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Date: Thu, 18 Jul 2013 19:23:15 +0300
From: Eli Zaretskii <eliz AT gnu DOT org>
Subject: Re: Set _IOERR if the file stream has been opened in wrong mode.
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> Date: Thu, 18 Jul 2013 00:59:32 -0400
> From: DJ Delorie <dj AT delorie DOT com>
> 
> I looked in the code and didn't see anything where we specifically set
> the error flag, other than read() failing.  So, it looks like this is
> a DOS thing - that file just isn't readable.

We could set the flag ourselves, just like we do now with returning
EOF (in filbuf.c):

  if ((f->_flag&_IOREAD) == 0)
    return EOF;

We could also set the _IOERR flag at that spot, if we decide that to
be TRT.

> But I couldn't find anything in the posix/ansi specs that said what
> conditions cause errors, just that if errors happen ferror() says so.

Right, same here.  So this is basically our decision: do we want the
undefined behavior in this case be "bug-compatible" to what the other
libraries do?