setsystz utility: set the kernel's system time zone

From: David Madore
Date: Mon Feb 19 2007 - 13:46:17 EST

// Hi. I felt the need to write the following little utility (which
// is mostly comments, really), to prevent my digital camera's image
// files to have incorrect modification when I mount them under Linux.
// Comments are welcome.

// Enjoy!

/// cut after ///
/* setsystz: set the Linux kernel's idea of the time zone */
/* David A. Madore <david.madore@xxxxxx>, 2007-02-19. Public Domain */

/* Rationale: the Linux kernel needs to have some idea of time zone,
notably because some filesystems (e.g. FAT) store file
modification/access times in local time rather than UTC(=GMT)
(which Unix uses internally for all timestamps). This kernel
(system) time zone is set through the settimeofday() system call;
unfortunately, there does not seem to be a practical way to do it,
and some (all?) Linux distributions get it wrong: e.g., simply
because my CMOS clock is set to GMT (as recommended), my Debian
init scripts apparently assume that any FAT filesystems I'll be
mounting will have GMT timestamps (uh?). Note: IMHO, the whole
idea of having a per-system global time zone is probably wrong, and
FAT mounts should probably better use an adhoc option to specify
GMT offset (defaulting to the libc time zone for the mount
process), and CMOS clock thingies should be kept separate. */

/* What this does: called without arguments, setsystz sets the
kernel's time zone to the userland's time zone (typically from the
/etc/localtime file, overridden by the TZ environment variable if
it exists). With an explicit argument, setsystz sets the kernel's
time zone to that many minutes west of GMT (see settimeofday(2) man
page for explanations). This program takes care _not_ to
change/warp the system clock while changing the time zone: see
comments on avoid_linux_braindeadness() below. */

/* How to use: probably just call "setsystz" (as root) before mounting
a FAT filesystem, if the files it contains are in your usual system
time zone. If they are, e.g., from the Shanghai time zone, then
use "TZ=Asia/Shanghai setsystz" before mounting. Note: it's
probably wiser not to do this while there are existing mounted FAT
filesystems. */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>

auto_minutes (void)
/* Determine localtime GMT offset and return it in minutes west
of GMT (as expected by a struct timezone). This will
typically use the TZ environment variable if it is defined or,
as a fallback, the contents of /etc/localtime (see libc
documentation for more details). */
time_t now = time(NULL);
struct tm *lt = localtime (&now);
long int gmtoff = lt->tm_gmtoff;
fprintf (stderr, "GMT offset=%lds\n", gmtoff);
if ( gmtoff%60 )
fprintf (stderr, "warning: GMT offset %lds "
"is not an integer number of minutes\n", gmtoff);
gmtoff /= 60;
return -gmtoff;

avoid_linux_braindeadness (void)
/* We ___DO NOT___ want to change the system time, only the
system time zone! Since Linux does something special
(warp_clock() semantics) the very first time settimeofday() is
called with tz!=NULL, we call it once with tz pointing to a
GMT-filled structure, i.e., tz->tz_minuteswest==0 (so the
clock won't be warped). The settimeofday(2) man page claims
that tz->tz_minuteswest==0 will not count toward cancelling
the warp_clock() semantics, i.e., that our trick does not
work: fortunately, it is wrong (at least under 2.6.19 and
whereabouts) and our trick works. Note however that this
still resets the time interpolator the first time:
unfortunately there does not seem to be a way around this
problem. See /usr/src/linux/kernel/time.c for details
about the whole mess. -- David A. Madore 2007-02-19 */
struct timezone tz;
memset (&tz, 0, sizeof(struct timezone));
tz.tz_minuteswest = 0;
tz.tz_dsttime = 0;
settimeofday (NULL, &tz);

main (int argc, char *argv[])
int minuteswest;
if ( argc == 1 )
minuteswest = auto_minutes();
else if ( argc == 2 )
if ( sscanf (argv[1], "%d", &minuteswest) != 1 )
fprintf (stderr, "invalid argument: %s\n", argv[1]);
exit (2);
fprintf (stderr, "wrong number or arguments\n");
exit (2);
struct timezone tz;
memset (&tz, 0, sizeof(struct timezone));
tz.tz_minuteswest = minuteswest;
tz.tz_dsttime = 0;
fprintf (stderr, "setting system time zone to tz_minuteswest=%d\n",
#if 1
avoid_linux_braindeadness ();
if ( settimeofday (NULL, &tz) == -1 )
perror ("settimeofday()");
return 0;
/// cut before ///
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