Formatted elapsed time in C - c

Question

I'm writing a function that, among other things, must print out an elapsed time counter. It receives by reference a start time _start, and compares it to current, both typed as time_t. I want to use strftime() to print out the observed time delta in ISO 8601 format. Here's what I attempted to do:
// Negative start time implies program has not begun
if (*_start > 0) {
time_t elapsed = current - *_start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
And here is the output I get immediately after running the program
01:00:00
01:00:00
01:00:01
01:00:01
01:00:01
The seconds work fine, if I let it run longer they get incremented as expected, so do the minutes, however the hour starts as 01 as opposed to 00. Why is this happening? How can I get the hours to start zeroed, like the minutes?

Answer 1

time_t typically (see edit) stores absolute timestamps (the number of seconds since midnight UTC, January 1, 1970). By calculating current - *_start, you're getting elapsed time in seconds (as desired), but by then passing that to localtime and strftime, you're telling the computer to take the time elapsed since the start of your program and treat it as the time elapsed since midnight UTC 1-1-1970.
I'm guessing that happens to be 01:00:00 in your system's local time zone.
I'm not aware of a C99 function to print elapsed time, but it's not hard to write one yourself.
void format_elapsed_time(char *time_str, int total_seconds_elapsed) {
int hours_elapsed = total_seconds_elapsed / 3600;
int minutes_elapsed = (total_seconds_elapsed % 3600) / 60;
int seconds_elapsed = total_seconds_elapsed % 60;
sprintf(time_str, "%02i:%02i:%02i", hours_elapsed, minutes_elapsed, seconds_elapsed);
}
Edit: As #chux points out, time_t doesn't have to store timestamps as seconds since 1-1-1970. See this answer for details.

Answer 2

To portably find the number of elapsed seconds between 2 time_t, use difftime() as subtracting 2 time_t values is not specified in C to be a difference in seconds.
double difftime(time_t time1, time_t time0); C11dr §7.26.2.2
The difftime function returns the difference expressed in seconds as a double.
double elapsed_seconds = difftime(current, *_start);
printf("elapsed_seconds: %f\n", elapsed_seconds);

Answer 3

If you want to use this method, you should be aware that time() returns UTC. If you then subtract that from your local time (as returned by localtime()), time zones will have an effect on the result (it's likely your particular time zone is removed from UTC by one hour).
Consider the following complete program, similar to your snippet:
#include <stdio.h>
#include <time.h>
#include <unistd.h>
int main(void) {
time_t start = time(0);
char time_str[100];
for (int i = 5; i > 0; i--) {
sleep (1);
time_t elapsed = time(0) - start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
return 0;
}
Also consider that my particular time zone is removed from UTC by eight hours:
pax> date ; date -u
Thursday 30 March 10:25:57 AWST 2017
Thursday 30 March 02:25:57 UTC 2017
When I run it, I see:
08:00:01
08:00:02
08:00:03
08:00:04
08:00:05
You can see there that the eight-hour time difference from UTC affects the value. The fix for that is actually quite simple: don't use local time at all. If you replace the localtime() call with gmtime(), you get the output you expect:
00:00:01
00:00:02
00:00:03
00:00:04
00:00:05

Answer 4

I realize this question already has an answer, but I thought I might shift the focus a little and expand on the accepted answer. Since the OP specified that they are dealing with elapsed times, one can avoid dealing with absolute timestamps altogether using the clock() function.
clock_t start = clock();
.
.
clock_t end = clock();
double elapsed = (end - start) / (double)CLOCKS_PER_SEC;
then you can call the notionally revised format_elapsed_time() function, like so:
void format_elapsed_time(char *time_str, double elapsed) {
int h, m, s, ms;
h = m = s = ms = 0;
ms = elapsed * 1000; // promote the fractional part to milliseconds
h = ms / 3600000;
ms -= (h * 3600000);
m = ms / 60000;
ms -= (m * 60000);
s = ms / 1000;
ms -= (s * 1000);
sprintf(time_str, "%02i:%02i:%02i.%03i", h, m, s, ms);
}