mktime adds hour unless extra timespec pointers declared

halestorm914 · 05-06-2010, 01:15 PM

I have a set of applications which convert ascii (YYYY-MM-DD) style time to unix time (time_t), and vice-versa. I was getting unexpected results so I put together a little test application which converts ascii string time (from the command line) to a tm struct, back again tm -> ascii (so far, so good), then into time_t calendar representation using mktime(). Here is where it breaks down. The time returned from mktime is an hour ahead. HOWEVER, by dumb luck I found that if I declare (but don't use) not one but two pointers to a timespec struct, then mktime returns the correct time.

What is happening there?

A copy of the test code is below.

Thanks.

Code:

/* timetest.c
 * ----------
 * compile with gcc -lrt -o timetest timetest.c
 *
 * d.hale, 2010-05-05
 *
 * call with YYYYMMDDTHHMMSS (e.g. timetest 20100505T120000)
 *
 */
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
#include <time.h>

int main(int argc, char **argv)
  {
/* !!! uncomment this line to make it report the correct time !!!
  struct timespec *p, *q;
*/
  struct timespec ts;
  time_t t;
  struct tm tm, *gmt;
  const char fm[]="%Y%m%dT%H%M%S";
  char buf[256];

  if (argc>1) {
    printf("\n    argv=%s\n", argv[1]);
    strptime(argv[1], fm, &tm);           /* convert argv string to tm struct */

    strftime(buf, sizeof(buf), "%Y%m%dT%H%M%S", &tm);       /* and back again */
    printf("    argv=%s (converted strptime->strftime)\n",buf);

    t = mktime(&tm);                /* now convert to calendar representation */

/* !!! The incorrect time will be reported here,
   !!!                            unless the *p, *q line is uncommented above */

    printf(" argv tm=%d ==> %d-%02d-%02d %02d:%02d:%02d (dst=%d)\n", t,
           tm.tm_year+1900, tm.tm_mon+1, tm.tm_mday, 
           tm.tm_hour, tm.tm_min, tm.tm_sec, tm.tm_isdst);

    strftime(buf, sizeof(buf), "%Y%m%dT%H%M%S", &tm);
    printf("         %s (converted mktime->strftime)\n",buf);

    gmt = gmtime(&t);                                   /* this is also wrong */

    printf("argv gmt=%d-%02d-%02d %02d:%02d:%02d\n", gmt->tm_year+1900, 
           gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec);

    printf(" asctime=%s\n", asctime(&tm));
    }

  clock_gettime(CLOCK_REALTIME, &ts);
  printf("timespec now = %d.%09d\n", ts.tv_sec, ts.tv_nsec);

  return 0;
  }

wje_lq · 05-07-2010, 09:12 AM

You're probably not going to like this answer.

First, I was able to reproduce the problem, even when uncommenting the declarations of p and q.

The problem boils down to strptime() not filling in the tm_isdst field of the struct. After I made a few changes to your source, the following script ...

Code:

#!/bin/bash

set -o verbose
##############################################################################
cat timetest.c
##############################################################################
gcc -lrt -o timetest timetest.c
timetest 20100505T120000

... yields this result:

Code:

##############################################################################
cat timetest.c
/* timetest.c
 * ----------
 * compile with gcc -lrt -o timetest timetest.c
 *
 * d.hale, 2010-05-05
 *
 * call with YYYYMMDDTHHMMSS (e.g. timetest 20100505T120000)
 *
 */
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
#include <time.h>

int main(int argc, char **argv)
  {
  struct timespec *p, *q;
  struct timespec ts;
  time_t t;
  struct tm tm, *gmt;
  const char fm[]="%Y%m%dT%H%M%S";
  char buf[256];

  if (argc>1) {
    printf("\n    argv=%s\n", argv[1]);
    strptime(argv[1], fm, &tm);           /* convert argv string to tm struct */

    strftime(buf, sizeof(buf), "%Y%m%dT%H%M%S", &tm);       /* and back again */
    printf("    argv=%s (converted strptime->strftime)\n",buf);

    printf("There's your trouble -> %d-%02d-%02d %02d:%02d:%02d (dst=%d)\n",
           tm.tm_year+1900, tm.tm_mon+1, tm.tm_mday, 
           tm.tm_hour, tm.tm_min, tm.tm_sec, tm.tm_isdst);

    t = mktime(&tm);                /* now convert to calendar representation */

/* !!! The incorrect time will be reported here,
   !!!                            unless the *p, *q line is uncommented above */

    printf(" argv tm=%d ==> %d-%02d-%02d %02d:%02d:%02d (dst=%d)\n", t,
           tm.tm_year+1900, tm.tm_mon+1, tm.tm_mday, 
           tm.tm_hour, tm.tm_min, tm.tm_sec, tm.tm_isdst);

    strftime(buf, sizeof(buf), "%Y%m%dT%H%M%S", &tm);
    printf("         %s (converted mktime->strftime)\n",buf);

    gmt = gmtime(&t);                                   /* this is also wrong */

    printf("argv gmt=%d-%02d-%02d %02d:%02d:%02d\n", gmt->tm_year+1900, 
           gmt->tm_mon+1, gmt->tm_mday, gmt->tm_hour, gmt->tm_min, gmt->tm_sec);

    printf(" asctime=%s\n", asctime(&tm));
    }

  clock_gettime(CLOCK_REALTIME, &ts);
  printf("timespec now = %d.%09d\n", ts.tv_sec, ts.tv_nsec);

  return 0;
  }
##############################################################################
gcc -lrt -o timetest timetest.c
timetest 20100505T120000

    argv=20100505T120000
    argv=20100505T120000 (converted strptime->strftime)
There's your trouble -> 2010-05-05 12:00:00 (dst=0)
 argv tm=1273089600 ==> 2010-05-05 13:00:00 (dst=1)
         20100505T130000 (converted mktime->strftime)
argv gmt=2010-05-05 20:00:00
 asctime=Wed May  5 13:00:00 2010

timespec now = 1273241321.034025332

What to do? Well, I hope someone can come along and recommend something more constructive than the following, but I've long been dissatisfied with all but the most trivial provided time conversion functions. I just go to the data source:

Code:

/usr/share/zoneinfo

and do my own conversions as I need them. For more info:

Code:

man tzfile

halestorm914 · 05-12-2010, 03:49 PM

What we ended up doing was setting the system clock to UTC and initializing the tm struct to zero.

Keyur-Mithawala · 12-03-2010, 11:34 AM

Set the struct tm.tm_isdst flag to a negative value before calling the mk/gm/local set of time functions.

Let me know if this works for you!

- Keyur