diff --git a/thirdparty/zlib/gzlib.c b/thirdparty/zlib/gzlib.c
index 4105e6a..eae3a38 100644
--- a/thirdparty/zlib/gzlib.c
+++ b/thirdparty/zlib/gzlib.c
@@ -3,6 +3,7 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#include <unistd.h>
 #include "gzguts.h"
 
 #if defined(_WIN32) && !defined(__BORLANDC__) && !defined(__MINGW32__)
diff --git a/thirdparty/zlib/gzread.c b/thirdparty/zlib/gzread.c
index 956b91e..66089b6 100644
--- a/thirdparty/zlib/gzread.c
+++ b/thirdparty/zlib/gzread.c
@@ -3,6 +3,7 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#include <unistd.h>
 #include "gzguts.h"
 
 /* Local functions */
diff --git a/thirdparty/zlib/gzwrite.c b/thirdparty/zlib/gzwrite.c
index c7b5651..e685f3e 100644
--- a/thirdparty/zlib/gzwrite.c
+++ b/thirdparty/zlib/gzwrite.c
@@ -3,6 +3,7 @@
  * For conditions of distribution and use, see copyright notice in zlib.h
  */
 
+#include <unistd.h>
 #include "gzguts.h"
 
 /* Local functions */