1、简介
sleep()-------以秒为单位
#include<unistd.h>
unsigned int sleep(unsigned int seconds);
return：若进程暂停到参数seconds 所指定的时间，成功则返回0，若有信号中断则返回剩余秒数。
在linux中，sleep是通过nanosleep实现的。在一些其他系统中（例如POSIX.1），它是通过alarm()来实现的。
usleep()----以微秒为单位
#include<unistd.h>
unsigned int usleep(unsigned int useconds);
return：若进程暂停到参数seconds 所指定的时间，成功则返回0，若有信号中断则返回剩余微秒数。
nanosleep( )---------以纳秒为单位
#include<time.h>
struct timespec
{
time_t tv_sec; /* 秒seconds /
long tv_nsec; / 纳秒nanoseconds */
};
int nanosleep(const struct timespec req, struct timespec rem);
return: 若进程暂停到参数req所指定的时间，成功则返回0，若有信号中断则返回-1，并且将剩余微秒数记录在rem中。
req->tv_sec是以秒为单位，而tv_nsec以毫微秒为单位（10的-9次方秒）。
由于调用nanosleep是是进程进入TASK_INTERRUPTIBLE,这种状态是会相应信号而进入TASK_RUNNING状态的。
2、注意
使用这些函数时一定要注意判断返回值。有时候会出现sleep函数被系统中断的情况，导致结果不符合预期。
while (nanosleep(&ts, &ts) == -1 && errno == EINTR) {}
3、精确度对比
低精度情况（100000us及以上）：usleep和nanosleep表现差不多。select和pselect表现较差。
高精度情况（100000us及以上）：四者表现差不多。
fuction time(usec) realtime reduce
----------------------------------------------------
usleep 500000 500091 91
nanosleep 500000 500089 89
select 500000 500540 540
pselect 500000 500549 549
--------------------------------
usleep 100000 100078 78
nanosleep 100000 100110 110
select 100000 100157 157
pselect 100000 100149 149
--------------------------------
usleep 50000 50091 91
nanosleep 50000 50107 107
select 50000 50111 111
pselect 50000 50084 84
--------------------------------
usleep 10000 10086 86
nanosleep 10000 10091 91
select 10000 10089 89
pselect 10000 10088 88
--------------------------------
usleep 1000 1089 89
nanosleep 1000 1065 65
select 1000 1065 65
pselect 1000 1066 66
--------------------------------
usleep 900 969 69
nanosleep 900 974 74
select 900 970 70
pselect 900 980 80
--------------------------------
usleep 500 569 69
nanosleep 500 565 65
select 500 569 69
pselect 500 569 69
--------------------------------
usleep 100 166 66
nanosleep 100 165 65
select 100 163 63
pselect 100 163 63
--------------------------------
usleep 10 73 63
nanosleep 10 76 66
select 10 73 63
pselect 10 78 68
--------------------------------
usleep 1 64 63
nanosleep 1 66 65
select 1 65 64
pselect 1 63 62
--------------------------------
4、测试代码
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<sys/time.h>
#include<errno.h>
#include<string.h>
#include<unistd.h>
#include<sys/types.h>
#include<sys/select.h>
int main(int argc, char **argv)
{
unsigned int nTimeTestSec = 0;
unsigned int nTimeTest = 0;
struct timeval tvBegin;
struct timeval tvNow;
int ret = 0;
unsigned int nDelay = 0;
struct timeval tv;
int fd = 1;
int i = 0;
struct timespec req;
unsigned int delay[20] =
{500000, 100000, 50000, 10000, 1000, 900, 500, 100, 10, 1, 0};
int nReduce = 0; //误差
fprintf(stderr, "%19s%12s%12s%12s\n", "fuction", "time(usec)", "realtime", "reduce");
fprintf(stderr, "----------------------------------------------------\n");
for (i = 0; i < 20; i++)
{
if (delay[i] <= 0)
break;
nDelay = delay[i];
//test sleep
gettimeofday(&tvBegin, NULL);
ret = usleep(nDelay);
if(ret == -1)
{
fprintf(stderr, "usleep error, errno=%d [%s]\n", errno, strerror(errno));
}
gettimeofday(&tvNow, NULL);
nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
nReduce = nTimeTest - nDelay;
fprintf (stderr, "\t usleep %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
//test nanosleep
req.tv_sec = nDelay/1000000;
req.tv_nsec = (nDelay%1000000) * 1000;
gettimeofday(&tvBegin, NULL);
ret = nanosleep(&req, NULL);
if (-1 == ret)
{
fprintf (stderr, "\t nanousleep %8u not support\n", nDelay);
}
gettimeofday(&tvNow, NULL);
nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
nReduce = nTimeTest - nDelay;
fprintf (stderr, "\t nanosleep %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
//test select
tv.tv_sec = 0;
tv.tv_usec = nDelay;
gettimeofday(&tvBegin, NULL);
ret = select(0, NULL, NULL, NULL, &tv);
if (-1 == ret)
{
fprintf(stderr, "select error. errno = %d [%s]\n", errno, strerror(errno));
}
gettimeofday(&tvNow, NULL);
nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
nReduce = nTimeTest - nDelay;
fprintf (stderr, "\t select %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
//pselcet
req.tv_sec = nDelay/1000000;
req.tv_nsec = (nDelay%1000000) * 1000;
gettimeofday(&tvBegin, NULL);
ret = pselect(0, NULL, NULL, NULL, &req, NULL);
if (-1 == ret)
{
fprintf(stderr, "select error. errno = %d [%s]\n", errno, strerror(errno));
}
gettimeofday(&tvNow, NULL);
nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
nReduce = nTimeTest - nDelay;
fprintf (stderr, "\t pselect %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
fprintf (stderr, "--------------------------------\n");
}
return 0;
}