diff -ur a/aigunconstraint.c b/aigunconstraint.c
--- a/aigunconstraint.c	2013-10-06 09:08:03.000000000 -0400
+++ b/aigunconstraint.c	2023-10-27 08:55:01.678566389 -0400
@@ -26,6 +26,7 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 static const char * USAGE =
 "usage: aigunconstraint [-h][-v] [<input> [<output>]]\n"