linux 下高精度时间

今天在公司代码中看到了使用select函数的超时功能作定时器的用法，便整理了如下几个Linux下的微秒级别的定时器。在我的Ubutu10.10 双核环境中，编译通过。

/*
 * @FileName: test_sleep.c
 * @Author: wzj
 * @Brief:
 *
 *
 * @History:
 *
 * @Date: 2012年02月07日星期二22:20:00
 *
 */
#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
", "fuction", "time(usec)", "realtime", "reduce");
    fprintf(stderr, "----------------------------------------------------
");
    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]
", 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, "	 usleep       %8u   %8u   %8d
", 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, "	 nanousleep   %8u   not support
", nDelay);
         }
         gettimeofday(&tvNow, NULL);
         nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
         nReduce = nTimeTest - nDelay;
         fprintf (stderr, "	 nanosleep    %8u   %8u   %8d
", 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]
", 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, "	 select       %8u   %8u   %8d
", 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]
", 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, "	 pselect      %8u   %8u   %8d
", nDelay, nTimeTest,nReduce);

         fprintf (stderr, "--------------------------------
");

    }

    return 0;
}

    老大建议我们在对精度要求较高的情况下使用select()作为定时器，最大的好处就是不会影响信号处理，线程安全，而且精度能得到保证。在这个实验中，当时间延时时间较长时，select和pselect表现较差，当时间小于1毫秒时，他们的精确度便提高了，表现与usleep、nanosleep不相上下，有时精度甚至超过后者