Re: [Libvir] PATCH: 5/10: public auth callback API

Tue, 04 Dec 2007 07:54:00 -0800 

Daniel Veillard wrote:

On Tue, Dec 04, 2007 at 12:57:39PM +0000, Richard W.M. Jones wrote:

Daniel P. Berrange wrote:

virConnectPtr           virConnectOpenAuth      (const char *name,
                                                 virConnectAuthPtr auth,
                                                 int flags);
I'm a fan of callers passing in the size of the structure (as they see it). Allows the structure to be expanded in future, and if done right can allow both forwards and backwards compatibility. 

cf: http://www.libvirt.org/html/libvirt-libvirt.html#virDomainInterfaceStats


  Hum, honnestly, that's not my preferred way. If you really think there
should be room for expansion, I would either:
   - add a version number to the structure and allocator/destructor
     functions as part of the API (prefered)



Hmmm ... what if they forget to set it?  Now we need an init function 
for the structure.



   - add padding at tyhe end of the structure which could allow 
     a future growth



This incurs an unnecessary memory penalty always, and limits us to a 
particular maximum size which we must choose now when we don't know what 
extra fields we might add.



adding the size of the structure as the argument moves the complexity away

from the library implementor to the library user, 



Well, it means they must append "sizeof (*auth)" to their list of 
arguments, which is hardly a lot of complexity.  There _is_ much more 
complexity in the library however -- cf the implementation of 
virDomainBlockStats & virDomainInterfaceStats for details.



Remember also the need for forwards compatibility: program which was 
compiled against an old version of the library, dynamically links to a 
new version of the library.  If the structure has increased in size in 
the meantime, then having the caller pass the size means that the 
library can do the right thing and only fill in the first fields.


Rich.

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