Easy way to convert a struct tm (expressed in UTC) to time_t type

Question

How do I do the above? There is mktime function but that treats the input as expressed in local time but how do i perform the conversion if my input tm variable happens to b

Answer 1

This is really a comment with code to address the answer by Leo Accend: Try the following:

#include 
#include 
#include     

/*
 *  A bit of a hack that lets you pull DST from your Linux box
 */

time_t timegm( struct tm *tm ) {           // From Leo's post, above
  time_t t = mktime( tm );
  return t + localtime( &t )->tm_gmtoff;
}
main()
{
    struct timespec tspec = {0};
    struct tm tm_struct   = {0};

    if (gettimeofday(&tspec, NULL) == 0) // clock_gettime() is better but not always avail
    {
        tzset();    // Not guaranteed to be called during gmtime_r; acquire timezone info
        if (gmtime_r(&(tspec.tv_sec), &tm_struct) == &tm_struct)
        {
            printf("time represented by original utc time_t: %s\n", asctime(&tm_struct));
            // Go backwards from the tm_struct to a time, to pull DST offset. 
            time_t newtime = timegm (&tm_struct);
            if (newtime != tspec.tv_sec)        // DST offset detected
            {
                printf("time represented by new time_t: %s\n", asctime(&tm_struct));

                double diff = difftime(newtime, tspec.tv_sec);  
                printf("DST offset is %g (%f hours)\n", diff, diff / 3600);
                time_t intdiff = (time_t) diff;
                printf("This amounts to %s\n", asctime(gmtime(&intdiff)));
            }
        }
    }
    exit(0);
}