On Tue, Feb 12, 2013 at 05:56:00PM +0100, Markus Armbruster wrote:
> Gerd Hoffmann <kra...@redhat.com> writes:
>
> > Hi,
> >
> >> But why nested discriminators?
> >>
> >> regular files: type=file
> >> serial : type=port, data.type=serial
> >> parallel : type=port, data.type=parallel
> >>
> >> Simpler, and closer to existing -chardev:
> >>
> >> regular files: type=file
> >> serial : type=serial
> >> parallel : type=parallel
> >
> > Matter of taste IMHO.
> > I can live with that too.
> > Feel free to send patches.
> >
> >> I also dislike the pointless '"data" : {}' required by type pty and
> >> null, but I can't figure out how to express 'void' in the schema.
> >
> > Someone mentioned it is possible to leave out empty data altogether.
> > Didn't try whenever our marshaller actually accepts that though.
>
> Looks like it doesn't :(
>
> Empty objects work fine here:
>
> { 'type': 'ChardevDummy', 'data': { } }
>
> Generates the obvious
>
> struct ChardevDummy
> {
> };
>
> They don't work here:
>
> { 'union': 'ChardevBackend', 'data': { 'file' : 'ChardevFile',
> 'hostdev': 'ChardevHostdev',
> 'socket' : 'ChardevSocket',
> 'pty' : 'ChardevDummy',
> 'null' : {} } }
>
> Generates
>
> struct ChardevBackend
> {
> ChardevBackendKind kind;
> union {
> void *data;
> ChardevFile * file;
> ChardevHostdev * hostdev;
> ChardevSocket * socket;
> ChardevDummy * pty;
> void null;
> };
> };
>
> which is kind of cute, but the compiler doesn't like it.
>
> Anthony, Mike, is this a bug?
Not exactly, but it's a relic that doesn't seem to be needed anymore.
The code that does this is in scripts/qapi.py:
def c_type(name):
...
elif name == None or len(name) == 0:
return 'void'
...
else:
return '%s *' % name
The 'name' param being the value/type of a particular param/key in a
QAPI dictionary that defines a schema-defined type.
The reason '{}' maps to 'void' is so that in qapi-commands.py, where we generate
stuff like the function signatures for qmp commands, we'd map something like:
{ 'command': 'my_cmd',
'data': { 'param1': 'Foo' },
'returns': {} }
to:
void my_cmd(Foo *param1);
At some point in development we rightly decided that:
{ 'command': 'my_cmd',
'data': { 'param1': 'Foo' } }
was more efficient, which triggers the 'name == None' case and has the same
effect.
So, as long as we stay consistent about defining commands in this fashion, we
can map 'param_name': {} to something like 'struct {}', and use it in place of
schema-defined dummy types.
Though, as I mentioned on IRC, I think just defining a:
{ 'type': 'Dummy', 'data' {} }
Somewhere in the schema and re-using that might actually be cleaner and have
the same affect.
