Why does the variable "seconds" get printed correctly the first time but not the second time?

Question

First thing to say, i'm an absolute noob in programming so it might be a very simple thing and i'm not getting it.

I want to know how much time has passed since the beginning of the day and to do that i used the time() function.

I then want to print it and here is my problem: with the first printf the variable seconds is printed correctly but in the second printf (where I print mills and seconds) it gives me a wrong output.

The code:

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

int main(void) {
  long long int mills, seconds;

  mills = time(NULL);   
  printf("Mills: %i\n", mills );

  seconds = mills / 1000;

  //here the variable is printed correctly
  printf("Seconds: %i\n", seconds );  

  //here mills gets printed correctly but seconds gets printed as 0
  printf("Milliseconds since midnight: %i\nSeconds since midnight: %i\n", mills, seconds);

  return 0;

}

The output:

Mills: 1486143107
Seconds: 1486143
Milliseconds since midnight: 1486143107
Seconds since midnight: 0

Why is the variable printed correctly the first time but not the second time? Shouldn't it be always the same?

Actually, `time` typically returns the number of seconds since an epoch (usually midnight January 1, 1970). — Some programmer dude, Feb 03 '17 at 17:36
Undefined behavior for passing a value of type `long long int` to `printf()` with a corresponding `%i` conversion. — EOF, Feb 03 '17 at 17:38
int mills, seconds; is the correct one, not long long int. Change it and it will work correct. — Konstantinos Monachopoulos, Feb 03 '17 at 17:39
The value returned is of type `time_t`. Then cast that to `unsigned long long` with the format specifier `%llu`. — Weather Vane, Feb 03 '17 at 17:40
might be helpful: http://stackoverflow.com/questions/2792551/what-primitive-data-type-is-time-t — yano, Feb 03 '17 at 17:43
`long long int mills, ... printf("Mills: %i\n", mills );` should generate a compiler warning. Insure all compiler warnings are enabled. Saves us all time. — chux - Reinstate Monica, Feb 03 '17 at 17:48
@Riccardo De Togni Rename it like firsts and it is possible it will be outputted correctly the second time. Though it can be said nothing about how it will be outputted the first time. — Vlad from Moscow, Feb 03 '17 at 17:55
Does Someone Know how did it output correct the first time.? — Suraj Jain, Feb 03 '17 at 18:29
@SurajJain asking why *undefined behaviour* works is as fruitless as asking why it waited until your big presentation to fail. — Weather Vane, Feb 03 '17 at 18:53

score 6 · Answer 1 · answered Feb 03 '17 at 17:45

6

Printing a long long int value through printf requires a %lld (or %lli) format. You are using %i, which is wrong. The behavior is undefined.

answered Feb 03 '17 at 17:45

AnT stands with Russia

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score 0 · Answer 2 · answered Feb 03 '17 at 18:17

To get the seconds since midnight you need to find the seconds since epoch and mod it by the seconds in a day and then subtract the number of seconds to convert to your local time zone:

#include <time.h>
#include <stdlib.h>
#include <stdio.h>

int main(void)
{
    time_t current_time;
    char* c_time_string;

    /* Obtain current time. */
    current_time = time(NULL);

    if (current_time == ((time_t)-1))
    {
        (void) fprintf(stderr, "Failure to obtain the current time.\n");
        exit(EXIT_FAILURE);
    }

    /* Convert to local time format. */
    c_time_string = ctime(&current_time);

    if (c_time_string == NULL)
    {
        (void) fprintf(stderr, "Failure to convert the current time.\n");
        exit(EXIT_FAILURE);
    }

    /* Print to stdout. ctime() has already added a terminating newline character. */
    (void) printf("Current time is %s", c_time_string);
    (void) printf("Seconds since midnight is %li", current_time%86400 - 18000);
    exit(EXIT_SUCCESS);
}

I subtracted 18000 to make it show seconds since midnight for eastern time zone :)

output: Current time is Fri Feb 3 13:13:57 2017 Seconds since midnight is 47637 — Mathew Cross, Feb 03 '17 at 18:18

score 0 · Answer 3 · edited May 23 '17 at 10:29

Please Use %lld or %lli for printing long long int , you are using %i which can only be used with int.It causes Undefined Behaviour.

Your Compiler Should Have Warned You

format %i expects argument of type int, but argument 2 has type long long int

If not then please enable [-Wformat=] Warning.

Also Look At This Question cross-platform printing of 64-bit integers with printf

You were unlucky with first printf for that to be working correctly , often things like these get unchecked and cause harm later , it is often much better to enable extra warning and correct your code.

Why does the variable "seconds" get printed correctly the first time but not the second time?

3 Answers3