-/* Next available channel key. */
-static unsigned long next_channel_key;
-static unsigned long next_session_id;
+/* Next available channel key. Access under next_channel_key_lock. */
+static uint64_t _next_channel_key;
+static pthread_mutex_t next_channel_key_lock = PTHREAD_MUTEX_INITIALIZER;
+
+/* Next available session ID. Access under next_session_id_lock. */
+static uint64_t _next_session_id;
+static pthread_mutex_t next_session_id_lock = PTHREAD_MUTEX_INITIALIZER;