tzsetで何が変わるのか

tzset_test.c

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

void print_time(const time_t t)
{
  char buf[128];
  size_t str_size = 0;
  
  str_size = strftime(buf, 127, "%Y/%m/%d %H:%M:%S", localtime(&t));
  buf[str_size] = 0;

  printf("%s\n",buf);
} 

int main(int argc, char *argv[])
{
  int i;

  printf("== Before tzset() ==\n");
  for(i=0;i<2;i++) {
    printf("tzname[%d]: %s\n",i,tzname[i]);
  }
  printf("timezone: %ld (h)\n",timezone/(60*60));
  print_time(time(NULL));
  tzset(); /* call tzset() */

  printf("== After tzset() ==\n");
  for(i=0;i<2;i++) {
    printf("tzname[%d]: %s\n",i,tzname[i]);
  }
  printf("timezone: %ld (h)\n",timezone/(60*60));
  print_time(time(NULL));

  return 0;
}
/*
time(NULL)の値が変わるわけではないことに注意。
== Before tzset() ==
tzname[0]: WILDABBR
tzname[1]: WILDABBR
timezone: 0 (h)
2013/02/12 19:34:10
== After tzset() ==
tzname[0]: JST
tzname[1]: JDT
timezone: -9 (h)
2013/02/12 19:34:10
*/