How do I find the current system timezone?

Question

On Linux, I need to find the currently configured timezone as an Olsen location. I want my (C or C++) code to be portable to as many Linux systems as possible.

For example. I live in London, so my current Olsen location is "Europe/London". I'm not interested in timezone IDs like "BST", "EST" or whatever.

Debian and Ubuntu have a file /etc/timezone that contains this information, but I don't think I can rely on that file always being there, can I? Gnome has a function oobs_time_config_get_timezone() which also returns the right string, but I want my code to work on systems without Gnome.

So, what's the best general way to get the currently configured timezone as an Olsen location, on Linux?

Answer 1

According to this page, it looks like if you #include <time.h> it will declare the following.

void tzset (void);
extern char *tzname[2];
extern long timezone;
extern int daylight;

Does that give you the information that you need?

Answer 2

There is no standard c or c++ function for this. However, GNU libc has an extention. its struct tm has two extra members:

long tm_gmtoff;           /* Seconds east of UTC */
const char *tm_zone;      /* Timezone abbreviation */

This means that if you use one of the functions which populates a struct tm (such as localtime or gmtime) you can use these extra fields. This is of course only if you are using GNU libc (and a sufficiently recent version of it).

Also many systems have a int gettimeofday(struct timeval *tv, struct timezone *tz); function (POSIX) which will fill in a struct timezone. This has the following fields:

struct timezone {
    int tz_minuteswest;     /* minutes west of Greenwich */
    int tz_dsttime;         /* type of DST correction */
};

Not exactly what you asked for, but close...

Answer 3

It's hard to get a reliable answer. Relying on things like /etc/timezone may be the best bet.

(The variable tzname and the tm_zone member of struct tm, as suggested in other answers, typically contains an abbreviation such as GMT/BST etc, rather than the Olson time string as requested in the question).

• On Debian-based systems (including Ubuntu), /etc/timezone is a file containing the right answer.
• On some Redhat-based systems (including at least some versions of CentOS, RHEL, Fedora), you can get the required information using readlink() on /etc/localtime, which is a symlink to (for example) /usr/share/zoneinfo/Europe/London.
• OpenBSD seems to use the same scheme as RedHat.

However, there are some issues with the above approaches. The /usr/share/zoneinfo directory also contains files such as GMT and GB, so it's possible the user may configure the symlink to point there.

Also there's nothing to stop the user copying the right timezone file there instead of creating a symlink.

One possibility to get round this (which seems to work on Debian, RedHat and OpenBSD) is to compare the contents of the /etc/localtime file to the files under /usr/share/zoneinfo, and see which ones match:

eta:~% md5sum /etc/localtime
410c65079e6d14f4eedf50c19bd073f8  /etc/localtime
eta:~% find /usr/share/zoneinfo -type f | xargs md5sum | grep 410c65079e6d14f4eedf50c19bd073f8
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/London
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Belfast
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Guernsey
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Jersey
410c65079e6d14f4eedf50c19bd073f8  /usr/share/zoneinfo/Europe/Isle_of_Man
...
...

Of course the disadvantage is that this will tell you all timezones that are identical to the current one. (That means identical in the full sense - not just "currently at the same time", but also "always change their clocks on the same day as far as the system knows".)

Your best bet may be to combine the above methods: use /etc/timezone if it exists; otherwise try parsing /etc/localtime as a symlink; if that fails, search for matching timezone definition files; if that fails - give up and go home ;-)

(And I have no idea whether any of the above applies on AIX...)

Answer 4

On Linux, I need to find the current timezone as an Olsen location. I want my (C or C++) code to be portable to as many Linux systems as possible.

If you want to be portable, then use only GMT internally. Due to multi-user heritge, *NIX system clock is normally is in GMT and there is no system wide timezone - because different users connected to the system might be living in different timezones.

The user specific timezone is reflected in TZ environment variable and you might need to use that only when converting internal date/time into the user readable form. Otherwise, localtime() takes care of it automatically for you.

Answer 5

Since tzselect was not mentioned by anyone and you do need a nearly goof-proof solution, work with what Olsen did. get the tzcode and tzdata files from elsie, plus tab files.

ftp://elsie.nci.nih.gov

Then get tzselect.ksh from the glibc download. Then you can see how to reverse engineer timezones. One sticking point: you WILL sometimes have to ask what country and city the linux box is in. You can serialize that data if you want, and then verify it against the timezone data you can find.

There is no way to do this reliably all the time without the possibility of user intervention, for example, as part of program installation.

Good luck on Arizona in general, and in Western Indiana.... hopefully your code is going to run elsewhere.