How can a process kill itself?

Question

#include<stdlib.h>
#include<unistd.h>
#include<signal.h>

int main(){

    pid_t pid = fork();

    if(pid==0){
            system("watch ls");
    }
    else{
            sleep(5);
            killpg(getpid(),SIGTERM);  //to kill the complete process tree.
    }
    return 0;
}

Terminal:

anirudh@anirudh-Aspire-5920:~/Desktop/testing$ gcc test.c
anirudh@anirudh-Aspire-5920:~/Desktop/testing$ ./a.out
Terminated

for the first 5 secs the output of the "watch ls" is shown and then it terminates because I send a SIGTERM.

Question: How can a process kills itself ? I have done kill(getpid(),SIGTERM);

My hypothesis: so during the kill() call the process switches to kernel mode. The kill call sends the SIGTERM to the process and copies it in the process's process table. when the process comes back to user mode it sees the signal in its table and it terminates itself (HOW ? I REALLY DO NOT KNOW ) (I think I am going wrong (may be a blunder) somewhere in my hypothesis ... so Please enlighten me)

This code is actually a stub which I am using to test my other modules of the Project. Its doing the job for me and I am happy with it but there lies a question in my mind how actually a process kills itself. I want to know the step by step hypothesis.

Thanks in advance

Anirudh Tomer

Answer 1

Your process dies because you are using killpg(), that sends a signal to a process group, not to a process.

When you fork(), the children inherits from the father, among the other things, the process group. From man fork:

   *  The child's parent process ID is the same as the parent's process ID.

So you kill the parent along with the child.

If you do a simple kill(getpid(), SIGTERM) then the father will kill the child (that is watching ls) and then will peacefully exit.