Counting the number of arguments in __VA_ARGS__

Counting the number of arguments in __VA_ARGS__

[Florian Weimer]

Is there a way to count the number of variadic macro arguments in C?

I found a way to check if the list is empty: examine 
sizeof(STR((__VA_ARGS__))) where STR is a macro that stringifies its 
argument.  If it is 3, __VA_ARGS__ is empty.  I wonder if there is a 
less hackish approach.

The macro arguments are expressions, but not of a uniform type, so an 
array-based approach does not work.

-- 
Florian Weimer / Red Hat Product Security Team

Re: Counting the number of arguments in __VA_ARGS__

[vijay nag]

On Thu, Dec 12, 2013 at 6:38 PM, Florian Weimer <fweimer@redhat.com> wrote:
> Is there a way to count the number of variadic macro arguments in C?
>
> I found a way to check if the list is empty: examine
> sizeof(STR((__VA_ARGS__))) where STR is a macro that stringifies its
> argument.  If it is 3, __VA_ARGS__ is empty.  I wonder if there is a less
> hackish approach.
>
> The macro arguments are expressions, but not of a uniform type, so an
> array-based approach does not work.
>
> --
> Florian Weimer / Red Hat Product Security Team

#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,[..],_61,_62,_63,N,...) N
#define PP_RSEQ_N() 63,62,61,60,[..],9,8,7,6,5,4,3,2,1,0

PP_NARG(A)  1
PP_NARG(A, B) 2
........

Re: Counting the number of arguments in __VA_ARGS__

[Florian Weimer]

On 12/12/2013 02:21 PM, vijay nag wrote:
> On Thu, Dec 12, 2013 at 6:38 PM, Florian Weimer <fweimer@redhat.com> wrote:
>> Is there a way to count the number of variadic macro arguments in C?
>>
>> I found a way to check if the list is empty: examine
>> sizeof(STR((__VA_ARGS__))) where STR is a macro that stringifies its
>> argument.  If it is 3, __VA_ARGS__ is empty.  I wonder if there is a less
>> hackish approach.
>>
>> The macro arguments are expressions, but not of a uniform type, so an
>> array-based approach does not work.
>>
>> --
>> Florian Weimer / Red Hat Product Security Team
>
> #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,[..],_61,_62,_63,N,...) N
> #define PP_RSEQ_N() 63,62,61,60,[..],9,8,7,6,5,4,3,2,1,0

Hmm.  I think this returns the 64th argument if the argument list is 
longer than 63.  I don't want to silently produce wrong results if some 
arbitrary limit is exceeded.

-- 
Florian Weimer / Red Hat Product Security Team

Re: Counting the number of arguments in __VA_ARGS__

[Alexander Monakov]

On Thu, 12 Dec 2013, Florian Weimer wrote:

> > #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,[..],_61,_62,_63,N,...) N
> > #define PP_RSEQ_N() 63,62,61,60,[..],9,8,7,6,5,4,3,2,1,0
> 
> Hmm.  I think this returns the 64th argument if the argument list is longer
> than 63.  I don't want to silently produce wrong results if some arbitrary
> limit is exceeded.

You can establish an upper bound on the number of arguments using the
following device:

#define PP_STRIP_1(_, ...) __VA_ARGS__
#define PP_STRIP_2(_, ...) PP_STRIP_1(__VA_ARGS__)
#define PP_STRIP_3(_, ...) PP_STRIP_2(__VA_ARGS__)

PP_STRIP_3(a, b, c, d, e)  ->  d, e
PP_STRIP_3(a)  ->  (empty)

So you can check that arg list count does not exceed 63 by examining
sizeof(STR((PP_STRIP_63(__VA_ARGS__))))

One downside to this is that the above simple device relies on a GNU extension
to the C preprocessor and produces a warning with -pedantic. 

Alexander