diff --git a/isisd/isis_misc.h b/isisd/isis_misc.h
index 330556a..91bd248 100644
--- a/isisd/isis_misc.h
+++ b/isisd/isis_misc.h
@@ -40,11 +40,11 @@
 /*
  * Printing functions
  */
-char *isonet_print (u_char *, int len);
-char *sysid_print (u_char *);
-char *snpa_print (u_char *);
-char *rawlspid_print (u_char *);
-char *time2string (u_int32_t);
+const char *isonet_print (u_char *, int len);
+const char *sysid_print (u_char *);
+const char *snpa_print (u_char *);
+const char *rawlspid_print (u_char *);
+const char *time2string (u_int32_t);
 /* typedef struct nlpids nlpids; */
 char *nlpid2string (struct nlpids *);