Re: [Qemu-devel] QAPI and empty structs

[Eric Blake]

On 12/23/2014 08:50 AM, Peter Maydell wrote:
> In qapi-schema.json we have a couple of entries that define
> empty structures, like this:
> 
> { 'type': 'ChardevDummy', 'data': { } }
> 
> In the generated qapi-types.h these are turned into empty C structs:
> 

> Although you could argue that we don't care about differences
> in C and C++ semantics, having a zero-sized struct floating
> around seems a bit risky to me, since for instance a naive
> attempt to g_malloc() space for it will return NULL.

Oh, good point.

> 
> How should we handle these? Should the qapi-types generator
> stick a dummy field in so they aren't zero sized?

Sounds like the best plan to me.

> Are they
> actually an error in the schema? (what's the point of them?)

No, they are valid - it is how you express a union between a branch that
needs no additional members compared to any other branch.

> Do we claim that actually all our code correctly handles
> zero sized structs and just suppress the clang warning?

I'd be fine with adding a dummy member.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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