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Date: Sun, 18 Jan 2009 15:26:49 -0700
From: Eric Blake <ebb9 AT byu DOT net>
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To: cygwin AT cygwin DOT com
Subject: Re: GNU coreutils does not work on Cygwin because of freopen() ?
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According to Ed Schouten on 1/18/2009 3:19 PM:
> But that's not a proper fix anyway. cat can't just decide to reopen the
> file descriptor of the standard output. That even breaks the first
> sentence of POSIX:
> 
> 	The cat utility shall read files in sequence and shall write
> 	their contents to the standard output in the same sequence.
> 
> If it says "the standard output" doesn't mean you can reopen the output
> descriptor. This means you end up with two different file descriptors.
> They can point to the same file, but they are not the same file
> descriptor.

That's where you are wrong.  freopen can be passed a NULL pointer instead
of a filename in order to reuse the same file descriptor, but with an
implementation-defined different mode.  In cygwin's case, this means you
can use freopen to swap a file between text and binary mode.

> 
> They fixed it by using SET_BINARY(), but not on Cygwin, because I assume
> SET_BINARY() is a Windows-thing, not Cygwin.

SET_BINARY is an upstream coreutils thing designed for cygwin and other
platforms that have O_BINARY in the first place (Debian doesn't).  It is a
macro that expands to either the non-standard setmode(), or the
standardized (but implementation-dependent) freopen().

> 
> I can't really understand why Cygwin just doesn't force O_BINARY to be
> set. File descriptors on UNIX are supposed to display raw file contents.

That's exactly what that freopen is doing - it is forcing O_BINARY to be
set, regardless of how stdin was inherited.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9 AT byu DOT net
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