Using scanf to read uint8_t data

Question

I have written a simple C program to read hours and minutes and then add them together. But it is not getting added and currentHrMin is printing only the value of minutes. However if getCurrentDate(&dateParams) is called after printing currentHrMin, there is no issue. I am not able to find out what is wrong with my code. Could be a silly issue. I am using MinGW C compiler.

#include <stdio.h>
#include <stdint.h>

#define BCD_TO_DEC(num) ((((num)&0xF0)>>4)*10+((num)&0x0F))
#define DEC_TO_BCD(num) ((((num)/10) << 4) | ((num) % 10))


struct RTC_TIME
{
    uint8_t hours;
    uint8_t minutes;
    uint8_t seconds;
    uint8_t twelveHourFormat:1; //1 = 12 hour format, 0=24 hour format.
    uint8_t AM_0_PM_1:1;
    uint8_t hours24Format;
    uint8_t alarm1State:1;
    uint8_t alarm2State:1;
};

struct RTC_DATE
{
    uint8_t date;
    uint8_t month;
    uint8_t dayOfWeek;
    uint8_t year;
};


void getCurrentTime(struct RTC_TIME* time)
{
    printf("Enter Hour: ");
    scanf("%d",&(time->hours));
    printf("Enter Min: ");
    scanf("%d",&(time->minutes));
}

void getCurrentDate(struct RTC_DATE* date)
{
    printf("Enter Month: ");
    scanf("%d",&(date->month));
}

int ar1[5]= {0x1253,0x1034,0x0804,0x1112,0x0409};

int main(void)
{
    struct RTC_DATE dateParams;
    struct RTC_TIME timeParams;

    getCurrentTime(&timeParams);
    getCurrentDate(&dateParams);
    uint16_t currentHrMin = timeParams.hours*60 + timeParams.minutes;
    printf("Current hour minute = %d\n",currentHrMin);

    return(0);

}

Answer 1

After including #include <inttypes.h>, change:

scanf("%d",&(time->hours));

to this:

scanf("%" SCNu8, &(time->hours));

in all your scanf's, so that you read for uint8_t, instead of an int.

---

The observation you made relates to that, you were reading with %d specifier, which designated for type int, which is usually 32 bits. So when you assign the read value to time->hours, then it would "overflow" to the adjacent struct fields as well.

---

Next time, please enable your compiler's warning, and you should get something like this:

Georgioss-MacBook-Pro:~ gsamaras$ gcc -Wall main.c 
main.c:32:16: warning: format specifies type 'int *' but the argument has type
      'uint8_t *' (aka 'unsigned char *') [-Wformat]
    scanf("%d",&(time->hours));
           ~~  ^~~~~~~~~~~~~~
           %s
main.c:34:16: warning: format specifies type 'int *' but the argument has type
      'uint8_t *' (aka 'unsigned char *') [-Wformat]
    scanf("%d",&(time->minutes));
           ~~  ^~~~~~~~~~~~~~~~
           %s
main.c:40:16: warning: format specifies type 'int *' but the argument has type
      'uint8_t *' (aka 'unsigned char *') [-Wformat]
    scanf("%d",&(date->month));
           ~~  ^~~~~~~~~~~~~~
           %s
3 warnings generated.

I used Wall compiler flag, as I discuss in this answer.