Re: Extended attributes and thread safety

On Aug 31, 2009, at 11:28, Graham Lee wrote:

> On Aug 31, 2009, at 10:19 , Christiaan Hofman wrote:
>
>>
>> On Aug 31, 2009, at 11:12, Graham Lee wrote:
>>
>>> On Aug 31, 2009, at 10:06 , Christiaan Hofman wrote:
>>>
>>>>
>>>> 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?
>
> Sorry, reading my reply it didn't make the main point clear.  
> Processes in Mac OS X have a _per-thread_ error variable which is  
> accessed through errno. Will another thread trash your errno?  
> Probably not. _Could_ another thread, through malicious design or  
> buggy behaviour, trash your errno? Yes. If that's an unacceptable  
> risk to you then the two threads need to live in separate processes.
>
> Cheers,
> Graham.

Thanks, that completely answers my question.
Christiaan