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From: "Wiktor S. (wswiktorSP AT Mpoczta DOT fm) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Fwd: A few DJGPP libc.a bugs
Date: Wed, 11 May 2016 23:53:20 +0200
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To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

> Can you debug the libc functions involved and see which one of them
> fails?

fopen() fails because open() fails because _open() fails.

I haven't yet determined why. I have two theories.
One is this (abbreviated) logic in open.c:

    fd = _open(real_name, oflag);

 /* Under multi-taskers, such as Windows, our file might be
    open by some other program with DENY-NONE sharing bit,
    which fails the `_open' call above.  Try again with
    DENY-NONE bit set, unless some sharing bits were already
    set in the initial call.  */

    if (fd == -1)
       fd = _open(real_name, oflag | SH_DENYNO);

Both calls to _open() fail. So maybe there is a problem with access flags.

Another possibility is _open function itself. It contains convoluted logic 
of OS version checking, detecting LFN, falling back to SFN, then falling 
back to LFN again...
Perhaps some assumptions are no longer true on exFAT and Windows 10.

-- 
Wiktor S.