1 files changed, 34 insertions, 0 deletions
diff --git a/src/corelib/global/qtenvironmentvariables.cpp b/src/corelib/global/qtenvironmentvariables.cpp
index 4f2a263cc4..cf5955902a 100644
--- a/src/corelib/global/qtenvironmentvariables.cpp
+++ b/src/corelib/global/qtenvironmentvariables.cpp
@@ -353,6 +353,10 @@ void qTzSet()
 time_t qMkTime(struct tm *when)
 {
     const auto locker = qt_scoped_lock(environmentMutex);
+#if defined(Q_OS_WIN)
+    // QTBUG-83881 MS's mktime() seems to need _tzset() called first.
+    _tzset();
+#endif
     return mktime(when);
 }
 
@@ -377,6 +381,7 @@ bool qLocalTime(time_t utc, struct tm *local)
     tzset();
     if (tm *res = localtime_r(&utc, local)) {
         Q_ASSERT(res == local);
+        Q_UNUSED(res);
         return true;
     }
     return false;
@@ -392,4 +397,33 @@ bool qLocalTime(time_t utc, struct tm *local)
 #endif
 }
 
+/* Access to the tzname[] global in one thread is UB if any other is calling
+   tzset() or anything that behaves as if it called tzset(). So also lock this
+   access to prevent such collisions.
+
+   Parameter dstIndex must be 1 for DST or 0 for standard time.
+   Returns the relevant form of the name of local-time's zone.
+*/
+QString qTzName(int dstIndex)
+{
+    char name[512];
+    bool ok;
+#if defined(_UCRT)  // i.e., MSVC and MinGW-UCRT
+    size_t s = 0;
+    {
+        const auto locker = qt_scoped_lock(environmentMutex);
+        ok = _get_tzname(&s, name, 512, dstIndex) != 0;
+    }
+#else
+    {
+        const auto locker = qt_scoped_lock(environmentMutex);
+        const char *const src = tzname[dstIndex];
+        ok = src != nullptr;
+        if (ok)
+            memcpy(name, src, std::min(sizeof(name), strlen(src) + 1));
+    }
+#endif // Q_OS_WIN
+    return ok ? QString::fromLocal8Bit(name) : QString();
+}
+
 QT_END_NAMESPACE