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Subject: Re: fopen with "a+" does not respect setting file read position to  start of file
References: <B0E0B6C348D8AB4BAD6290DC0BDE99D47427D7442A AT GVW1115EXC DOT americas DOT hpqcorp DOT net> <4BB482FA DOT 3030201 AT ltu DOT se> <op DOT vahh3o181e62zd AT balu DOT cs DOT uni-paderborn DOT de> <4BB4CAFE DOT 5070605 AT redhat DOT com>
Date: Thu, 01 Apr 2010 19:13:49 +0300
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From: "Matthias Andree" <matthias DOT andree AT gmx DOT de>
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Eric Blake wrote on 2010-04-01:

> On 04/01/2010 05:53 AM, Matthias Andree wrote:
>>> Well if that worked previously it was a bug.
>>
>> Not a Cygwin defect, but an application bug, and what Gavin named "work
>> around" is actually the right fix.
>>
>> POSIX explicitly states that the initial position beginning/end of file
>> in append mode is implementation-defined, and the Cygwin documentation
>> doesn't make promises either way. Linux and FreeBSD document BOF and
>> EOF, respectively.
>
> I agree that it is not a cygwin bug.  And I actually prefer Linux'
> interpretation of BOF, since it matches with the POSIX requirement that
> open(,O_APPEND|O_RDWR) be positioned at offset 0 and not at the end (it
> is only fopen that is allowed, but not required, to be at the end).  In
> fact, the release of m4 1.4.10 worked on cygwin and Linux but was broken
> on BSD because of this very portability pitfall.

Preference seconded; Solaris 10 for instance also comes up with  
ftell[o](fopen("nonemptyfile", "a+")) == 0.

The FreeBSD argument (in the libc sources) is that their approach  
applications gets use ftell[o]() right - and how this isn't reliable at  
the same time:

	/*
	 * When opening in append mode, even though we use O_APPEND,
	 * we need to seek to the end so that ftell() gets the right
	 * answer.  If the user then alters the seek pointer, or
	 * the file extends, this will fail, but there is not much
	 * we can do about this.  (We could set __SAPP and check in
	 * fseek and ftell.)
	 */
	if (oflags & O_APPEND)
		(void)_sseek(fp, (fpos_t)0, SEEK_END);

(<http://www.freebsd.org/cgi/cvsweb.cgi/src/lib/libc/stdio/fopen.c?rev=1.14>  
bottom)

-- 
Matthias Andree

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