usleep

Quoted 2 lines

I would think the following code would wait for 1 second each itteration before printing hello,

well, usleep takes microseconds, so as you've written it, you should expect it to take 1ms per usleep or 50ms between hello's. further, the man page doesn't claim that it'll actually sleep for exactly the specified microseconds.

here:
#include <unistd.h>
#include <stdio.h>

int main() {
    int c = 20;
    while (c--) {
        int i;
        printf("hello\n");
        for(i = 0; i < 50; i++) {
            usleep(1000);
        }
    }
    return 0;
}

(not that a correct program needs both the stdio and a return from main.)

| [hahn@hahn hahn]$ /usr/bin/time ./chin
| hello
...

| hello
| 0.00user 0.00system 0:19.99elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
| 0inputs+0outputs (71major+10minor)pagefaults 0swaps
| [hahn@hahn hahn]$ setrealtime time ./chin
| hello
...
| hello
| 0.13user 0.87system 0:01.00elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
| 0inputs+0outputs (71major+10minor)pagefaults 0swaps

hmm, so on its own, 20 iterations of your loop take 20 seconds, one second per iteration, and the usleep is actually lasting 20ms. this is the traditional behavior (and correct, since the man page permits this kind of fuzziness).

if run in realtime mode, 20*50 usleep(1000)'s take 1.00 seconds, cool.

also, if I add:
#include <sys/time.h>

void myusleep(unsigned us) {
    struct timeval tv;
    tv.tv_sec = 0;
    tv.tv_usec = us;
    select(0,0,0,0,&tv);
}

then it behaves pretty much exactly as expected. (good code would handle the case where us>1000000 and where select is interrupted by a signal.)

Any way... what is setrealtime? I dont have it on my linux machine