http://lists.gnu.org/archive/html/bug-commoncpp/2004-05/msg00006.html
本意是找一个线程安全并可重入的 求线程运行时间的函数。
time(), gettimeofday() and possibly localtime() are not thread safe functions. time() is, on some platforms at least, implemented by using gettimeofday().
可重入函数 必然是 线程安全函数 和 异步信号安全函数；
线程安全函数不一定是可重入函数。（http://blog.csdn.net/jiange_zh/article/details/52071428）
The localtime() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.
#include <stdio.h> #include <time.h> int main() { time_t time_seconds = time(0); struct tm start; localtime_r(&time_seconds, &start); sleep(3); time_t time_seconds2 = time(0); struct tm end; localtime_r(&time_seconds2, &end); //printf("%d-%d-%d %d:%d:%d\n", now_time.tm_year + 1900, now_time.tm_mon + 1,now_time.tm_mday, now_time.tm_hour, now_time.tm_min, now_time.tm_sec); //printf("%d-%d-%d %d:%d:%d\n", now_time2.tm_year + 1900, now_time2.tm_mon + 1,now_time2.tm_mday, now_time2.tm_hour, now_time2.tm_min, now_time2.tm_sec); time_t t = mktime(&end) - mktime(&start); printf("t = %d\n",t); }
原文：http://www.cnblogs.com/csun/p/6246055.html