[PATCH 2/4] gsm0411_utils: Add helper function to get the gmt offset of a time_t

The function uses localtime as well as gmtime to calculate the timezone offset at a specific time. This is useful for the *_scts functions so we're not dependent on non-portable features (tm_gmtoff). --- src/gsm/gsm0411_utils.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/src/gsm/gsm0411_utils.c b/src/gsm/gsm0411_utils.c index ad9753e..d857e41 100644 --- a/src/gsm/gsm0411_utils.c +++ b/src/gsm/gsm0411_utils.c @@ -72,6 +72,24 @@ uint8_t gsm411_unbcdify(uint8_t value) return ret; } +/* Figure out the timezone offset in a portable way. + * The idea is to convert the time_t into local and UTC struct tm + * representations and then calculate the difference of both. */ +static time_t gmtoffset_from_ts(time_t time) +{ + struct tm tm_local, tm_utc; + time_t ts_local, ts_utc; + + localtime_r(&time, &tm_local); + gmtime_r(&time, &tm_utc); + tm_utc.tm_isdst = 0; + tm_local.tm_isdst = 0; + ts_utc = mktime(&tm_utc); + ts_local = mktime(&tm_local); + + return ts_local - ts_utc; +} + /* Generate 03.40 TP-SCTS */ void gsm340_gen_scts(uint8_t *scts, time_t time) { -- 1.8.4.2