usleep() suspends execution of the process for the specified number of microseconds (in this, it is a "microsleep", hence the name; think
This function is even less standardised than
nanosleep(), and its behaviour in the presence of signals is less specified that that system call's. However, it may be more common on older platforms.
nanosleep() and even
usleep() may well sleep for longer than specified. The time given is only the minimal length of (real, wall clock time) to sleep.