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From: "Stephen J. Turnbull" <turnbull AT sk DOT tsukuba DOT ac DOT jp>
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Subject: Re: Set _IOERR if the file stream has been opened in wrong mode.
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Date: Sat, 20 Jul 2013 23:15:01 +0900
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Eli Zaretskii writes:

 > > From: "Stephen J. Turnbull" <turnbull AT sk DOT tsukuba DOT ac DOT jp>
 > > Date: Sat, 20 Jul 2013 19:54:47 +0900
 > > 
 > > Eli Zaretskii writes:
 > >  > > From: "Stephen J. Turnbull" <turnbull AT sk DOT tsukuba DOT ac DOT jp>
 > >  > > Date: Sat, 20 Jul 2013 17:20:25 +0900
 > >  > > 
 > >  > > I would assume that
 > >  > > 
 > >  > >     ifp = fopen(infile, "r"); ofp = fopen(outfile, "w");
 > >  > > 
 > >  > > produces an unwritable and an unreadable file respectively
 > >  > 
 > >  > There's no such thing as "unreadable file" on DOS.
 > > 
 > > Sorry, I didn't mean "file" (too much Python programming, obviously),
 > > I meant "FILE*".  Can you read from ofp on DOS?
 > 
 > That last question is no longer in DOS-land, it is in the libc-land.
 > Because DOS knows nothing of 'FILE *'.

*sigh* Try "Can you read from a (FILE*|file descriptor) opened in mode
'w' in a DJGPP program?" instead, which is what I intended.  (My
original question was "what semantics should this C code have?" and
above is my answer, imprecisely stated.)  Or you could ask the same
question for a set of relevant DOS implementations.

 > So we are back at square one: is there any good reason, apart of
 > "other libraries do that", which mandates that a FILE stream open
 > with "w" be unreadable?

If you can write to ifp and read from ofp as above, why is there a
mode argument to the open functions at all?  In fact the man pages for
open(2) on both Linux and BSD specify the that O_RDONLY and O_WRONLY
flags mean open in "read-only" and "write-only" modes respectively.

I would assume that the designers wanted to avoid situations where
FILE* or fd is passed around and somebody gets confused and overwrites
a file that was supposed to be readonly.

I really think the lack of a specification here is due simply to the
fact that nobody considered that anything else would be reasonable,
and it's just an embarrassing oversight (for the standards committees).