Linux: blit image on screen without stuttering at a stable frame rate

Question

I'm working on a very simple game, and one of the project goals is to minimise the use of libraries.

I'm drawing every frame into a buffer, which is then displayed using XPutImage (see the code of the main loop). This simple code is used to limit FPS:

// Limit FPS
{
  u64 current_timestamp = LinuxGetWallClock();
  u64 ns_elapsed = LinuxGetWallClock() - last_timestamp;

  if (ns_elapsed < target_nspf) {
    struct timespec ts;
    ts.tv_sec = 0;
    ts.tv_nsec = target_nspf - ns_elapsed;  // time to sleep
    clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, NULL);

    while (ns_elapsed < target_nspf) {
      ns_elapsed = LinuxGetWallClock() - last_timestamp;
    }
  } else {
    // printf("Frame missed\n");
  }

  last_timestamp = LinuxGetWallClock();
}

Everything works fine and as expected, but every now and then the image stutters, as if the X server has buffered up a few frames' worth of images and then flushed them all in a single go. This happens irrespective of frame misses.

Note: I'm not talking about image tearing here.

I realise that the code above is not supposed to provide a stable frame rate, and occasionally one frame might be displayed for too long, and another frame might get lost and never displayed, but I don't see how that thing alone can cause the terrible occasional stuttering.

My only guess would be that clock_nanosleep may sleep for longer than I asked for.

Questions:

• Is my guess above correct?
• If not, what might be the reason?
• Are there any ways to address this problem without having to use OpenGL to wait on vblank?

Thanks.