diff --git a/lib/if2ip.c b/lib/if2ip.c
index 63a634099..94de81f8a 100644
--- a/lib/if2ip.c
+++ b/lib/if2ip.c
@@ -81,7 +81,7 @@
 
 #define SYS_ERROR -1
 
-char *Curl_if2ip(char *interface, char *buf, int buf_size)
+char *Curl_if2ip(const char *interface, char *buf, int buf_size)
 {
   int dummy;
   char *ip=NULL;
@@ -125,8 +125,11 @@ char *Curl_if2ip(char *interface, char *buf, int buf_size)
 
 /* -- end of if2ip() -- */
 #else
-char *Curl_if2ip(char *interface, char *buf, int buf_size)
+char *Curl_if2ip(const char *interface, char *buf, int buf_size)
 {
+    (void) interface;
+    (void) buf;
+    (void) buf_size;
     return NULL;
 }
 #endif
diff --git a/lib/if2ip.h b/lib/if2ip.h
index f2c3a541f..30a6540a4 100644
--- a/lib/if2ip.h
+++ b/lib/if2ip.h
@@ -26,7 +26,7 @@
 
 #if !defined(WIN32) && !defined(__BEOS__) && !defined(__CYGWIN32__) && \
     !defined(__riscos__) && !defined(__INTERIX)
-extern char *Curl_if2ip(char *interface, char *buf, int buf_size);
+extern char *Curl_if2ip(const char *interface, char *buf, int buf_size);
 #else
 #define Curl_if2ip(a,b,c) NULL
 #endif