Questions about strptime()/strftime()

[Richard Harvey Chapman <hchapman at 3gfp dot com>]

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]ÃÂÂÂÂÂÂÂÂÂÂÂUÂÃ]ÃÂÂÂÂÂÂÂÂÂÂÂUÂÃÂU
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]ÃÂÂÂÂÂÂÂÂÂÂÂUÂÃ]ÃÂÂÂÂÂÂÂÂÂÂÂUÂÃÂU
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);
}