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Date: Sun, 24 Jul 2011 18:29:26 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: close(2) failure cases (was: some fixes to musl)
On Sun, Jul 24, 2011 at 09:49:03PM +0400, Vasiliy Kulikov wrote:
> Rich,
>
> On Sun, Jul 24, 2011 at 08:24 -0400, Rich Felker wrote:
> > > Even if close() fails, the fd is freed. So fd leakage is impossible.
> >
> > Is this true even in the case of EINTR?
>
> For all types of fd the fd is deleted from fd table, then
> FS-specific function is called. Any error would be returned to the
> program, but fd would be already deregistered.
Good to know. I may need to investigate and verify that this behaves
as expected in conjunction with thread cancellation while blocked at
close(), and if not, work on a work-around...
Do you know any reliable way to setup the kernel to block/sleep for a
measurable length of time on close() so that I could test this?
> I agree with POSIX in part that close() should somehow signal about
> failed IO (e.g. no free disk space) and error return code is good
> enough. However, I feel it was wrong to leave undefined behaviour of fd in
> case of error. If the file is so important that the error must be
> handled by the program, it really should do *sync() and react on its
> error. IMO close() should unconditionally close fd. (The same for
> fclose(3), etc.)
Note that the way POSIX leaves the state of the fd indeterminate if
close fails makes it impossible to write robust portable
multi-threaded programs that use files in any non-trivial way. You
can't retry closing a file descriptor you already passed to close,
because it might get assigned to a new file opened in another thread,
in which case you would close the other thread's newly-opened file. I
consider this a major flaw in the standard (one of the many oversights
of not considering the interaction of certain behaviors with threads)
and hope to raise the issue as a defect report and push for the next
version of the standard to define the behavior that the fd always be
freed.
Rich
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