diff --git a/xmodmap.c b/xmodmap.c

index 58a8e70..0aafed5 100644

--- a/xmodmap.c

+++ b/xmodmap.c

@@ -34,6 +34,8 @@ from The Open Group.

 #include <stdarg.h>

 #include "xmodmap.h"

+extern int asprintf (char **, const char *, ...);

+

 const char *ProgramName;

 Display *dpy = NULL;

 int min_keycode, max_keycode;