diff --git a/src/os/macosx/osx_stdafx.h b/src/os/macosx/osx_stdafx.h
--- a/src/os/macosx/osx_stdafx.h
+++ b/src/os/macosx/osx_stdafx.h
@@ -8,6 +8,7 @@
 /* We need to include this first as that "depends" on the compiler's setting
  * of __LP64__. So before we define __LP64__ so it can be used. */
 #include <sys/cdefs.h>
+#include <unistd.h>
 
 /* __LP64__ only exists in 10.5 and higher */
 #if defined(__APPLE__) && !defined(__LP64__)