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From: Eric Blake <eblake@redhat.com>
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Date: Thu, 21 Dec 2017 19:15:24 -0600
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v1 1/1] block: Add numeric
 errno field to BLOCK_IO_ERROR events
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To: Jack Schwartz <jack.schwartz@oracle.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org
Cc: kwolf@redhat.com, armbru@redhat.com, konrad.wilk@oracle.com, mreitz@redhat.com

On 12/21/2017 07:08 PM, Eric Blake wrote:

>> =C2=A0 #
>> +# @errno: int describing the error cause, provided for applications.
>> +#=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (Note: while most e=
rrnos are posix compliant between OSs, it
>> +#=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 is possible some er=
rno values can vary among different OSs.)
>> +#=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (since 2.12)
>=20
> The proof is in the pudding - if your documentation has to give this bi=
g=20
> disclaimer, then what you are adding is not portable and should not be=20
> added in that manner.

To follow up to myself, POSIX explicitly says that errno values are=20
implementation dependent, and there is NO requirement that errno value 1=20
be EPERM, for example.  And while qemu does not target GNU Hurd, that is=20
a classic example of a system where errno values intentionally do not=20
fit in 8 bits.  So you can't argue that there are "POSIX-compliant errno=20
values", because POSIX doesn't mandate specific values.

--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org