sigwait() does not work in multithreaded program

Question

I'm trying to write a multithreaded program which one thread (variable thread in below) is responsible to any asynchronous signals that might be set to this process.

I am facing thread that uses sigwait() but does not react to any signals have been sent to process. (like SIGUSR1 in below).

static void * signal_thread(void *arg = nullptr)
{
    int sig = -1;
    sigset_t sigset;
    sigfillset(&sigset);
    pthread_sigmask(SIG_BLOCK, &sigset, NULL);
    while(1)
    {
       int s = sigwait(&sigset, &sig);
       if(s == 0)
          printf("SIG %d recieved!...\n", sig);
       usleep(20);
    }
}

int main()
{
    sigset_t signalset;
    pthread_t thread;
    pthread_create(&thread, NULL, &signal_thread, nullptr);
    sigfillset(&signalset);
    pthread_sigmask(SIG_BLOCK, &signalset, NULL);

    while(1)
    {
       raise(SIGUSR1);
       usleep(20);
    }
}

Answer 1

The problem is concerned to two issues:

First, call of raise in main sent signal only to main thread not whole process.

Secondly, std::cout should be used instead of printf in signal_thread.

Answer 2

raise(sig) is the equivalent of calling pthread_kill(pthread_self(), sig) .

Since the main thread raise() s the signal, the SIGUSR1 will be generated for that thread and not for any other. Thus, your signal_thread will be unable to sigwait() for the USR1, which will be held pending for the thread that generated it.