diff --git a/config.h b/config.h
index 57fce54..633c79b 100644
--- a/config.h
+++ b/config.h
@@ -1,6 +1,7 @@
 #ifdef	STDC_HEADERS
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #else	/* Not STDC_HEADERS */
 #ifdef HAVE_STRINGS_H
 #include <strings.h>