diff --git a/compat/msvc.h b/compat/msvc.h
index c64d5a56b2..926e2fd5d6 100644
--- a/compat/msvc.h
+++ b/compat/msvc.h
@@ -26,6 +26,9 @@ static __inline int strcasecmp (const char *s1, const char *s2)
 
 #ifdef _MSC_VER
 typedef int sigset_t;
+/* open for reading, writing, or both (not in fcntl.h) */
+#define O_ACCMODE     (_O_RDONLY | _O_WRONLY | _O_RDWR)
+
 #endif
 
 #include "compat/mingw.h"