On Wed, Aug 22, 2007 at 03:42:49PM +0200, Matthias Hopf wrote: > On Aug 20, 07 19:51:31 +0300, Daniel Stone wrote: > > On Mon, Aug 20, 2007 at 05:27:43PM +0200, Matthias Hopf wrote: > > > On Aug 12, 07 17:50:12 +0200, [EMAIL PROTECTED] wrote: > > > Please be sure that if you design this using ioctls the design should be > > > forward and backward compatible. > > > > > > AFAICS the only way to ensure this is some sort of tag-based parameter > > > space (parameters are a list of tag,value pairs, terminated by a special > > > tag). Otherwise you're stuck with one parameter list, which you cannot > > > enhance easily, and version control both in user and kernel space will > > > be a nightmare. With tags you can just ignore the tags you don't know > > > about. > > > > Well, either that, or you can have a version field as the very first > > member (think: X11 requests), or simply bump the ioctl number every time > > you change the API (think: shared libraries). > > But that exactly makes version control difficult (lots of cases, > especially if the exact requirements aren't clear from day one).
I honestly don't understand the problem. Your ioctl number is _that_ request. It's not referring to a vague concept, but a single structure you've already specified. If you have ioctl number 32, your next free one is 43, and you want to change the semantics of UPDATE_WINDOW, then you rename 32 to UPDATE_WINDOW_OLD, create UPDATE_WINDOW as 43 with the new structure, and handle both cases. This is what we've done for ioctl BC in our kernel tree, and it's worked fine. > Without a version field, this is made impossible, something that must be > avoided at all costs. Except that your ioctl then becomes variably-sized unless you add loads of padding and hope for the best. Cheers, Daniel
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