struct timeval和gettimeofday

struct timeval和gettimeofday()

struct timeval结构体在time.h中的定义为：

struct timeval { time_t tv_sec; /* Seconds. */ suseconds_t tv_usec; /* Microseconds. */ }; 

其中，tv_sec为Epoch到创建struct timeval时的秒数，tv_usec为微秒

struct timeval结构体在time.h中的定义为：

struct timeval
{
time_t tv_sec; /* Seconds. */
suseconds_t tv_usec; /* Microseconds. */
};

其中，tv_sec为Epoch到创建struct timeval时的秒数，tv_usec为微秒数，即秒后面的零头。比如当前我写博文时的tv_sec为1244770435，tv_usec为442388，即当前时间距Epoch时间1244770435秒，442388微秒。需要注意的是，因为循环过程，新建结构体变量等过程需消耗部分时间，我们作下面的运算时会得到如下结果：

int i;
for (i = 0; i < 4; ++i)
{
gettimeofday(&tv, NULL);
printf("%d\t%d\n", tv.tv_usec, tv.tv_sec);
sleep(1);
}

442388 1244770435
443119 1244770436
443543 1244770437
444153 1244770438

前面为微秒数，后面为秒数，可以看出，在这个简单运算中，只能精确到小数点后面一到两位，或者可以看出，每进行一次循环，均需花费0.005秒的时间，用这个程序来作计时器显然是不行的，除非精确计算产生的代码消耗时间。

gettimeofday() -- 获取当前时间(保存在结构体timeval中)

#include <stdio.h>
#include <sys/time.h>
#include <time.h>

int main(int argc, char * argv[]){

struct timeval tv; //(1)
while(1){
gettimeofday(&tv, NULL); //(2)
printf("time %u:%u\n", tv.tv_sec, tv.tv_usec);
sleep(2);
}
return 0;

}

(1) struct--timeval

struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
millisecond 毫秒
microsecond 微秒

timeval表示一个时间点，比如:
timeval.tv_sec = 1 (s)
timevat.tv_usec = 500 000 (μs)
1:500 = 1s500000μs = 1.5s

(2) gettimeofday()

int gettimeofday(struct timeval *tv, struct timezone *tz);

The functions gettimeofday() and settimeofday() can get and set the time as well as a timezone.
The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL.

(3) 运行结果:

time 1181788367:991487
time 1181788369:991602

表示睡眠2秒经过的精确时间为： 2s115μs

#include <sys/time.h>
#include <stdio.h>

int main(void)
{
    struct timeval before,after;
	long us_t = 100; //us
	long max=1000000;
	max = max + us_t;

    while(1){
        gettimeofday(&before,NULL);
        sleep(1);
        gettimeofday(&after,NULL);
        if(((after.tv_sec*1000000 + after.tv_usec ) - (before.tv_sec*1000000 + before.tv_usec)) > max){
        	printf("before us:%ld,after us:%ld,value=%ldus > %ld us \n",(before.tv_sec+before.tv_usec),(after.tv_sec+after.tv_usec),((after.tv_sec+after.tv_usec) - (before.tv_sec+before.tv_usec)), us_t);
            break;
		}else{
        	printf("before us:%ld,after us:%ld,value=%ldus\n",(before.tv_sec+before.tv_usec),(after.tv_sec+after.tv_usec),((after.tv_sec+after.tv_usec) - (before.tv_sec+before.tv_usec)));
		}
    }
	return 0;
}