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Re: [libvirt] [PATCH] libvirt, logging: cleanup VIR_DEBUG0() VIR_INFO0() VIR_WARN0() VIR_ERROR0()



On 05/10/2011 09:29 AM, Eric Blake wrote:
>> ##__VA_ARGS__ is a GNU extension; it does not work on all C99 compilers.
>>
>> I'm worried that this is not portable.
> 
> But, we CAN do:
> 
> #ifdef __GNUC__
> # define high_level(fmt, ...) low_level(fmt, ##__VA_ARGS__)
> #else
> 
> suitably expanded (and we can also place a cap on the maximum number of
> arguments we ever expect to see in VIR_DEBUG, rather than going all the
> way to the C99 limit of 63).

Even simpler, using ideas from
https://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/
(and actually tested this time):

#define ARG_8(_0, _1, _2, _3, _4, _5, _6, _7, ...) _7
#define HAS_COMMA(...) ARG_8(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 0)
# define f(...) f_(f, HAS_COMMA(__VA_ARGS__), __VA_ARGS__)
# define f_(a, b, ...) f__(a, b, __VA_ARGS__)
# define f__(a, b, ...) a##b(__VA_ARGS__)
# define f0(x) printf("%s", x)
# define f1(x, ...) printf(x, __VA_ARGS__)

I'll work up a followup patch along these lines; it doesn't seem too
impossible to guarantee C99 portability for the four macros.

> Yeah, it's gross, but we limit the grossness to one header, and the rest
> of the code is more maintainable as a result.

-- 
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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