Mail Archives: cygwin/2010/04/01/12:14:10
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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