1.3 How can I sleep for less than a second?

The sleep() function, which is available on all Unixes, only allows for a duration specified in seconds. If you want finer granularity, then you need to look for alternatives:

• Many systems have a function usleep()
• You can use select() or poll(), specifying no file descriptors to test; a common technique is to write a usleep() function based on either of these (see the comp.unix.questions FAQ for some examples)
• If your system has itimers (most do), you can roll your own usleep() using them (see the BSD sources for usleep() for how to do this)
• If you have POSIX realtime, there is a nanosleep() function

Of the above, select() is probably the most portable (and strangely, it is often much more efficient than usleep() or an itimer-based method). However, the behaviour may be different if signals are caught while asleep; this may or may not be an issue depending on the application.

Whichever route you choose, it is important to realise that you may be constrained by the timer resolution of the system (some systems allow very short time intervals to be specified, others have a resolution of, say, 10ms and will round all timings to that). Also, as for sleep(), the delay you specify is only a minimum value; after the specified period elapses, there will be an indeterminate delay before your process next gets scheduled.

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