On 05/10/2011 08:52 AM, Eric Blake wrote:
> On 05/09/2011 03:24 AM, Lai Jiangshan wrote:
>> These VIR_XXXX0 APIs make us confused, use the non-0-suffix APIs instead.
>>
>> How these coversions works? The magic is using ##.
>> #define high_levle_api(fmt, ...) low_levle_api(fmt, ##__VA_ARGS__)
>> When __VA_ARGS__ is empty, "##" will swallow the "," in "fmt," to avoid compile error.
>
> ##__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
# define PP_NARG(...) \
PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
# define PP_NARG_(...) \
PP_ARG_N(__VA_ARGS__)
# define PP_ARG_N( \
_1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
_11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
_21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
_31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
_41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
_51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
_61,_62,_63,N,...) N
# define PP_RSEQ_N() \
63,62,61,60, \
59,58,57,56,55,54,53,52,51,50, \
49,48,47,46,45,44,43,42,41,40, \
39,38,37,36,35,34,33,32,31,30, \
29,28,27,26,25,24,23,22,21,20, \
19,18,17,16,15,14,13,12,11,10, \
9,8,7,6,5,4,3,2,1,0
# define VIR_DEBUG(...) VIR_DEBUG ## PP_NARG(__VA_ARGS__) (__VA_ARGS__)
# define VIR_DEBUG1(xxx) xxx
# define VIR_DEBUG2(xxx, xxx) xxx
...
# define VIR_DEBUG63(xxx, ..., xxx) xxx
#endif
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).
Thanks to:
https://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5
for the idea.
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@xxxxxxxxxx +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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