--- gzlib.c
+++ gzlib.c
@@ -3,6 +3,9 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#if !defined _WIN32
+#include <unistd.h>
+#endif
 #include "gzguts.h"
 
 #if defined(_WIN32) && !defined(__BORLANDC__)
--- gzread.c
+++ gzread.c
@@ -3,6 +3,9 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#if !defined _WIN32
+#include <unistd.h>
+#endif
 #include "gzguts.h"
 
 /* Use read() to load a buffer -- return -1 on error, otherwise 0.  Read from
--- gzwrite.c
+++ gzwrite.c
@@ -3,6 +3,9 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#if !defined _WIN32
+#include <unistd.h>
+#endif
 #include "gzguts.h"
 
 /* Initialize state for writing a gzip file.  Mark initialization by setting