Question 19.37

How can I implement a delay, or time a user's response, with sub-second resolution?

Unfortunately, there is no portable way. V7 Unix, and derived systems, provided a fairly useful ftime routine with resolution up to a millisecond, but it has disappeared from System V and POSIX. Other routines you might look for on your system include clock, delay, gettimeofday, msleep, nap, napms, setitimer, sleep, times, and usleep. (A routine called wait, however, is at least under Unix not what you want.) The select and poll calls (if available) can be pressed into service to implement simple delays. On MS-DOS machines, it is possible to reprogram the system timer and timer interrupts.

Of these, only clock is part of the ANSI Standard. The difference between two calls to clock gives elapsed execution time, and if CLOCKS_PER_SEC is greater than 1, the difference will have subsecond resolution. However, clock gives elapsed processor time used by the current program, which on a multitasking system may differ considerably from real time.

If you're trying to implement a delay and all you have available is a time-reporting function, you can implement a CPU-intensive busy-wait, but this is only an option on a single-user, single-tasking machine as it is terribly antisocial to any other processes. Under a multitasking operating system, be sure to use a call which puts your process to sleep for the duration, such as sleep or select, or pause in conjunction with alarm or setitimer.

For really brief delays, it's tempting to use a do-nothing loop like

	long int i;
	for(i = 0; i < 1000000; i++)
		;

but resist this temptation if at all possible! For one thing, your carefully-calculated delay loops will stop working next month when a faster processor comes out. Perhaps worse, a clever compiler may notice that the loop does nothing and optimize it away completely.

References: H&S Sec. 18.1 pp. 398-9
PCS Sec. 12 pp. 197-8,215-6
POSIX Sec. 4.5.2

Read sequentially: prev next up top