Re: Questions about strptime()/strftime() [text format]

["Ryan Arnold" <ryan dot arnold at gmail dot com>]

Sorry,

This was supposed to be a moderation queue reject to be sent to
libc-help for comment.

Further comment to this thread will be rejected.  Please carry on this
conversation on libc-help.

On Fri, Jun 20, 2008 at 12:33 PM, Richard Harvey Chapman
<hchapman@3gfp.com> wrote:
> (Sorry for the double post, but I didn't realize that Thunderbird defaults
> to HTML formatting for e-mails)
>
> Hello all and thanks in advance.
>
> I'm trying to transmit date and time in a user readable format from one
> machine to another. My goal is a format like this:
>
> "17:25:58 2008-06-20 -0500"
>
>
> I am thinking of using the strftime() and strptime() functions except
> that I don't get consistent results. I've attached a sample program and
> included here the output on a recent 10.5.3 OS X machine and an Ubuntu
> Linux machine using libc-2.3.6. The Linux version segfaults when
> strftime() uses the tm struct that was filled using strptime().
>
> Can someone suggest a reliable method for this?
> Am I supposed to be initializing the tm struct?
>
> OS X output:
>
> harveybook:coding_tests hchapman$ make time_formats && ./time_formats
> g++ -ggdb time_formats.cc -o time_formats
> -----strftime(localtime(time())) output-----
> Time now is "17:25:58 2008-06-20 -0500 UTC" (Local)
> Time now is "17:25:58 2008-06-20 -0500 UTC" (GM)
>
> -----printf() of strptime(strftime(...))-----
> Reverse time is "17:25:58 2008-06-20 -0500 (null) DST" (Local)
> Reverse time is "17:25:58 2008-06-20 -0500 (null) DST" (GM)
>
> -----printf() of original tm structs used with strftime()-----
> Reverse time is "17:25:58 2008-06-20 00000 UTC " (Local)
> Reverse time is "17:25:58 2008-06-20 00000 UTC " (Local)
>
> -----printf() of strftime(strptime(strftime(...)))-----
> Time now is "17:25:58 2008-06-20 -0400 EDT" (Local)
> Time now is "17:25:58 2008-06-20 " (GM)
>
> -----Crude attempt to go back-and-forth using time() and ctime()-----
> Time in seconds is "1213982758"
> Time reversed is "Fri Jun 20 13:25:58 2008
> "
>
>
> Ubuntu output:
>
> dev1:~# g++ -ggdb time_formats.cc -o time_formats && ./time_formats
> -----strftime(localtime(time())) output-----
> Time now is "17:25:03 2008-06-20 +0000 GMT" (Local)
> Time now is "17:25:03 2008-06-20 +0000 GMT" (GM)
>
> -----printf() of strptime(strftime(...))-----
> Reverse time is
> "17:25:03 2008-06-20 00000 UâÃ]à           UâÃ]à           UâÃâU
> âÃtâEâ DST" (Local)
> Reverse time is "17:25:03 2008-06-20 00000 DST" (GM)
>
> -----printf() of original tm structs used with strftime()-----
> Reverse time is "17:25:03 2008-06-20 00000 GMT " (Local)
> Reverse time is "17:25:03 2008-06-20 00000 GMT " (Local)
>
> -----printf() of strftime(strptime(strftime(...)))-----
> Time now is "17:25:03 2008-06-20 UâÃ]à           UâÃ]à           UâÃâU
> âÃtâEâ" (Local)
> Segmentation fault
>
>
>
> #include <stdio.h>
> #include <time.h>
>
> // size_t
> //     strftime(char *restrict s, size_t maxsize, const char *restrict
> format,
> //         const struct tm *restrict timeptr);
