Linux RTC application code waits at read() & current year is displayed incorrectly

Question

I wrote a simple RTC Test application, compiled & ran. But I am unable to decode two things from the output I got:

Output:

====== RTC Test  ====
Open & release
opened
get RTC time
Current RTC date/time is 9-5-114, 18:17:55
read RTC time

Issue #1) The year is incorrect as 114 & not 2014 or 14 as expected.

Issue #2) The code waits at read()

Why is this happening? And how to resolve this? Here is the code:-

#include <stdio.h>
#include <linux/rtc.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>


int main()
{
        int fd, ret;
        struct rtc_time rtc_tm;
        unsigned long data;

        printf("====== RTC Test  ====\n");

        printf("Open & release\n");
//open function is used to establish the connection between the RTC device with the file descriptor
        fd = open("/dev/rtc0", O_RDWR, 0);      //open for reading & writing
        if (fd == -1)
        {
                perror("/dev/rtc0");
                exit(errno);
        }
        else
        {
                printf("opened\n");
        }

//ioctl() to configure the RTC device
        printf("get RTC time\n");
        ret = ioctl(fd, RTC_RD_TIME, &rtc_tm);  //ioctl command RTC_RD_TIME is used to read the current timer
        if (ret == -1)
        {
                perror("rtc ioctl RTC_RD_TIME error");
        }
        printf("Current RTC date/time is %d-%d-%d, %02d:%02d:%02d\n",
                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year,
rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);

        /* Set the RTC time/date */
       /* rtc_tm.tm_mday = 31;
        rtc_tm.tm_mon = 4;      // for example Sep 8
        rtc_tm.tm_year = 104;
        rtc_tm.tm_hour = 2;
        rtc_tm.tm_min = 30;
        rtc_tm.tm_sec = 0;
        */

// call the read function to wait the Alarm interrupt
        printf("read RTC time\n");
        ret = read(fd, &data, sizeof(unsigned long));
        if (ret == -1)
        {
                perror("rtc read error");
        }
        printf("rtc read\n");

// set RTC time
        printf("set RTC time\n");
        ret == ioctl(fd, RTC_SET_TIME, &rtc_tm);
        if (ret == -1)
        {
                perror("rtc ioctl RTC_SET_TIME error");
        }
        printf("Set Current RTC date/time to %d-%d-%d, %02d:%02d:%02d\n",
                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year,
                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
        printf("Get RTC time\n");
        ret = ioctl(fd, RTC_RD_TIME, &rtc_tm);
        if (ret == -1)
        {
                perror("rtc ioctl RTC_RD_TIME error");
        }
        printf("Current RTC date/time is %d-%d-%d, %02d:%02d:%02d\n",
                rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year,
                rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);

        rtc_tm.tm_sec += 50;
        if (rtc_tm.tm_sec >= 60)
        {
                rtc_tm.tm_sec %= 60;
                rtc_tm.tm_min++;
        }
        if (rtc_tm.tm_min == 60)
        {
                rtc_tm.tm_min = 0;
                rtc_tm.tm_hour++;
        }
        if (rtc_tm.tm_hour == 24)
        {
                rtc_tm.tm_hour = 0;
        }
        printf("RTC tests done");
        close(fd);

        return 0;
}

As far as issue #2 is concerned, it could be since the device running the RTC is waiting for an interrupt. Please confirm whether my guess is correct, because if yes, then I need not worry about it.

Answer 1

regarding #1: you might need to decode the year:

If the year according to the RTC's Epoch and the year register is less than 1970 it is assumed to be 100 years later, that is, between 2000 and 2069. http://man7.org/linux/man-pages/man4/rtc.4.html

which is implemented like

        if (wtime.tm_year < 70)
            wtime.tm_year += 100;

so you can just subtract 100 to get 14 (for 2014)

Answer 2

Yes, the read is blocking waiting for the next interrupt. Details:

https://www.kernel.org/doc/Documentation/rtc.txt

Also, man rtc and man gmtime for the structure of the time structure. tm_year is defined as:

tm_year   The number of years since 1900.