[Issue 14505] File doesn't rewind read pointer for a+ mode on Windows DMC

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14505

Steven Schveighoffer <schveiguy at yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #7 from Steven Schveighoffer <schveiguy at yahoo.com> ---
(In reply to Vladimir Panteleev from comment #6)
> (In reply to Steven Schveighoffer from comment #5)
> > It tells the current position of the file descriptor. There is no different
> > "read" or "write" pointer. When writing, the C library (or the OS in the
> > case of Unixen) seeks the stream to the end.
> 
> Is that part of the specification? If so, then that contradicts with your
> above claims and one of the libcs is clearly buggy.

That's for "a", and the specification says what happens only on write, not what
happens on open (at least clearly). That means it's implementation defined.

Here is a quote from my C 90 spec:

"r" open text file for reading
"w" create text file for writing; discard previous contents if any
"a" append; open or create text file for writing at end of file
"r+" open text file for update (i.e. reading and writing)
"w+" create text file for update; discard previous contents if any
"a+" append; open or create text file for update, writing at end

Update mode permits reading and writing the same file; fflush or a
file-positioning function must be called between a read and a write or vice
versa. 

END QUOTE

It does not specifically say what ftell should return immediately after
opening. Only that append writes at the end of the file. Now, one could play
lawyer and say that "at end of file" could describe how to open and not how to
write, but I think that is not what was intended.

The K&R book I have shows an example of how to implement fopen as an example of
how to use unix system interface functions, and shows using lseek to the end of
the file on 'a'. It leaves out how to implement the '+' version. This, however,
is NOT part of the C specification, it's just an example implementation.

Now, clearly it does say that you have to seek between reads and writes, but it
does not say what you have to do on the first read or write. But I think this
doesn't apply to the "a" style opens, as those say writing at end of file.

I'm pretty sure the spec is ambiguous enough to allow all the implementations
here. I'm going to close as WONTFIX, as technically it's a difference in
implementations but I don't think we should fix it in D-land. If someone wants
to fix DMC (and Walter's OK with that), then sure. If someone thinks MSVCRT is
buggy, good luck with that one :)

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