epoll_wait() blocks prints to stdout

Question

When using epoll_wait , it seems to "eat" everything that's written to stdout and delay the print until after epoll_wait has received an event, even though I tried to print before calling on anything related to epoll (it could even be at the beginning of my main method, it still won't get printed).

An example of prints that will not show until after epoll_wait has received an event:

printf("This doesn't get printed. ");
fprintf(stdout, "This doesn't get printed either.");
ev.events = EPOLLIN;
ev.data.fd = some_sock_fd; // Same with STDIN_FILENO
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, some_sock_fd, &ev) == -1) {
    perror("epoll_ctl");
    exit(EXIT_FAILURE);
}

for (;;) {
    rc = epoll_wait(epoll_fd, &ev, 1, -1);
    // This is where it gets printed

Writing to stderr works as normal, but how can I write to stdout ? How do I prevent epoll_wait from blocking prints to stdout ?

Answer 1

The issue seems to be unrelated to epoll_wait . Here's a summary of the offending code:

// Since there's no newline, the following stays in the buffer
printf("Some print without newline.");

for (;;) {
    // At this point, the buffer has not been flushed,
    // and epoll_wait blocks the output
    rc = epoll_wait(epoll_fd, &ev, 1, -1);

Using fflush(stdout) is the solution to this code, since the buffering has nothing to do with epoll_wait, but with how the user-space buffers stdout:

// Since there's no newline, the following stays in the buffer
printf("Some print without newline.");

// Forces a write of user-space buffered data for stdout
fflush(stdout);

for (;;) {
    // At this point, the buffer has not been flushed,
    // and epoll_wait blocks the output
    rc = epoll_wait(epoll_fd, &ev, 1, -1);

To sum it up, this seems to be a case of looking in the wrong place for an issue that should have been obvious.