mktime, localtime 関数は環境変数 TZ を参照する

result.md

実行結果

$ gcc prog/c/timezone.c && TZ=UTC ./a.exe
mktime() = 0
gmtime((t=0,&t))         : Thu Jan 01 00:00:00 1970
localtime((t=0,&t))      : Thu Jan 01 00:00:00 1970
gmtime((t=32400,&t))     : Thu Jan 01 09:00:00 1970
localtime((t=32400,&t))  : Thu Jan 01 09:00:00 1970

$ gcc prog/c/timezone.c && TZ=UTC+9 ./a.exe
mktime() = 32400
gmtime((t=0,&t))         : Thu Jan 01 00:00:00 1970
localtime((t=0,&t))      : Wed Dec 31 15:00:00 1969
gmtime((t=32400,&t))     : Thu Jan 01 09:00:00 1970
localtime((t=32400,&t))  : Thu Jan 01 00:00:00 1970

$ gcc prog/c/timezone.c && TZ= ./a.exe
mktime: Invalid argument

リンク

• mktime : http://msdn.microsoft.com/ja-jp/library/d1y53h2a.aspx
• gmtime : http://msdn.microsoft.com/ja-jp/library/0z9czt0w.aspx
• localtime : http://msdn.microsoft.com/ja-jp/library/bf12f0hc.aspx

timezone.c

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

int main(int argc, char *argv[])
{
  struct tm stm;
  time_t t;

  tzset();

  // 1970-01-01T00:00:00
  stm.tm_year = 1970 - 1900;
  stm.tm_mon  = 1 - 1;
  stm.tm_mday = 1;
  stm.tm_hour = 0;
  stm.tm_min  = 0;
  stm.tm_sec  = 0;
  stm.tm_isdst = -1;

  t = mktime(&stm);
  if (t == -1) {
    perror("mktime");
    exit(EXIT_FAILURE);
  }
  printf("mktime() = %ld\n", (long)t);

#define PRINT(x) printf(#x "\t : %s", asctime((x)));

  PRINT(gmtime((t=0,&t)));
  PRINT(localtime((t=0,&t)));
  PRINT(gmtime((t=32400,&t)));
  PRINT(localtime((t=32400,&t)));

  return 0;
}