C C portable algorithm to convert time stamp to 8 chars? <->

[Spartrekus]

Hello,

I would glad to make my time stamp string smaller, for windows, bsd,... 8 chars would be ok.
The simple, universal, tiny, algorithm shall convert back and forth (<->) the time stamp <-> 8 chars.


some basic starting point:

char *strtimestamp()
{
      long t;
      struct tm *ltime;
      time(&t);
      ltime=localtime(&t);
      char charo[50];  int fooi ; 
      fooi = snprintf( charo, 50 , "%04d%02d%02d%02d%02d%02d",
    1900 + ltime->tm_year, ltime->tm_mon +1 , ltime->tm_mday, 
    ltime->tm_hour, ltime->tm_min, ltime->tm_sec 
    );


    size_t siz = sizeof charo ; 
    char *r = malloc( sizeof charo );
    return r ? memcpy(r, charo, siz ) : NULL;
}

 

[tobik@]

See strftime(3) and strptime(3).

 

[mrclksr]

Hi Spartrekus,

The simple, universal, tiny, algorithm shall convert back and forth (<->) the time stamp <-> 8 chars.

If I have understood correctly, maybe something like this:

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

#define T0 1500000000   /* Fri Jul 14 04:40:00 2017 */

/*
 * We can represent every 32bit number using 8 (hex) digits:
 * [log(2^32 - 1) / log(16)] = 8
 * Using offsets from T0, we can use 8 char (32bit) timestamps until
 * Mon Aug 20 11:08:15 2153 if the future machine uses an unsigned 32bit
 * integer for time_t or a (signed) 64bit integer.
 */

char *
strtimestamp()
{
   char *buf;
   time_t delta = time(NULL) - T0;

   if ((buf = malloc(9)) != NULL)
       (void)sprintf(buf, "%08x", (unsigned int)delta);
   return (buf);
}

time_t
timestamptotime(const char *ts)
{
   return ((time_t)(strtol(ts, NULL, 16) + T0));
}

int
main(int ragc, char *argv[])
{
   char *ts = strtimestamp();
   time_t t;
 
   if (ts == NULL)
       return (EXIT_FAILURE);
   t = timestamptotime(ts);
   (void)printf("Timestamp: %s\n", ts);
   (void)printf("Date: %s", ctime(&t));

   return (EXIT_SUCCESS);
}