How can I be sure I'm not losing signals when using pause()?

I'm writing a program that uses fork to create child processes and count them when they're done. How can I be sure I'm not losing signals? what will happen if a child sends the signal while the main program still handles the previous signal? is the signal "lost"? how can I avoid this situation?

void my_prog()
{
    for(i = 0; i<numberOfDirectChildrenGlobal; ++i) {    
        pid = fork();
        if(pid > 0)//parent
            //do parent thing
        else if(0 == pid) //child
            //do child thing
        else
            //exit with error
    }

    while(开发者_开发百科numberOfDirectChildrenGlobal > 0) {
        pause(); //waiting for signal as many times as number of direct children
    }

    kill(getppid(),SIGUSR1);
    exit(0);
}

void sigUsrHandler(int signum)
{
    //re-register to SIGUSR1
    signal(SIGUSR1, sigUsrHandler);
    //update number of children that finished
    --numberOfDirectChildrenGlobal;
}

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It's recommended to use sigaction instead of signal, but in both cases it won't provide what you need. If a child sends a signal while the previous signal is still being handled, it will become a pending signal, but if more signals are sent they will be discarded (on systems that are not blocking incoming signals, the signals can be delivered before reestablishment of the handler and again resulting in missing signals). There is no workaround for this.

What one usually does is to assume that some signals are missing, and lets the handler take care of exiting children.

In your case, instead of sending a signal from your children, just let the children terminate. Once they terminate, the parent's SIGCHLD handler should be used to reap them. Using waitpid with WNOHANG option ensures that the parent will catch all the children even if they all terminate at the same time.

For example, a SIGCHLD handler that counts the number of exited children can be :

 pid_t pid;

 while((pid = waitpid(-1, NULL, WNOHANG)) > 0) {
       nrOfChildrenHandled++;
 }

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To avoid this situation you can use the posix real-time signals.

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Use sigaction instead of signal to register your handlers, and the delivery of the signals is assured.