[RFC 1/1] gsm0411_utils: Fix timezone offs calculation in gsm340_gen_scts()

[Daniel Willmann]

The current code uses gmtime which returns the time in UTC. This is not
the intended behaviour (otherwise the offset could always be set to
zero).

This uses localtime as well as gmtime to calculate the timezone offset
instead of relying on tm_gmtoff.
This means three extra function calls (mktime * 2, gmtime_r), but this
is completely independent of non-standard features.
---
 src/gsm/gsm0411_utils.c | 41 +++++++++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/src/gsm/gsm0411_utils.c b/src/gsm/gsm0411_utils.c
index bb59a10..18664dc 100644
--- a/src/gsm/gsm0411_utils.c
+++ b/src/gsm/gsm0411_utils.c
@@ -77,23 +77,32 @@ uint8_t gsm411_unbcdify(uint8_t value)
 /* Generate 03.40 TP-SCTS */
 void gsm340_gen_scts(uint8_t *scts, time_t time)
 {
-	struct tm *tm = gmtime(&time);
-
-	*scts++ = gsm411_bcdify(tm->tm_year % 100);
-	*scts++ = gsm411_bcdify(tm->tm_mon + 1);
-	*scts++ = gsm411_bcdify(tm->tm_mday);
-	*scts++ = gsm411_bcdify(tm->tm_hour);
-	*scts++ = gsm411_bcdify(tm->tm_min);
-	*scts++ = gsm411_bcdify(tm->tm_sec);
-#ifdef HAVE_TM_GMTOFF_IN_TM
-	if (tm->tm_gmtoff >= 0)
-		*scts++ = gsm411_bcdify(tm->tm_gmtoff/(60*15));
+	struct tm tm_local, tm_utc;
+	time_t ts_local, ts_utc, gmtoffset;
+
+	localtime_r(&time, &tm_local);
+
+	*scts++ = gsm411_bcdify(tm_local.tm_year % 100);
+	*scts++ = gsm411_bcdify(tm_local.tm_mon + 1);
+	*scts++ = gsm411_bcdify(tm_local.tm_mday);
+	*scts++ = gsm411_bcdify(tm_local.tm_hour);
+	*scts++ = gsm411_bcdify(tm_local.tm_min);
+	*scts++ = gsm411_bcdify(tm_local.tm_sec);
+
+	/* 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. */
+	gmtime_r(&time, &tm_utc);
+	tm_utc.is_dst = 0;
+	tm_local.is_dst = 0;
+	ts_utc = mktime(&tm_utc);
+	ts_local = mktime(&tm_local);
+	gmtoffset = ts_local - ts_utc;
+
+	if (gmtoffset >= 0)
+		*scts++ = gsm411_bcdify(gmtoffset/(60*15));
 	else
-		*scts++ = gsm411_bcdify(-tm->tm_gmtoff/(60*15)) | 0x80;
-#else
-#warning find a portable way to obtain timezone offset
-	*scts++ = 0;
-#endif
+		*scts++ = gsm411_bcdify(-gmtoffset/(60*15)) | 0x80;
 }
 
 /* Decode 03.40 TP-SCTS (into utc/gmt timestamp) */
-- 
1.8.4.2