Question

My current program is creating child processes and giving them work (CPU intensive work). The main() sits there and waits for the child processes to send data via pipes (using select).

What I wanted to do is when the program is processing data I could press CTRL+C to stop the child processes from working and asking the user if he wants to quit or resume work.

If user wants to quit, the program would kill all the processes. If user wants to resume work, it would tell the child processes to resume the computation.

I already have the code in place but it’s not quite working right.

In main I have signal(SIGINT, pausar); to handle SIGINT (CTRL+C).

This is the pausar() function:

void pausar(int signum){
    signal(SIGINT, pausar);

    int i;
    // pid[] contains all the child processes
    for(i = 0; i<CORES; i++)
    {
        kill(pid[i], SIGSTOP);
    }

    char option[2];
    printf("\n Computacao pausada.\n'S' para sair ou 'C' para continuar: ");

    scanf("%1s", option);
    if (option[0] == 's' || option[0] == 'S') {
        printf("A desligar...\n");

        //if user wants to quit, kill all the child processes
        for(i = 0; i<CORES; i++)
        {
            kill(pid[i], SIGKILL);
        }

        exit(0);
    }
    else
    {
        printf("[%d] A resumir computacao...\n",getpid());
        kill(getpid(), SIGCONT);

        //if user wants to resume work, send signal to continue
        for(i = 0; i<CORES; i++)
        {
            kill(pid[i], SIGCONT);
            printf("%d resumiu\n", pid[i]);
        }
    }
}

The problem is that sometimes I press CTRL+C and nothing shows in the console (but the processes STOP because I’m paying attention to the process manager). The other problem is that after I enter ‘C’ to resume work, I get errors in select() and the children never resume work.

Answer 1

First, for what you’re trying to-do, your signal handler is way too complex. Secondly, calling signal() inside your signal handler is not a good idea … it’s not an asynchronous signal-safe function.

What you can do is the following:

1. In your main, set the signal handler function using signal() like you’ve done.
2. Block the SIGINT signal via sigprocmask(). This prevents a spurious signal from arriving before the call to pselect().
3. Inside your signal handler only set a simple global flag that is a sig_atomic_t
4. Use pselect() instead of select(). This will allow you to change the process signal mask to allow a SIGINT signal to arrive, and it will do-so in an atomic manner with respect to signals. Otherwise, you could have your SIGINT arrive before the call to select(), and then you have “lost” that signal, even though it does set the flag in the handler.
5. When the pselect() call returns, detect whether the flag has been set.
6. If the global sig_atomic_t flag was set, and you returned from pselect because of a caught signal, then launch another function that will actually do all the ending of the child-processes and prompt the user, etc.

Doing these steps will simplify your signal-handling code and reduce the chances of race-conditions or other unexpected results because of the asynchronous nature of signal arrival.

If you’d like some more information on pselect(), you there is a nice article on that here.