usleep,nanosleep,select定时比较

#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>;

#define PRINT_USEAGE { \
   fprintf(stderr,"\n Usage: %s usec ",argv[0]); \
   fprintf(stderr,"\n\n");\
}

int
main (int argc, char **argv)
{
unsigned int nTimeTestSec = 0; /* sec */
unsigned int nTimeTest = 0; /* usec */
struct timeval tvBegin;
struct timeval tvNow;
int ret = 0;
unsigned int nDelay = 0; /* usec */
fd_set rfds;
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 */

#if 0
if (argc < 2)
    {
      PRINT_USEAGE;
      exit (1);
    }
nDelay = atoi (argv[1]);
#endif

fprintf (stderr, "%18s%12s%12s%12s\n", "function", "time(usec)", "realTime",
           "reduce");
fprintf (stderr,
           "-------------------------------------------------------------------\n");

for (i = 0; i < 20; i++)
    {
      if (delay[i] <= 0)
        break;
      nDelay = delay[i];

      /* test usleep */
      gettimeofday (&tvBegin, NULL);
      ret = usleep (nDelay);
      if (-1 == ret)
        {
          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 */
      gettimeofday (&tvBegin, NULL);
      req.tv_sec = nDelay / 1000000;
      req.tv_nsec = (nDelay % 1000000) * 1000;
      ret = nanosleep (&req, NULL);
      if (-1 == ret)
        {
          fprintf (stderr, "\t nanosleep %8u not support\n", nDelay);
        }
      else
        {
          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 */
      gettimeofday (&tvBegin, NULL);
      FD_ZERO (&rfds);
      FD_SET (fd, &rfds);
      tv.tv_sec = 0;
      tv.tv_usec = nDelay;
      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);

    }

return 0;
}