Send child pid using sigqueue and SIGUSR1 to father

Question

I am trying to send a child's pid to his father using SIGUSR1 and sigqueue. But the signal is never sent, or it appears not to be sent.

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <stdlib.h>

void handler(int signum, siginfo_t *info, void *extra)
{
    void *ptr_val = info->si_value.sival_ptr;
    int int_val = info->si_value.sival_int;
    printf("Child: Father, I am %d!\n",int_val);
}

int main()
{
    int pid;

    printf("Father: I am %d\n",getpid());

    if ( (pid=fork()) <0 )
    {
        perror("fork");
        exit(EXIT_FAILURE);
    }
    else
        if ( pid==0 )
        {
            printf("Child: I am My father is %d.\n",getppid());
            sigqueue(getppid(),SIGUSR1,(const union sigval) getpid());
        }
        else
        {
            struct sigaction action;
            sigset_t mask;
            sigemptyset(&mask);
            sigaddset(&mask,SIGUSR1);
            action.sa_flags = SA_SIGINFO;
            action.sa_mask =mask;
            action.sa_sigaction = &handler;

            if (sigaction(SIGUSR1,&action,NULL)==-1)
            {
                perror("sigaction");
                exit(EXIT_FAILURE);
            }
            printf("Father: Welcome home, son!\n");
        }

    return 0;
}

Running the code above I get the following output:

Father: I am 18990
Father: Welcome home, son!
Child: My father is 18990.

And that's not all. If I run it again, all my apps close(editor,terminal,etc.) and I need to sign in again. (kind of a switch user operation) What's wrong with the code? Thanks.

Answer 1

The reason the signal is not caught is that parent is exiting before the child queues up the signal. Just add a delay inside the parent and you can see that it catches the signal.

This fact is further strengthened by a different output that I get on my system for your same program

Father: I am 18990
Father: Welcome home, son!
Child: My father is 1.

The child notes the father's pid as 1, because the father has exited leaving the child as orphan and being adopted by init.

A very related question

getpid and getppid returns two different values