今天在公司代碼中看到了使用select函數的超時功能作定時器的用法，便整理了如下幾個Linux下的微秒級別的定時器。在我的Ubutu10.10 雙核環境中，編譯通過。

1. /*
2. * @FileName: test_sleep.c
3. * @Author: wzj
4. * @Brief:
5. *
6. *
7. * @History:
8. *
9. * @Date: 2012年02月07日星期二22:20:00
10. *
11. */
12. #include<stdio.h>
13. #include<stdlib.h>
14. #include<time.h>
15. #include<sys/time.h>
16. #include<errno.h>
17. #include<string.h>
18. #include<unistd.h>
19. #include<sys/types.h>
20. #include<sys/select.h>
23. int main(int argc, char **argv)
24. {
25. unsigned int nTimeTestSec = 0;
26. unsigned int nTimeTest = 0;
27. struct timeval tvBegin;
28. struct timeval tvNow;
29. int ret = 0;
30. unsigned int nDelay = 0;
31. struct timeval tv;
32. int fd = 1;
33. int i = 0;
34. struct timespec req;
36. unsigned int delay[20] =
37. {500000, 100000, 50000, 10000, 1000, 900, 500, 100, 10, 1, 0};
38. int nReduce = 0; //誤差
40. fprintf(stderr, "%19s%12s%12s%12s\n", "fuction", "time(usec)", "realtime", "reduce");
41. fprintf(stderr, "----------------------------------------------------\n");
42. for (i = 0; i < 20; i++)
43. {
44. if (delay[i] <= 0)
45. break;
46. nDelay = delay[i];
47. //test sleep
48. gettimeofday(&tvBegin, NULL);
49. ret = usleep(nDelay);
50. if(ret == -1)
51. {
52. fprintf(stderr, "usleep error, errno=%d [%s]\n", errno, strerror(errno));
53. }
54. gettimeofday(&tvNow, NULL);
55. nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
56. nReduce = nTimeTest - nDelay;
58. fprintf (stderr, "\t usleep %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
60. //test nanosleep
61. req.tv_sec = nDelay/1000000;
62. req.tv_nsec = (nDelay%1000000) * 1000;
64. gettimeofday(&tvBegin, NULL);
65. ret = nanosleep(&req, NULL);
66. if (-1 == ret)
67. {
68. fprintf (stderr, "\t nanousleep %8u not support\n", nDelay);
69. }
70. gettimeofday(&tvNow, NULL);
71. nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
72. nReduce = nTimeTest - nDelay;
73. fprintf (stderr, "\t nanosleep %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
75. //test select
76. tv.tv_sec = 0;
77. tv.tv_usec = nDelay;
79. gettimeofday(&tvBegin, NULL);
80. ret = select(0, NULL, NULL, NULL, &tv);
81. if (-1 == ret)
82. {
83. fprintf(stderr, "select error. errno = %d [%s]\n", errno, strerror(errno));
84. }
86. gettimeofday(&tvNow, NULL);
87. nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
88. nReduce = nTimeTest - nDelay;
89. fprintf (stderr, "\t select %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
91. //pselcet
92. req.tv_sec = nDelay/1000000;
93. req.tv_nsec = (nDelay%1000000) * 1000;
95. gettimeofday(&tvBegin, NULL);
96. ret = pselect(0, NULL, NULL, NULL, &req, NULL);
97. if (-1 == ret)
98. {
99. fprintf(stderr, "select error. errno = %d [%s]\n", errno, strerror(errno));
100. }
102. gettimeofday(&tvNow, NULL);
103. nTimeTest = (tvNow.tv_sec - tvBegin.tv_sec) * 1000000 + tvNow.tv_usec - tvBegin.tv_usec;
104. nReduce = nTimeTest - nDelay;
105. fprintf (stderr, "\t pselect %8u %8u %8d\n", nDelay, nTimeTest,nReduce);
107. fprintf (stderr, "--------------------------------\n");
109. }
111. return 0;
112. }

老大建議我們在對精度要求較高的情況下使用select()作為定時器，最大的好處就是不會影響信號處理，線程安全，而且精度能得到保證。在這個實驗中，當時間延時時間較長時，select和pselect表現較差，當時間小於1毫秒時，他們的精確度便提高了，表現與usleep、nanosleep不相上下，有時精度甚至超過後者

linux 下高精度時間