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bug#18449: "cat x >> x" error even when x is empty

From: Eric Blake
Subject: bug#18449: "cat x >> x" error even when x is empty
Date: Thu, 11 Sep 2014 08:56:10 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0

On 09/11/2014 07:20 AM, Pádraig Brady wrote:

>> while there's no reason to return an error in this case: the file
>> should just remain empty.

Testing for an empty file is enough of an additional special case over
the existing check for same files that I don't think it is worth it.

>> BTW, when x isn't empty, I wonder whether an error is correct if
>> POSIXLY_CORRECT is set. The result will typically depend on the
>> implementation and possibly be non-deterministic, but POSIX doesn't
>> seem to allow an error (except FS errors, such as disk full).

I've raised this as an issue for the Austin Group to rule on:

> This is just a protection against non deterministic usage.
> I.E. if there is data in the file then it'll probably run
> until the file system is exhausted.  This is what happens
> on FreeBSD BTW, though I see that Solaris has the same
> protections as GNU.

I'm hoping that POSIX standardizes the existing practice of forbidding
known disk-filling operations.

> I wouldn't be on for removing this protection which
> has existed from the initial GNU implementation.

My thoughts, too.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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