The Self-Pipe Trick

https://news.ycombinator.com/item?id=27185522

The linux programming interface 63.5.2 The Self-Pipe Trick

static int pfd[2]; /* File descriptors for pipe */
static void handler(int sig)
{
    int savedErrno; /* In case we change 'errno' */
    savedErrno = errno;
    if (write(pfd[1], "x", 1) == -1 && errno != EAGAIN)
        errExit("write");
    errno = savedErrno;
}
int main(int argc, char *argv[])
{
    fd_set readfds;
    int ready, nfds, flags;
    struct timeval timeout;
    struct timeval *pto;
    struct sigaction sa;
    char ch;
    /* ... Initialize 'timeout', 'readfds', and 'nfds' for select() */
    if (pipe(pfd) == -1)
        errExit("pipe");
    FD_SET(pfd[0], &readfds); /* Add read end of pipe to 'readfds' */
    nfds = max(nfds, pfd[0] + 1); /* And adjust 'nfds' if required */
    flags = fcntl(pfd[0], F_GETFL);
    if (flags == -1)
        errExit("fcntl-F_GETFL");
    flags |= O_NONBLOCK; /* Make read end nonblocking */
    if (fcntl(pfd[0], F_SETFL, flags) == -1)
        errExit("fcntl-F_SETFL");
    flags = fcntl(pfd[1], F_GETFL);
    if (flags == -1)
        errExit("fcntl-F_GETFL");
    flags |= O_NONBLOCK; /* Make write end nonblocking */
    if (fcntl(pfd[1], F_SETFL, flags) == -1)
        errExit("fcntl-F_SETFL");
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART; /* Restart interrupted read()s */
    sa.sa_handler = handler;
    if (sigaction(SIGINT, &sa, NULL) == -1)
        errExit("sigaction");
    while ((ready = select(nfds, &readfds, NULL, NULL, pto)) == -1 && errno == EINTR)
        continue; /* Restart if interrupted by signal */
    if (ready == -1) /* Unexpected error */
        errExit("select");
    if (FD_ISSET(pfd[0], &readfds))
    { /* Handler was called */
        printf("A signal was caught\n");
        for (;;)
        { /* Consume bytes from pipe */
            if (read(pfd[0], &ch, 1) == -1)
            {
                if (errno == EAGAIN)
                    break; /* No more bytes */
                else
                    errExit("read"); /* Some other error */
            }
            /* Perform any actions that should be taken in response to signal */
        }
    }
    /* Examine file descriptor sets returned by select() to see
     which other file descriptors are ready */
}