[TxMt] Atomic Saving / Saving Question

Chris Thomas chris at m-audio.com
Tue Feb 8 17:43:54 UTC 2005

On Feb 8, 2005, at 9:00 AM, Sune Foldager wrote:

> On 7. feb 2005, at 18:29, Chris Thomas wrote:
>> However, it appears that this discussion is academic, because HFS+'s 
>> implementation of exchangedata actually returns an error if one of 
>> the files is hardlinked.
> Huh? If I have file A hard linked with B and I "atomically" replace A 
> (i.e. write A.new, move A.new to A) then I will break the link between 
> A and B. That's the issue here, and that's certainly what happens in 
> the scenario I just described :-). No HFS+ errors here.

Right, but I'm talking specifically about the exchangedata system call 
(or FSExchangeObjects). exchangedata is a Darwin VFS operation. It 
calls into the file system implementation to perform the atomic 
exchange. For HFS+, it explicitly preserves the file ID. I had assumed 
it would preserve hardlinks as well, but it turns out that it just 
errors out if one of the files is hardlinked (or has ever been 


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