>
> int main()
> {
>  char str[100];
>  char *result = NULL;
>  time_t t = time(NULL);
>  struct tm *tt = localtime(&t);
>  struct tm *tt2 = gmtime(&t);
>  struct tm tt_r;
>  struct tm tt2_r;
>  char format[] = "%T %F %z %Z"; // time, date, zone // "15:02:18 2008-06-20
> -0500 UTC"
>  int len = 0;
>
>  // First, test conversion to a string
>  printf("-----strftime(localtime(time())) output-----\n");
>
>  len = strftime(str, 100, format, tt);
>  printf("    Time now is \"%s\"   (Local)\n", str);
>
>  len = strftime(str, 100, format, tt2);
>  printf("    Time now is \"%s\"   (GM)\n", str);
>
>  printf("\n");
>
>  // Second, test converting back and verifying the results
>  printf("-----printf() of strptime(strftime(...))-----\n");
>
>  result = strptime(str, format, &tt_r);
>  printf("Reverse time is \"%02u:%02u:%02u %4u-%02u-%02u %05ld %s %s\"
> (Local)\n",
>         tt_r.tm_hour,
>         tt_r.tm_min,
>         tt_r.tm_sec,
>         tt_r.tm_year + 1900,
>         tt_r.tm_mon + 1,
>         tt_r.tm_mday,
>         tt_r.tm_gmtoff / 36,
>         tt_r.tm_zone == NULL ? "(null)" : tt_r.tm_zone,
>         tt_r.tm_isdst ? "DST" : "");
>
>  result = strptime(str, format, &tt2_r);
>  printf("Reverse time is \"%02u:%02u:%02u %4u-%02u-%02u %05ld %s %s\"
> (GM)\n",
>         tt2_r.tm_hour,
>         tt2_r.tm_min,
>         tt2_r.tm_sec,
>         tt2_r.tm_year + 1900,
>         tt2_r.tm_mon + 1,
>         tt2_r.tm_mday,
>         tt2_r.tm_gmtoff / 36,
>         tt2_r.tm_zone == NULL ? "(null)" : tt2_r.tm_zone,
>         tt2_r.tm_isdst ? "DST" : "");
>
>  printf("\n");
>
>  // Try printing the original tm structs
>  printf("-----printf() of original tm structs used with strftime()-----\n");
>
>  printf("Reverse time is \"%02u:%02u:%02u %4u-%02u-%02u %05ld %s %s\"
> (Local)\n",
>         tt->tm_hour,
>         tt->tm_min,
>         tt->tm_sec,
>         tt->tm_year + 1900,
>         tt->tm_mon + 1,
>         tt->tm_mday,
>         tt->tm_gmtoff / 36,
>         tt->tm_zone == NULL ? "(null)" : tt->tm_zone,
>         tt->tm_isdst ? "DST" : "");
>
>  printf("Reverse time is \"%02u:%02u:%02u %4u-%02u-%02u %05ld %s %s\"
> (Local)\n",
>         tt2->tm_hour,
>         tt2->tm_min,
>         tt2->tm_sec,
>         tt2->tm_year + 1900,
>         tt2->tm_mon + 1,
>         tt2->tm_mday,
>         tt2->tm_gmtoff / 36,
>         tt2->tm_zone == NULL ? "(null)" : tt2->tm_zone,
>         tt2->tm_isdst ? "DST" : "");
>
>  printf("\n");
>
>  // Can we take the reversed tm struct and then convert it back into a
> string?
>  printf("-----printf() of strftime(strptime(strftime(...)))-----\n");
>
>  len = strftime(str, 100, format, &tt_r);
>  printf("    Time now is \"%s\"   (Local)\n", str);
>
>  len = strftime(str, 100, format, &tt2_r);
>  printf("    Time now is \"%s\"   (GM)\n", str);
>
>  printf("\n");
>
>  // What about time in seconds?
>
>  printf("-----Crude attempt to go back-and-forth using time() and
> ctime()-----\n");
>  time_t t_s = time(NULL);
>  printf("Time in seconds is \"%lu\"\n", t_s);
>  printf("Time reversed is \"%s\"\n", ctime(&t_s));
>
>  return(0);
> }
>
>