Unexpected parent process id in output

Question

My code is forking a process and printing each process' PID and PPID. I was expecting the child's PPID to be same as the parent's PID, but it is not coming up as such.

I'm using Ubuntu 14.04.

#include <stdio.h>
#include <sys/wait.h>

int main(){
    int pid;
    pid = fork();
    if(pid==0){
        printf("\nI am the child and my parent id is %d and my id %d\n", getppid(), getpid());
    }
    else
        printf("\nI am the parent and my pid is %d and my parent id is %d\n", getpid(), getppid());

    return 0;
}

Here is the output I am getting:

I am the parent and my pid is 29229 and my parent id is 27087
I am the child and my parent id is 1135 and my id is 29230

John WH Smith · Accepted Answer · 2016-10-01T20:51:20.560

5

My guess is: the parent returned before the child, which became an orphan. PID 1135 must be your user init process, which became the process' new parent. (there are 2 subreapers in a Ubuntu user session).

$ ps -ef | grep init
you    1135    ...    init --user

If you want your parent to wait for its child, use wait. You actually have the include already:

#include <stdio.h>
#include <sys/wait.h>

int main(){
    int pid;
    pid = fork();
    if(pid == 0)
        printf("\nI am the child and my parent id is - %d and mine id %d\n",getppid(),getpid());
    else{
       printf("\nI am the parent and my pid is %d and my parent id is %d\n",getpid(),getppid());
       wait(NULL);
    }
    return 0;
}

This will ensure that the parent doesn't exit before the child's printf. You can see this behaviour more clearly by inserting a few sleep() calls here and there to see in which order things occur.

For more information on subreapers, have a look here.

edited Oct 01 '16 at 20:51

answered Oct 01 '16 at 20:44

John WH Smith

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Thanks,Earlier I was doing examples on zombie process and was not able to understand the ppid of 1135,Now I get it – Beginner Oct 01 '16 at 20:56
Note to you and the OP: `fork()` may return -1 in case of error. You should handle that in your code. – ott-- Oct 01 '16 at 21:26
@ott Indeed! However I will keep that out of the answer's code (based on the OP's), since it's also outside of the question's scope. The [`fork`](https://linux.die.net/man/2/fork) and [`wait`](https://linux.die.net/man/2/wait) man pages will provide information on such things. – John WH Smith Oct 01 '16 at 21:28

Unexpected parent process id in output

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