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Full Discussion: Reading Daylight Saving Time in Linux using C/C++
Operating Systems Linux Reading Daylight Saving Time in Linux using C/C++ Post 302383093 by jim mcnamara on Monday 28th of December 2009 10:40:12 AM
Old 12-28-2009
I stayed out of this one for a while. Anytime you make assumptions about timezone or date you get in trouble - of all applications, calendrics and time get mishandled the most, IMO.

The std C runtime plus the environment is the final arbiter on what the system thinks about time and date.

This code is a rather stupid (with no error checking) brute force solution. It has assumptions: 32 bit system, years from 1970-2037, 2 ds time changes per year, and we are N of the equator. That way "start ds" is the first change we will hit if we begin Jan 01. Use at your own risk.

Code:
#include <stdlib.h>
#include <time.h>
#include <stdio.h>
#include <string.h>

#define FULL_FMT "%m-%d-%Y"
#define DT_LEN 80
#define TZ_LEN 28
#define DAY 86400

typedef
struct
{
  char start[DT_LEN];
  char tz1[TZ_LEN];  
  char end[DT_LEN];
  char tz2[TZ_LEN];
  
} dsavings_t;

char *sec2dt(char *dest, time_t sec, const char *fmt, char *cmp, dsavings_t *ds)
{
	struct tm *p=localtime(&sec);
	*dest=0x0;

	strftime(dest, DT_LEN, fmt, p);
	if(cmp!=NULL && strcmp(cmp, dest))
	{
		strcpy(   (*ds->tz1)?   ds->tz2: ds->tz1, cmp);
		strftime( (*ds->start)? ds->end: ds->start, DT_LEN, FULL_FMT, p);
		strcpy(cmp, dest);
	}
	return dest;
}

int tzshift(dsavings_t *ds, const int year)
{
	  struct tm w={0,0,0,0,0,0,0,0,0};
	  int i=0;
	  char tmp[DT_LEN]={0x0};
	  char tz[DT_LEN]={0x0};
	  time_t lt=0;
	  memset(ds, 0x0, sizeof(dsavings_t));	 	  
    sprintf(tmp,"01-01-%4d-23:00", year);
	  strptime(tmp, "%d-%m-%Y-%H:%M", &w);
	  lt=mktime(&w);
	  sec2dt(tz, lt, "%Z", NULL, NULL);
	  /*lt+=DAY; */
	  for(i=0; *(sec2dt(tmp, lt, "%Z", tz, ds)) && i<366; i++, lt+=DAY);	    
	  return (*ds->start && *ds->end)? 1 : 0;
}

int main(int argc, char **argv)
{
	  int yr=0;
	  dsavings_t ds={ {0x0}, {0x0} };
	  int retval=0;
	  for(yr=1970; yr < 2038; yr++)
	  {
	  		retval=tzshift(&ds, yr);
	  		printf("%d: %s (%s)  %s (%s)\n", yr, 
	  		       (retval) ? ds.start : "None",
	  		       (retval) ? ds.tz1 : "",
	  		       (retval) ? ds.end : "None",
	  		       (retval) ? ds.tz2 : "");
	  }
	  return 0;
}

 

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LEARN ABOUT LINUX

string

STRING(3)						     Linux Programmer's Manual							 STRING(3)

NAME

       stpcpy, strcasecmp, strcat, strchr, strcmp, strcoll, strcpy, strcspn, strdup, strfry, strlen, strncat, strncmp, strncpy, strncasecmp, strp-
       brk, strrchr, strsep, strspn, strstr, strtok, strxfrm, index, rindex - string operations

SYNOPSIS

       #include <strings.h>

       int strcasecmp(const char *s1, const char *s2);

       int strncasecmp(const char *s1, const char *s2, size_t n);

       char *index(const char *s, int c);

       char *rindex(const char *s, int c);

       #include <string.h>

       char *stpcpy(char *dest, const char *src);

       char *strcat(char *dest, const char *src);

       char *strchr(const char *s, int c);

       int strcmp(const char *s1, const char *s2);

       int strcoll(const char *s1, const char *s2);

       char *strcpy(char *dest, const char *src);

       size_t strcspn(const char *s, const char *reject);

       char *strdup(const char *s);

       char *strfry(char *string);

       size_t strlen(const char *s);

       char *strncat(char *dest, const char *src, size_t n);

       int strncmp(const char *s1, const char *s2, size_t n);

       char *strncpy(char *dest, const char *src, size_t n);

       char *strpbrk(const char *s, const char *accept);

       char *strrchr(const char *s, int c);

       char *strsep(char **stringp, const char *delim);

       size_t strspn(const char *s, const char *accept);

       char *strstr(const char *haystack, const char *needle);

       char *strtok(char *s, const char *delim);

       size_t strxfrm(char *dest, const char *src, size_t n);

DESCRIPTION

       The string functions perform string operations on null-terminated strings.  See the individual man pages for descriptions of each function.

SEE ALSO

       index(3), rindex(3), strcasecmp(3), stpcpy(3), strcat(3), strchr(3), strcmp(3), strcoll(3), strcpy(3),  strcspn(3),  strdup(3),	strfry(3),
       strlen(3),  strncasecmp(3),  strncat(3),  strncmp(3),  strncpy(3),  strpbrk(3),	strrchr(3),  strsep(3),  strspn(3),  strstr(3), strtok(3),
       strxfrm(3)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

								    2010-02-25								 STRING(3)
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