Plea for clarification on bz #12724

[Eric Blake]

On 07/28/2011 07:30 AM, Eric Blake wrote:
>> f = fdopen (fd2, "w");
>> assert (f);
>> + printf("%ld\n", ftell(f));
>
> So far, so good. fdopen() is required to set the stream position to the
> same as the underlying fd position, so this should always print 1.
>
>> assert(lseek(fd, 4, SEEK_SET) == 4);
>> + printf("%ld\n", ftell(f));
>
> Undefined behavior. POSIX states:
>
>> The result of function calls involving any one handle (the "active
>> handle") is defined elsewhere in this volume of POSIX.1-2008, but if
>> two or more handles are used, and any one of them is a stream, the
>> application shall ensure that their actions are coordinated as
>> described below. If this is not done, the result is undefined.

>
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05
>
>
> Your code has two handles (fd and f), you are starting with fd as the
> active handle and did a reposition, then switched to an action on f.

>
> The POSIX wording for fclose() effectively requires an fflush(), and
> since fflush() is one of the functions that resync's the stream position
> back to the fd position as modified by the first handle, before making f
> the active handle, I still stand by my analysis that at this point in
> the program, the offset of the file description should remain at 4, and
> not be reset to 1 (that is, since the transition from the first handle
> which set the offset to 4 to the second handle which used fclose() is
> clean, the action on the second handle must not corrupt the fd position
> back to 1 merely because the FILE* position had not been advanced from
> 1). Solaris behavior is correct, glibc behavior is buggy.

This topic came up again in today's Austin Group meeting, reaffirming 
that Solaris behavior is correct and glibc 2.14 behavior, while fixing 
one aspect of fclose, caused an an unintentional regression in another 
aspect when compared to 2.13 behavior:

http://austingroupbugs.net/view.php?id=480
http://austingroupbugs.net/view.php?id=87#c895

The latest wording is that with fclose():

the file offset of the underlying open file description shall be set to 
the file position of the stream if the stream is the active handle to 
the underlying file description.

Rereading section 2.5.1 tells you when a stream can become the active 
handle to an underlying file description - only when the previous active 
handle has gotten to a sane state (either because the previous handle 
was an fd, or because it was a stream but was unbuffered, at eof, or 
just flushed), and the new handle does I/O after either ensuring that 
the old handle did no additional lseek/fseek or the new handle starts 
I/O with an explicit fseek.

That is, fclose() should _not_ call lseek() to the stream position 
unless the stream has done any I/O or fseek() which would qualify as 
making the stream an active handle, such that the stream position has a 
reason to be reflected back into the file description position.

> When dealing with multiple handles to a file description, then you must
> ensure that you can properly transition from one active handle to
> another. As long as the active handle transition rules are defined, then
> yes, a file offset change will be observable as an implicit change in
> the file position. As soon as the active handle transition rules are
> broken (such as calling ftell() without an intervening fflush()), then

This should have read:
such as calling ftell() without an intervening fseek()

> behavior is undefined.

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
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