Question

I have one daemon written in C. I am logging the events in a log file, but now I want to add date and time while writing event to log file. How can I achieve that?

Current log file:-

Event one occurred: result:
Event two occurred: result:

I want the log file to look like:-

Sep 14 11:35:55 Event one occurred: result:
Sep 14 11:35:55 Event two occurred: result:

My environment is C and Linux.

Answer 1

You need to look into using date and either gmtime or localtime to get the actual date and time.

Then strftime can format it for you.

Sample program follows:

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

int main (void) {
    char buff[20];
    struct tm *sTm;

    time_t now = time (0);
    sTm = gmtime (&now);

    strftime (buff, sizeof(buff), "%Y-%m-%d %H:%M:%S", sTm);
    printf ("%s %s\n", buff, "Event occurred now");

    return 0;
}

This outputs:

2011-09-14 04:52:11 Event occurred now

I prefer the use of UTC rather than local time since it allows you to tie together events from geographically separated machine without worrying about timezone differences. In other words, use gmtime rather than localtime unless you’re very certain you won’t be crossing timezones.

I also tend to prefer the YYYY-MM-DD HH:MM:SS format since it’s easier to sort than month names, vital for extraction and manipulation tools.

---

If you have an implementation that provides the optional bounds-checking functions (as per Appendix K of C11), you can probably use gmtime_s in preference. It allows you to specify your own buffer for receiving the result and is thus safer in re-entrant and/or threaded code.

To use that, you need to change your code to something like:

#include <stdio.h>
#define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>

int main (void) {
    char buff[20];
    struct tm sTm;

    time_t now = time (0);
    gmtime_s (&now, &sTm);

    strftime (buff, sizeof(buff), "%Y-%m-%d %H:%M:%S", &sTm);
    printf ("%s %s\n", buff, "Event occurred now");

    return 0;
}

Although you should be aware that the folks at Microsoft have somehow managed to get the arguments for gmtime_s around the wrong way. You’ll need to take that into account.

POSIX (and Linux) also provides a gmtime_r function which performs in the same way as the standard gmtime_s function (with the arguments in the correct order).