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.
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