This patch allows the 'asprintf' declaration to be visible, by including <config.h>, which defines _GNU_SOURCE. --- xmodmap-1.0.7/xmodmap.c 2013-11-25 23:25:25.000000000 +0100 +++ xmodmap-1.0.7/xmodmap.c 2013-11-25 23:25:27.000000000 +0100 @@ -26,6 +26,7 @@ from The Open Group. */ +#include <config.h> #include <X11/Xos.h> #include <X11/Xlib.h> #include <stdio.h>