首先要注意的是，你只能指定 delay 的「最短」時間；實際上會 delay 多久和 系統的 scheduling 方式有關，例如系統當時的負載。如果你倒楣的話，它還可 能會 delay 蠻長的時間。
並沒有一個標準函式能夠在「小睡」（很短的 sleep）期間提供你計數的功能。 某些系統有提供 usleep(n) 的函式，它能夠暫停執行 n 微秒 (microsecond) 的時間。如果你所使用的系統沒有提供 usleep() 函式，那麼以下有可在 BSD， System V 使用中的作法。
接下來的這段程式碼是 Doug Gwyn 在 System V 中模擬 4BSD 並利用 4BSD 中的 select() 系統呼叫。Doung 自己都叫它為 'nap()' ；你也可以把它叫做 "usleep()"；
/*
usleep -- support routine for 4.2BSD system call emulations
last edit: 29-Oct-1984 D A Gwyn
*/
extern int select();
int
usleep( usec ) /* returns 0 if ok, else -1 */
long usec; /* delay in microseconds */
{
static struct /* `timeval' */
{
long tv_sec; /* seconds */
long tv_usec; /* microsecs */
} delay; /* _select() timeout */
delay.tv_sec = usec / 1000000L;
delay.tv_usec = usec % 1000000L;
return select( 0, (long *)0, (long *)0, (long *)0, &delay );
}
On System V you might do it this way:
/*
subseconds sleeps for System V - or anything that has poll()
Don Libes, 4/1/1991
The BSD analog to this function is defined in terms of
microseconds while poll() is defined in terms of milliseconds.
For compatibility, this function provides accuracy "over the long
run" by truncating actual requests to milliseconds and
accumulating microseconds across calls with the idea that you are
probably calling it in a tight loop, and that over the long run,
the error will even out.
If you aren't calling it in a tight loop, then you almost
certainly aren't making microsecond-resolution requests anyway,
in which case you don't care about microseconds. And if you did,
you wouldn't be using UNIX anyway because random system
indigestion (i.e., scheduling) can make mincemeat out of any
timing code.
Returns 0 if successful timeout, -1 if unsuccessful.
*/
#include <poll.h>
int
usleep(usec)
unsigned int usec; /* microseconds */
{
static subtotal = 0; /* microseconds */
int msec; /* milliseconds */
/* 'foo' is only here because some versions of 5.3 have
* a bug where the first argument to poll() is checked
* for a valid memory address even if the second argument is 0.
*/
struct pollfd foo;
subtotal += usec;
/* if less then 1 msec request, do nothing but remember it */
if (subtotal < 1000) return(0);
msec = subtotal/1000;
subtotal = subtotal%1000;
return poll(&foo,(unsigned long)0,msec);
}
在 System V 或其他 非-BSD 的 Unix 中要使用這類的「小睡」程式，可以用
Jon Zeeff 發表在 comp.sources.misc, volume 4 中的 s5nap， 它需要安裝
一個驅動程式，但是裝好後就可以跑得很好。（它的精確度會受到 kernel 中
HZ 這個變數的影響，因為它是用到了 kernel 中的 delay() 函式。）
現在很多較新版本的 Unix 都有提供這類的「小睡」功能了。