usleep(useconds) unsigned useconds;
The routine is implemented by setting an interval timer and pausing until it occurs. The previous state of this timer is saved and restored. If the sleep time exceeds the time to the expiration of the previous timer, the process sleeps only until the signal would have occurred, and the signal is sent a short time later.
This routine is implemented using setitimer(2); it requires eight system calls each time it is invoked. A similar but less compatible function can be obtained with a single select(2); it would not restart after signals, but would not interfere with other uses of setitimer.
Note: This routine should not be used in multithreaded programs.