what is the relation between SIGTSTP and SIGCHLD

Question

I have tow handlers for each one of them (SIGTSTP, SIGCHLD), the thing is that when I pause a process using SIGTSTP the handler function of SIGCHLD run too. what should I do to prevent this .

signal handlers :

void signalHandler(int signal) {
int pid, cstatus;
if (signal == SIGCHLD) {
    susp = 0;
    pid = waitpid(-1, &cstatus, WNOHANG);
    printf("[[child %d terminated]]\n", pid);
    DelPID(&JobsList, pid);
}
}

void ctrlZsignal(int signal){
    kill(Susp_Bg_Pid, SIGTSTP);
    susp = 0;
    printf("\nchild %d suspended\n", Susp_Bg_Pid);
}

Susp_Bg_Pid used to save the paused process id.
susp indicates the state of the "smash" the parent process if it is suspended or not .

It appears that with the right flags you can tell which child got paused, and which signal it received. So that would not only let you detect the actual exit, but also take some action for pause/resume. — Ben Voigt, Nov 10 '13 at 00:35

Duck · Accepted Answer · 2013-11-10T00:15:33.990

Set up your SIGCHLD handler using sigaction with SA_NOCLDSTOP.

From sigaction (2)

SA_NOCLDSTOP - If signum is SIGCHLD, do not receive notification when child processes stop (i.e., when they receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or resume (i.e., they receive SIGCONT)(see wait(2)). This flag is only meaningful when establishing a handler for SIGCHLD.

update

void signalHandler(int sig)
{
  //...
}

struct sigaction act;

act.sa_handler = signalHandler;
sigemptyset(&act.sa_mask);
act.sa_flags = SA_NOCLDSTOP;

if (sigaction(SIGCHLD, &act, 0) == -1)
{
  perror("sigaction");
  exit(1);
}

If you aren't familiar with sigaction you should read up on it because it has several different options and behaviors that are vastly superior to signal but come at the cost of complexity and confusion before you figure out how to use it. I took my best guess at the minimum of what you seem to want to do but you'll need to learn this sooner rather than later.

BTW, though SA_NOCLDSTOP is available on *nix platforms it is implementation dependent and may not work as expected on all platforms. It works on linux. — Duck, Nov 10 '13 at 00:21

what is the relation between SIGTSTP and SIGCHLD

1 Answers1