diff --git a/compat/msvc.h b/compat/msvc.h
index ec4518eebf..ae1c58ec13 100644
--- a/compat/msvc.h
+++ b/compat/msvc.h
@@ -25,6 +25,8 @@ static __inline int strcasecmp (const char *s1, const char *s2)
 #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"