Signal handling in C

Question

I am figuring how signals work in C. Here is one of the examples taken from old exams:

#include<signal.h>
#include<unistd.h>
#include<stdio.h>
#include<errno.h>

//#include <sys/types.h>

void handler1(int sig) {
 printf("Phantom");
 exit(0);
}
int main()
{
   pid_t pid1;
   signal(SIGUSR1, handler1); //installing signal handler
   if((pid1 = fork()) == 0) { //forking
       printf("Ghost");
       exit(0);
   }
   kill(pid1, SIGUSR1);
   printf("Ninja");
   return 0;
}

So far, GCC gives me two answers Ghost Ninja & Ninja Phantom. Could it produce Ghost Phantom Ninja or any other combination made of 3 names ?

One way I see it could possibly produce 3 names is: Fork, run in Child, print Ghost, exit(0) => in Parent, accept/process signal and from signal Handler print Phantom, kill child, print Ninja. But I am not sure if my "theory" holds the ground.

Also, would the kill(pid1, SIGUSR1) invoke handler() ?

Thanks !

Answer 1

Let's examine this line-by line. Set up a signal handler, then fork. The child prints "Ghost" and exits. The parent makes the child print "Phantom" and exit. Then the parent prints "Ninja".

So you've really got a race condition. If the parent fires its SIGUSR1 before the child prints "Ghost", then you'll get Phantom Ninja, or perhaps Ninja Phantom (does kill block?)

But, if you can't get the signal off in time, then you'll get Ghost Ninja as the child finishes before the parent signals. I don't think the reverse is possible.

Now it is conceivable that the signal could be exactly on time to hit between the printf and the exit, in which case Ghost would finish, followed by Phantom then Ninja - or the reverse again, I think.

It's really finicky and sensitive to OS timing.

@Everyone - not tested! Feel free to contradict me, but I'll be as interested to know why as the OP.