// 適当に システム時間を RTC に同期(？)させるプログラム。

#define _BSD_SOURCE

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
#include <errno.h>

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

#include <linux/rtc.h>

time_t rtc_time2time_t(struct rtc_time * rtc)
{
	struct tm temp_tm;

	memset(&temp_tm, 0, sizeof(temp_tm));

	temp_tm.tm_sec = rtc->tm_sec;
	temp_tm.tm_min = rtc->tm_min;
	temp_tm.tm_hour = rtc->tm_hour;
	temp_tm.tm_mday = rtc->tm_mday;
	temp_tm.tm_mon = rtc->tm_mon;
	temp_tm.tm_year = rtc->tm_year;

	return mktime(&temp_tm);
}

int main(void)
{
	int fd, res;
	struct rtc_time cur_rtc;
	struct timeval cur_sys;
	time_t cur_rtc_sec;
	struct timeval delta, old_delta;
	unsigned long tmp;

	fd = open("/dev/rtc", O_RDONLY);
	if(fd == -1)
	{
		fprintf(stdout, "open rtc device error :%d [%s]\n", errno, strerror(errno));
		return(errno);
	}

	if(ioctl(fd, RTC_UIE_ON, 0) == -1) // RTC interrupt 1/s on.
	{
		fprintf(stdout, "ioctl RTC_UIE_ON error :%d [%s]\n", errno, strerror(errno));
		close(fd);
		return(errno);
	}

	res = read(fd, &tmp, sizeof(tmp)); // Wait RTC update interrupt.
	if (res == -1) {
		fprintf(stdout, "read error :%d [%s]\n", errno, strerror(errno));
		close(fd);
		return(errno);
        }

	if(ioctl(fd, RTC_UIE_OFF, 0) == -1) // RTC interrupt 1/s off.
	{
		fprintf(stdout, "ioctl RTC_UIE_OFF error :%d [%s]\n", errno, strerror(errno));
		close(fd);
		return(errno);
	}

	if(ioctl(fd, RTC_RD_TIME, &cur_rtc) == -1) // Read RTC.
	{
		fprintf(stdout, "ioctl RTC_RD_TIME error :%d [%s]\n", errno, strerror(errno));
		close(fd);
		return(errno);
	}

	gettimeofday(&cur_sys, NULL);
	cur_rtc_sec = rtc_time2time_t(&cur_rtc);

	delta.tv_sec = (long)cur_rtc_sec - (long)cur_sys.tv_sec;
	delta.tv_usec = (long)delta.tv_sec <= 0 
				? (long)cur_sys.tv_usec * (-1)
				: (long)(1000000 - cur_sys.tv_usec);

	if(adjtime(&delta, &old_delta)) // Adjust system time.
	{
		fprintf(stdout, "adjtime error :%d [%s]\n", errno, strerror(errno));
		close(fd);
		return(errno);
	}
	fprintf(stdout, "adjtime :sys = %ld.%06ld, rtc = %ld, (old delta = %s%ld.%06ld)\n", 
			(long) cur_sys.tv_sec, (long) cur_sys.tv_usec, cur_rtc_sec,
			(long)old_delta.tv_sec < 0 || (long)old_delta.tv_usec < 0 ? "-" : "+",
			abs((long)old_delta.tv_sec), abs((long)old_delta.tv_usec));

	close(fd);

	return(0);
}