From: Philip Oakley <redacted>

This constant is not defined in MSVC's headers.

In UCRT's fcntl.h, _O_RDONLY, _O_WRONLY and _O_RDWR are defined as 0, 1
and 2, respectively. Yes, that means that UCRT breaks with the tradition
that O_RDWR == O_RDONLY | O_WRONLY.

It is a perfectly legal way to define those constants, though, therefore
we need to take care of defining O_ACCMODE accordingly.

This is particularly important in order to keep our "open() can set
errno to EISDIR" emulation working: it tests that (flags & O_ACCMODE) is
not identical to O_RDONLY before going on to test specifically whether
the file for which open() reported EACCES is, in fact, a directory.

Signed-off-by: Philip Oakley <redacted>
Signed-off-by: Johannes Schindelin <redacted>
---
 compat/msvc.h | 2 ++
 1 file changed, 2 insertions(+)

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"
 

-- 
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