Re: Extended attributes and thread safety

On Aug 31, 2009, at 11:12, Graham Lee wrote:

> On Aug 31, 2009, at 10:06 , Christiaan Hofman wrote:
>
>>
>> On Aug 31, 2009, at 8:36, Chris Suter wrote:
>>
>>> Hi Christian,
>>>
>>> On Sun, Aug 30, 2009 at 9:48 PM, Christiaan Hofman<[email protected] 
>>> > wrote:
>>>> Does anybody know whether it is safe to use the extended  
>>>> attribute C-library
>>>> at sys/xattr.h (getxattr, setxattr, etc) on background threads? The
>>>> documentation does not mention anything about thread safety, but  
>>>> I could
>>>> imagine it to run into problems if it'd use global resources, and  
>>>> the
>>>> functions seem to use at least one global variable.
>>>
>>> Those functions will be thread safe. It's a bug if they're not.
>>>
>>> Kind regards,
>>>
>>> Chris
>>
>> Thanks Chris. However I am still worried about the fact that they  
>> access the global variable errno.
>
> On Mac OS X, errno is a macro wrapping access to a thread-safe  
> variable:
>
> extern int * __error(void);
> #define errno (*__error())
>
> Cheers,
> Graham.

Good, thanks! However, I do think that still leaves the problem that I  
don't see why another thread cannot change this same variable / 
between/ the time that the xattr functions set the variable and my  
code reads it. In other words, I can't be sure the value I get has  
anything to do with the error that occurred. Right?

Christiaan