diff --git a/test/posix-test.h b/test/posix-test.h
index 9bee85e7..19b3e76f 100644
--- a/test/posix-test.h
+++ b/test/posix-test.h
@@ -33,6 +33,8 @@
 
 #ifdef _WIN32
 # include <windows.h>
+#else
+# include <sys/types.h>  // for ssize_t
 #endif
 
 #ifndef _MSC_VER