diff --git a/include/openbsc/gsm_subscriber.h b/include/openbsc/gsm_subscriber.h
index 90117d3..27726a8 100644
--- a/include/openbsc/gsm_subscriber.h
+++ b/include/openbsc/gsm_subscriber.h
@@ -30,8 +30,12 @@
 	GSM_SUBSCRIBER_TMSI,
 };
 
-struct gsm_subscriber *subscr_alloc();
 struct gsm_subscriber *subscr_get(struct gsm_subscriber *subscr);
 struct gsm_subscriber *subscr_put(struct gsm_subscriber *subscr);
+struct gsm_subscriber *subscr_get_by_tmsi(const char *tmsi);
+struct gsm_subscriber *subscr_get_by_imsi(const char *imsi);
+
+/* internal */
+struct gsm_subscriber *subscr_alloc(void);
 
 #endif /* _GSM_SUBSCR_H */
diff --git a/src/gsm_subscriber.c b/src/gsm_subscriber.c
index 6a25009..390979b 100644
--- a/src/gsm_subscriber.c
+++ b/src/gsm_subscriber.c
@@ -53,7 +53,7 @@
 	free(subscr);
 }
 
-struct gsm_subscriber *subscr_get_by_tmsi(char *tmsi)
+struct gsm_subscriber *subscr_get_by_tmsi(const char *tmsi)
 {
 	struct gsm_subscriber *subscr;
 
@@ -66,7 +66,7 @@
 	return db_get_subscriber(GSM_SUBSCRIBER_TMSI, tmsi);
 }
 
-struct gsm_subscriber *subscr_get_by_imsi(char *imsi)
+struct gsm_subscriber *subscr_get_by_imsi(const char *imsi)
 {
 	struct gsm_subscriber *subscr;