What's the practical use of the error close() returns?

[Shachar Shemesh]

On 07/27/2012 02:52 PM, Elazar Leibovich wrote:
>
> (as mentioned earlier, the "no space left" could just as well happen
> after the file was closed, so I don't mind that much it's not reported
> on a close())
>
Ehm, no.

First of all, please note a subtle but important difference between your
question and Orna's answer. You asked about close, she answered about
fclose.

Fclose is an stdio function, and performs a flush of the (user space)
buffers. As such, it is possible for it to run out of disk space. Close,
on the other hand, might only run out of disk space on remote file
systems (such as nfs), and even then, it depends on the local cache
coherency policy. I fail to see how an "out of disk space" might
possibly happen AFTER close, as all that's left then are caches.

Please note that just because the data is in the OS caches, rather than
on disk, this does not mean that anything can change as far as file
system notion of the data goes. By the time close returns, the OS has
already allocated space for the data, and decided where on the disk this
data should go. It is not possible for it to fail after that point
because of lack of disk space.

As I said before, for local file systems, this is also true of "write".
The main reason we need to check the return code of "close" is because
you never know who will run your program over NFS. Over NFS, for
performance reasons, it is possible for a "write" command to fail after
the kernel has already returned to user space with a success indication.
Doing it otherwise will result in horrible latency for anyone working
over NFS. All such errors will get reported on the next operation over
the file descriptor: close.

Shachar

-- 
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com

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