How do I find the current system timezone?

Question

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

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Answer 1

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...