【Linux】延时函数sleep、usleep、nanosleep、select、pselect的比较

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         91nanosleep      500000     500089         89select         500000     500540        540pselect        500000     500549        549
--------------------------------usleep         100000     100078         78nanosleep      100000     100110        110select         100000     100157        157pselect        100000     100149        149
--------------------------------usleep          50000      50091         91nanosleep       50000      50107        107select          50000      50111        111pselect         50000      50084         84
--------------------------------usleep          10000      10086         86nanosleep       10000      10091         91select          10000      10089         89pselect         10000      10088         88
--------------------------------usleep           1000       1089         89nanosleep        1000       1065         65select           1000       1065         65pselect          1000       1066         66
--------------------------------usleep            900        969         69nanosleep         900        974         74select            900        970         70pselect           900        980         80
--------------------------------usleep            500        569         69nanosleep         500        565         65select            500        569         69pselect           500        569         69
--------------------------------usleep            100        166         66nanosleep         100        165         65select            100        163         63pselect           100        163         63
--------------------------------usleep             10         73         63nanosleep          10         76         66select             10         73         63pselect            10         78         68
--------------------------------usleep              1         64         63nanosleep           1         66         65select              1         65         64pselect             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 sleepgettimeofday(&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 nanosleepreq.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 selecttv.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);//pselcetreq.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;
}

参考博客：https://www.jianshu.com/p/42abcc2c9e50