Re: [Xen-devel] [PATCH] libxen-3.0 (libxc rewrite)

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ian Pratt wrote:

> Personally, I think the best approach is to stick with the existing C
> convention that everyone is already well familiar with, and have a
> separate errno variable. This means we can then have functions return
> pointers etc rather than having to pass them by reference, which is
> undeniably ugly. 
>  
There's only one problem with this: thread-safety.  I believe errno is 
commonly implemented in thread-local storage.  We'd have to jump through 
major hoops to get our own proper errno.
Returning the error as part of the interface makes life a lot easier for 
threading.  However, it sounds like a lot of people don't like this.
The best alternative would be to have a context for the library (not 
just a simple file descriptor, but an actual struct that contained the 
file descriptors and the current error condition).  I really was hoping 
to avoid requiring a context (it simplifies application programming 
tremendously).
Does anyone have an idea for making a libxen errno thread safe that 
wouldn't require a context?
> I notice that you store the fd of the priv_cmd in a static variable. I
> guess this is OK, but I think I still prefer a way of explicit way of
> closing the fd. You'd also have to be a little bit careful about someone
>  
Ok.  Not a problem.

> forking then two guys trying to open the fd at the same time.
>  
Yes, I was actually considering the case of two threads when I wrote the 
library.  That's why I exposed hcall_init() as part of the public 
interface.  It allows a threaded app (or a forking() app) to ensure that 
the library is only initialized once.
Regards,
Anthony Liguori

> Ian
>
>
>  


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