diff --git a/gzguts.h b/gzguts.h
index ca13f47aff77594a322c375f41b9a4499aa8b212..593dfee1111af630e02d37c6f6641ffabe40f45d 100644
--- a/gzguts.h
+++ b/gzguts.h
@@ -25,6 +25,10 @@
 #  include <stdlib.h>
 #  include <limits.h>
 #endif
+
+#ifndef _POSIX_SOURCE
+#  define _POSIX_SOURCE
+#endif
 #include <fcntl.h>
 
 #ifdef _WIN32