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From: "Stephen J. Turnbull" <turnbull AT sk DOT tsukuba DOT ac DOT jp>
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Cc: Eli Zaretskii <eliz AT gnu DOT org>, Juan Manuel Guerrero
 <juan DOT guerrero AT gmx DOT de>
Subject: Re: Set _IOERR if the file stream has been opened in wrong mode.
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Date: Thu, 18 Jul 2013 13:22:43 +0900
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CCing Eli and Juan because I might not be getting through to the DJGPP
lists due to an address change or something.

Eli Zaretskii writes: 

 > Thanks, but I've already read all that, and found nothing about
 > reading from a stream open as "w".  Who said that is an error?

Mac OS X (ie, *BSD) man page for fgets(3):

    ERRORS
     [EBADF]            The given stream is not a readable stream.

and read(2):

    ERRORS
     The pread(), read(), and readv() calls will succeed unless:

     [EBADF]            fildes is not a valid file or socket descriptor
                        open for reading.

I don't claim this is required for conformance to any independent
standard such as ISO C or POSIX, just that it's documented in the BSD
man pages.  IIRC, DJGPP libc is derived from BSD to avoid (L)GPL
implications of merely using the compilation suite, so BSD man pages
should be considered carefully, if not as authoritative.