, and when flushed by each child process on exit, it flushes the same data left in the buffer that it inherited.

To fix, do the following:

1. fflush(stdout) before forking
2. Exit the forked child processes with _exit, not exit (this can prevent buffers from being flushed, though it's implementation dependent so you can't make guarantees, but it also blocks atexit functions from being executed in the child process, which is usually the desired behavior)
3. As an optional alternative or complement to #1, you can use tcflush to clear the buffered output prior to _exit-ing.