Why does this ppid == 1? Am I missing something about fork()?

Question

#include <stdio.h>

int main(void)
{
    fork();
    fork();
    printf("ppid %d pid %d\n", getppid(), getpid());
    return 0;
}

The above code sometimes prints

$ ./a.out 
ppid 3389 pid 3883
ppid 3883 pid 3885
ppid 1 pid 3884
ppid 3884 pid 3886

Why is process 3884's ppid 1? Doesn't it supposed to be 3883?

Answer 1

I'd guess the parent process had already completed running and exited by the time the third child checked for the parent's PID. That would have caused the child to be re-parented under init, which has process ID 1.

Answer 2

Taken from:

http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=%2Fapis%2Fgetppid.htm

"The value returned by getppid() is the process ID of the parent process for the calling process. A process ID value of 1 indicates that there is no parent process associated with the calling process."

That printf instruction was executed within the parent process, so it returned 1 because it does not have a parent process. It's perfectly normal that this was the 3rd line to be printed, since the fork run its process concurrently and no particular order is guarantied.

Answer 3

To build upon Jamey's answer, when a process finishes before the child's execution and exits, the child becomes an orphan i.e. orphan process so the kernel maps that orphan to the init process.

Using wait() makes sure that the parent waits until the child has finished execution.