Re: [Qemu-devel] [PATCH] error: error_setg_errno(): errno gets preserved

[Eric Blake]

On 07/28/2016 04:46 AM, Halil Pasic wrote:

> The implementation of preserve errno seems inconsistent to me.  The
> function error_setv is static, and I guess it is supposed to provide
> this indirect errno preservation and is used for both error_setg und
> error_setg_errno, yet error_setg_ errno_internal does extra save-restore
> itself while error_setg_iternal relies on 'indirect', what is not OK in
> my opinion.

As long as errno gets saved where it is documented as saved, I don't
care whether it is direct or indirect (indirect is probably more
efficient, where we can prove that nothing is called that is allowed to
clobber errno).

> 
> As Sascha pointed out, in C11 any library functions may change errno
> unless explicitly told otherwise for the particular function.  Since
> start_va and end_va has nothing on preserving errno it is guaranteed by
> the standard that they persevere errno, and we should assume they don't.

You mean va_start, not start_va.  And actually, C11 is clear that errno
is unspecified after library functions (but not macros) that don't
explicitly state otherwise.  Since va_start() is a macro and not a
library function, that means va_start does NOT have carte blanche
permission to modify errno.  For more reading on the topic:

http://austingroupbugs.net/view.php?id=384

There are several related POSIX bug reports of other functions that have
been requested to explicitly document that they don't modify errno, and
I'm happy to submit even more, if we find other standard interfaces
whose semantics are easier when they guarantee that errno is not clobbered.

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

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