diff --git a/guetzli/guetzli.cc b/guetzli/guetzli.cc
index 08a2b3ae..020b4115 100644
--- a/guetzli/guetzli.cc
+++ b/guetzli/guetzli.cc
@@ -20,6 +20,7 @@
 #include <memory>
 #include <string>
 #include <string.h>
+#include <unistd.h>
 #include "gflags/gflags.h"
 #include "png.h"
 #include "guetzli/processor.h"