diff --git a/src/getopt.c b/src/getopt.c
index 4744e43..19776f3 100644
--- a/src/getopt.c
+++ b/src/getopt.c
@@ -208,7 +208,7 @@ static char *posixly_correct;
    whose names are inconsistent.  */
 
 #ifndef getenv
-extern char *getenv ();
+extern char *getenv (const char *);
 #endif
 
 static char *
diff --git a/src/getopt.h b/src/getopt.h
index b0147e9..6f1e784 100644
--- a/src/getopt.h
+++ b/src/getopt.h
@@ -133,7 +133,7 @@ struct option
    errors, only prototype getopt for the GNU C library.  */
 extern int getopt (int __argc, char *const *__argv, const char *__shortopts);
 # else /* not __GNU_LIBRARY__ */
-extern int getopt ();
+extern int getopt (int, char *const*, const char *);
 # endif /* __GNU_LIBRARY__ */
 
 # ifndef __need_getopt