백만분의 1초 까지 계산할 수 있을 까요?

glibc랑 인터넷 검색 해서 간단하게(?) 만들어 봤습니다.

struct timeval 사용하면 마이크로초 (백만분의 1초) 단위로 시간 가져오는 것 같습니다.

예로,
current time: 2004 09 05 15:46:17.919181
current time: 2004 09 05 15:46:17.920041
는 while loop 안에서 한 루프 돌때마다 시간이 지난 결과입니다.

맨 아래 결과 보면 clock() 이용해서 CPU 시간차 가져온 값과
gettimeofday() 값의 차이를 보면 소숫점 3째자리 이하입니다.

/*
   sample-cpu-time.c
   2004/09/03 19:02 

   example of getting CPU time 
*/

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

void do_something(const double limit);
void do_something2(void);
void elapsed_time(struct timeval start, struct timeval end);



void do_something(const double limit)
{
		clock_t start, end;
		double cpu_time_used;
		double i = 0;

		start = clock();
		while ( i++ < limit )
			;
		end = clock();

		cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;

		printf("%f loop - elapsed time is %lf\n", limit, cpu_time_used);
}



void do_something2(void)
{
		int i; 
		struct timeval now;
		struct tm brokentime;
		char 	timestr[80];

		for (i = 0; i < 20; i++)
		{
				gettimeofday(&now, NULL);
				localtime_r (&now.tv_sec, &brokentime);
				strftime (timestr, sizeof(timestr), "%Y %m %d %T", &brokentime);
				printf("current time: %s.%d\n", timestr, now.tv_usec);
		}
}


void do_something3(const double limit)
{
		struct timeval start, end;
		double i = 0;

		gettimeofday(&start, NULL);
		while ( i++ < limit )
			;

		gettimeofday(&end, NULL);

		elapsed_time(start, end);
}


void elapsed_time(struct timeval start, struct timeval end)
{
		struct timeval et;

		if ( end.tv_usec < start.tv_usec )
		{
				end.tv_sec = end.tv_sec - 1;
				end.tv_usec = end.tv_usec + 1000000;
		}
		et.tv_sec = end.tv_sec - start.tv_sec;
		et.tv_usec = end.tv_usec - start.tv_usec;
		printf("elapsed time is %d.%06d\n", et.tv_sec, et.tv_usec);

		
}

int main()
{
		struct timeval start, end;
		struct timezone zone;
		double i = 0; 

		do_something(500000);
		do_something(1000000);

		do_something3(500000);
		do_something3(1000000);
		do_something3(8000000);
		do_something3(118000000);
		printf("Is it microsec?\n");
		do_something2();

		return 0;
}
		
/*
500000.000000 loop - elapsed time is 0.010000
1000000.000000 loop - elapsed time is 0.020000
elapsed time is 0.010263
elapsed time is 0.020312
elapsed time is 0.161543
elapsed time is 2.369592
Is it microsec?
current time: 2004 09 05 15:46:17.919181
current time: 2004 09 05 15:46:17.920041
current time: 2004 09 05 15:46:17.920075
current time: 2004 09 05 15:46:17.920104
current time: 2004 09 05 15:46:17.920147
current time: 2004 09 05 15:46:17.920180
current time: 2004 09 05 15:46:17.920208
current time: 2004 09 05 15:46:17.920247
current time: 2004 09 05 15:46:17.920273
current time: 2004 09 05 15:46:17.920298
current time: 2004 09 05 15:46:17.920349
current time: 2004 09 05 15:46:17.920379
current time: 2004 09 05 15:46:17.920407
current time: 2004 09 05 15:46:17.920432
current time: 2004 09 05 15:46:17.920459
current time: 2004 09 05 15:46:17.920487
current time: 2004 09 05 15:46:17.920519
current time: 2004 09 05 15:46:17.920548
current time: 2004 09 05 15:46:17.920577
current time: 2004 09 05 15:46:17.920605
*/