• Prototype:

int kill(pid_t pid, int signum);

• Analogous to the /bin/kill shell command.
  • Unfortunately named, since kill implies SIGKILL, which implies death.
  • So named because the default action of most signals in early UNIX implementations was to just terminate the target process (identified by pid).
• Returns -1 if the call fails (generally because the calling process doesn't have permission to fire signals at target process), 0 if all is swell.
• pid parameter is overloaded to provide many signalling strategies:
  • When pid is a positive number, the target is a process with that pid.
  • When pid is a negative number less than -1, the targets are all processes within the process group abs(pid).
  • pid can also be 0 or -1, but we don't need to worry about those (though it's documented in the B&O textbook if you're curious).

• We'll first work through a small puzzle to confirm you all understand the workflow of the processes and understand how kill triggers various signal handlers to be executed. (Error checking is omitted from this example, since it's so small, and we'll assume, for simplicity, that nothing ever fails).
• Putting it all together. What're the possible outputs (plural!) of the following program?

static pid_t pid;
static int counter = 0;

static void parentHandler(int unused) {       static void childHandler(int unused) {
  counter++;                                    counter += 3;
  printf("counter = %d\n", counter);            printf("counter = %d\n", counter);
  kill(pid, SIGUSR1);                         }
}



int main(int argc, char *argv[]) {
  signal(SIGUSR1, parentHandler);
  if ((pid = fork()) == 0) {
    signal(SIGUSR1, childHandler);
    kill(getppid(), SIGUSR1);
    return 0;
  }

  waitpid(pid, NULL, 0);
  counter += 7;
  printf("counter = %d\n", counter);
  return 0;
}

• Make sure you understand why the program on the previous slide is capable of producing two different outputs, depending on processor scheduling. (Hint: the child process may or may not exit before parentHandler has a chance to kill it!)

myth22> ./kill-puzzle
counter = 1
counter = 8
myth22> ./kill-puzzle
counter = 1
counter = 3
counter = 8
myth22>