Linux system（）和信号处理程序导致竞争状况？

Question

I came across a problem with code like:

void sighandler(int signo)
{
    printf("sighandler() called\n");

    pid_t pid;
    pid = waitpid(-1, NULL, WNOHANG);
    if(pid >= 0)
    {
        printf("Caught by sighandler(): pid = %d\n", pid);
    }
    else
    {
        perror("sighandler pid failed");
    }
}


int main(void)
{
    int ret = 0;
    pid_t pid;

    signal(SIGCHLD, sighandler);

    ret = system("ls -al");

    if(ret < 0)
    {
        perror("system failed");
        printf("return value is %d\n", ret);
    }

    return 0;
}

1. In Linux(Cent OS) environment, sighandler() will be triggered after system() finishes. But on Mac OS X, sighandler() is not called in the same condition. Is this a known difference between Linux and BSD/UNIX systems?

2. The real problem is that in a GTK(C Language) program, SIGCHLD is bind with sighandler() in the main routine. But later I discover that a system() called in a callback function of child window GTK Button always returns -1. I'm fully sure that SIGCHLD is not bind with SIG_IGN and it is still bind sighandler() . Is it possible that waitpid in sighandler() catch the dead child process before waitpid() in system() which is intended to handle the dead child process?

Answer 1

Here's what the POSIX specification of system says regarding SIGCHLD :

The system() function shall ignore the SIGINT and SIGQUIT signals, and shall block the SIGCHLD signal, while waiting for the command to terminate.

Blocking SIGCHLD while waiting for the child to terminate prevents the application from catching the signal and obtaining status from system()'s child process before system() can get the status itself.

Note that if the application is catching SIGCHLD signals, it will receive such a signal before a successful system() call returns.

The last point is because when system() unblocks SIGCHLD , the pending signal is delivered, and it goes to the application's handler.