diff --git a/public/zlib/gzguts.h b/public/zlib/gzguts.h
index d87659d0..1296e524 100644
--- a/public/zlib/gzguts.h
+++ b/public/zlib/gzguts.h
@@ -29,6 +29,8 @@
 
 #ifdef _WIN32
 #  include <stddef.h>
+#else
+#  include <unistd.h>
 #endif
 
 #if defined(__TURBOC__) || defined(_MSC_VER) || defined(_WIN32)