linux-kernel - Re: Expose 'array_length()' macro in <sys/param.h>

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Message-ID: <ebe90ee6-b227-d1cf-c1a4-6b03b0037049@gmail.com>
Date:   Tue, 22 Sep 2020 12:35:21 +0200
From:   Alejandro Colomar <colomar.6.4.3@...il.com>
To:     Florian Weimer <fweimer@...hat.com>,
        Jonathan Wakely <jwakely@...hat.com>
Cc:     Ville Voutilainen <ville.voutilainen@...il.com>, gcc@....gnu.org,
        libstdc++ <libstdc++@....gnu.org>,
        Libc-alpha <libc-alpha@...rceware.org>,
        LKML <linux-kernel@...r.kernel.org>,
        libc-coord@...ts.openwall.com
Subject: Re: Expose 'array_length()' macro in <sys/param.h>

Thanks again for your improvements.

I think this might be ready for a patch already.
Any more thoughts?

Thanks,

Alex

------------------------


#if defined(__cplusplus)
# include <cstddef>

# if __cplusplus >= 201103L
template<typename _Tp, std::size_t _Len>
constexpr inline std::size_t
__array_length(const _Tp(&)[_Len]) __THROW
{
	return	_Len;
}

template<typename _Tp, std::size_t _Len>
constexpr inline std::ptrdiff_t
__array_slength(const _Tp(&)[_Len]) __THROW
{
	return	_Len;
}
# else /* __cplusplus < 201103L */
template<typename _Tp, std::size_t _Len>
char (&__array_length(const _Tp(&)[_Len]))[_Len];
#  define __array_length(_Arr)	(sizeof(__array_length(_Arr)))
#  define __array_slength(_Arr)						\
	(static_cast<std::ptrdiff_t>(__array_length(_Arr)))
# endif /* __cplusplus >= 201103L */


#else /* !defined(__cplusplus) */
#include <stddef.h>

# define __is_same_type(_A, _B)						\
	__builtin_types_compatible_p(__typeof__(_A), __typeof__(_B))
# define __is_array(_Arr)	(!__is_same_type((_Arr), &(_Arr)[0]))

# if __STDC_VERSION__ >= 201112L
#  define __must_be(_Expr, _Msg)	(				\
	0 * (int)sizeof(						\
		struct {						\
			_Static_assert((_Expr), _Msg);			\
			char _ISO_C_forbids_a_struct_with_no_members;	\
		}							\
	)								\
)
# else
#  define __must_be(_Expr, _Msg)	(				\
	0 * (int)sizeof(						\
		struct {						\
			int	: (-!(_Expr));				\
			char _ISO_C_forbids_a_struct_with_no_members;	\
		}							\
	)								\
)
# endif

# define __must_be_array(_Arr)	__must_be(__is_array(_Arr), "Must be an 
array!")

# define __array_len(_Arr)	(sizeof(_Arr) / sizeof((_Arr)[0]))
# define __array_length(_Arr)	(__array_len(_Arr) + __must_be_array(_Arr))
# define __array_slength(_Arr)	((ptrdiff_t)__array_length(_Arr))
#endif /* defined(__cplusplus) */


static int a[5];
static int v[__array_slength(a)];
static int w[__array_length(v)];
static int *p;

int main(void)
{
	int aa[5];
	const int xx = 6;
	int vv[xx];
	int yy = 7;
	int ww[yy];
	int *pp;

	(void)p;
	(void)pp;
	(void)ww;

	return	__array_slength(a)
		+ __array_length(v)
		+ __array_slength(w)
/*		+ __array_length(p) */ /* Always breaks :) */
		+ __array_length(aa)
		+ __array_slength(vv)
		+ __array_length(ww) /* Not in C++ */
/*		+ __array_length(pp) */ /* Always breaks :) */
		;
}