How to Cause a Child Process to Exit When the Parent Does

How can I cause a child process to exit when the parent does?

There is no tie between a child process and its parent. They may know each others process ID, but there's no hard connection between them. What you're talking about a orphan process. And it's an OS level concern. Meaning any solution is probably platform dependent.

About the only thing I can think of is to have the child check its parents status periodically, exiting if the parent's shutdown. I don't think this would be all that reliable though.

How to make child process die after parent exits?

Child can ask kernel to deliver SIGHUP (or other signal) when parent dies by specifying option PR_SET_PDEATHSIG in prctl() syscall like this:

prctl(PR_SET_PDEATHSIG, SIGHUP);

See man 2 prctl for details.

Edit: This is Linux-only

exit(...): Will it cause parent process to terminate?

exit does not cause the parent to also exit. It causes normal process termination for the current process.

How to kill all child processes after parent process termination?

Here's a probably more portable solution.

The fork(2) system call will return the PID of your child processes, you can store the PIDs, and then you can use kill(2) to send signal to the children and terminates them.

Notice that SIGKILL and SIGTERM signal may require some privileges of the parent process. If it doesn't have such privileges, you can send a SIGCONT to the child process, and modify the SIGCONT signal handler in your child process.

!!! Warning sign

From a signal handler using exit() is not safe. I've just checked the manual man 7 signal and found that it is not async safe. You can use _exit, _Exit or abort

Some pseudo code:

#include <stdio.h> 
#include <unistd.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
void* handler(int sig){
    _exit(0);
}
int main() 
{ 
    pid_t children[6];
    for(int i=0;i<6;i++) // loop will run 6 times(there are 6 child processes.) 
    { 
        if((children[i] = fork()) == 0) 
        { 
            signal(SIGCONT,handler);
            printf("Started [son] pid %d from [parent] pid %d\n",getpid(),getppid()); 

            sleep(10); //child waits 10 seconds,then it exitted.

            printf("Exitted [son] pid %d from [parent] pid %d\n",getpid(),getppid()); 

            exit(0); 
        } 
    } 

    //parent
    sleep(5);  //parent will wait 5 seconds than it will exit
    for(int i=0;i<6;i++)
        kill(children[I],SIGCONT);
    printf("Parent terminated\n");
    exit(0); //parent terminated.(how can I exit the the other 6 child processes too?)

}

Why child process not getting exited before parent process calls wait() function?

Upon exit, the child leaves an exit status that should be returned to the parent. So, when the child finishes it becomes a zombie.

Whenever the child exits or stops, the parent is sent a SIGCHLD signal.
The parent can use the system call wait() or waitpid() along with the macros WIFEXITED and WEXITSTATUS with it to learn about the status of its stopped child.

If the parent exits, than you can see your children still as zombie processes (unwaited children ).

wait() just tells you which child exited so you can get the exit code. If you have more children running, then of course, others could have terminated in the meantime as well.

If you don't care about the exit status, then wait() is just fine, but you still have to wait on all children you started.

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