struct timeval 和 struct timespec

timeval

　　DESCRIPTION
　　The functions gettimeofday and settimeofday can get and set the time as
　　well as a timezone. The tv argument is a timeval struct, as specified in <sys/time.h>:

struct timeval {
　　 time_t tv_sec; /* seconds */
　　 suseconds_t tv_usec; /* microseconds */
　　};
其中返回的timeval值为Epoch（00:00:00 1970-01-01 UTC）到创建struct timeval时的时间，tv_sec为秒数部分，tv_usec为微秒数部分（10的-6次方秒）。比如当前程序运行的tv_sec为1244770435，tv_usec为442388，即当前时间距Epoch时间1244770435秒，442388微秒。

测试代码如下：
　　#include <stdio.h>
　　#include <sys/time.h>
　　#include <time.h>
　　int gettimeofday(struct timeval *tv, struct timezone *tz);
　　int main(int argc,char * argv[]){
　　struct timeval tv;
　　 while(1){
　　 gettimeofday(&tv,NULL);
printf("time %u:%u/n",tv.tv_sec,tv.tv_usec);
　 　sleep(2);
　　 }
　 return 0;
　　 }

运行结果如下：
time 1259471701:970193
time 1259471703:971529
time 1259471705:973225
time 1259471707:974921

timespec
/* POSIX.1b structure for a time value. This is like a `struct timeval' but
has nanoseconds instead of microseconds. */
struct timespec
{
__time_t tv_sec; /* Seconds. */
long int tv_nsec; /* Nanoseconds. */
};

与struct timeval 不同，将结构体内成员微秒变换成纳秒