diff --git a/compat/msvc.h b/compat/msvc.h
index 04b4750b87..d336d80670 100644
--- a/compat/msvc.h
+++ b/compat/msvc.h
@@ -19,6 +19,8 @@
 #undef ERROR
 
 typedef int sigset_t;
+/* open for reading, writing, or both (not in fcntl.h) */
+#define O_ACCMODE     (_O_RDONLY | _O_WRONLY | _O_RDWR)
 
 #include "compat/mingw.h"