Re: [Xen-devel] [PATCH XEN v8 02/29] tools: Refactor /dev/xen/evtchn wrappers into libxenevtchn.

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2016-01-25 at 14:35 +0000, Ian Jackson wrote:
> Ian Campbell writes ("Re: [Xen-devel] [PATCH XEN v8 02/29] tools:
> Refactor /dev/xen/evtchn wrappers into libxenevtchn."):
> > Various of the tools/libs/*/include/*.h have a
> > 
> > ÂÂÂÂ/* Callers who don't care don't need to #include <xentoollog.h> */
> > ÂÂÂÂtypedef struct xentoollog_logger xentoollog_logger;
> > 
> > but since that typedef matches in all cases I think it is allowed to be
> > repeated like this, isn't it?
> 
> No, I'm afraid not.

:-/

> ÂÂIt is permitted to repeatedly mention `struct
> xentoollog_logger', but the typedef must only occur once in any
> translation unit.

For some reason I thought the typedef's could be repeated so long as they
were identical.

Seems at least Debian's gcc tolerates such things (which makes me wonder
how many more instances of this might have snuck in, I guess folks like
Boris would have spotted them pretty quick).

> If it is desirable to let callers avoid including xentoollog.h, then
> all those headers need to say:
> 
> Â struct xentoollog_logger;
> Â int some_function(..., struct xentoollog_logger *lg, ...);
> 
> So in your patches something like:
> 
> Â struct xentoollog_logger;
> 
> Â xenevtchn_handle *xenevtchn_open(struct xentoollog_logger *logger,
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂunsigned open_flags);

I'll do something like this for tools/libs/*/include.

FYI the underlying patches are all in since Friday.


> 
> The separate `struct xentoollog_logger;' is needed because otherwise
> the `struct xentoollog_logger *logger' in the formal parameters of
> xenevtchn_open is a declaration, rather than a reference to a
> previously-declared thing, and if it is a declaration its scope is
> only the contained function prototype, so other mentions of `struct
> xentoollog_logger' are treated as references to a different type.
> This is, of course, insane.
> 
> Ian.

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