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Date: Tue, 22 Sep 2020 10:40:20 +0100
From: Jonathan Wakely <jwakely@...hat.com>
To: Alejandro Colomar <colomar.6.4.3@...il.com>
Cc: Ville Voutilainen <ville.voutilainen@...il.com>,
	Florian Weimer <fweimer@...hat.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>

On 22/09/20 11:10 +0200, Alejandro Colomar via Libstdc++ wrote:
>[[ CC += LKML ]]
>
>Thanks for all your input.  I learned some C++ :)
>
>The following code works for all C and C++ standards:
>g++ --std={c++98, c++03, c++11, c++14, c++17, c++20}
>gcc --std={c89, c99, c11, c18, c2x}
>With `-Wall -Wextra -Werror -pedantic -Wno-vla -Wno-sizeof-pointer-div`.
>It doesn't compile when '+ __array_length(p)' is uncommented.
>It compiles, and returns the correct value (18).
> With some exceptions:
>c++ doesn't accept the VLA (w[]):
>
>array_length.c: In function 'int main()':
>array_length.c:101:22: error: no matching function for call to 
>'__array_slength(int [y])'
>  101 |   + __array_slength(w)
>      |                      ^
>array_length.c:38:1: note: candidate: 'template<class T, long int N> 
>std::ptrdiff_t __array_slength(const T (&)[N])'
>   38 | __array_slength(const T(&)[N])
>      | ^~~~~~~~~~~~~~~
>array_length.c:38:1: note:   template argument deduction/substitution 
>failed:
>array_length.c:101:22: note:   variable-sized array type 'long int' is 
>not a valid template argument
>  101 |   + __array_slength(w)
>      |                      ^
>
>But we can live with limited support for VLAs in C++.
>So I needed to comment '+ __array_slength(w)',
>and then the program correctly returns 11.
>
>As Ville suggested, I renamed the function/macro to __array_[s]length().
>However, (some) BSDs already provide nitems() in <sys/param.h>,
>so it probably wouldn't be very drastic to
>provide this function/macro with the name '[s]nitems()' there.
>
>Would you like to add anything else before I write the patch?
>
>
>BTW, I should note one more thing:
>
>Linux has a macro named '__must_be_array()' with the same API,
>but slightly different internal implementation,
>so they should be aware of this change.
>However, I don't think they include <sys/param.h> a lot,
>so maybe it doesn't break anything; but they should be aware anyway.
>I CC'd the LKML so they are aware and can give any suggestions.
>
>Thanks,
>
>Alex
>
>
>------------------------------------------------------------------------
>
>#if defined(__cplusplus)
>
># include <cstddef>
># if __cplusplus >= 201703L
>#  include <iterator>
># endif
>
>template<typename T, std::size_t N>

You need to use reserved names here.

># if __cplusplus >= 201103L
>constexpr
># endif
>inline std::size_t
># if __cplusplus >= 201703L
>__array_length(const T(&arr)[N])
># else
>__array_length(const T(&)[N])
># endif
># if __cplusplus >= 201103L
>noexcept

If this is going to be part of Glibc then you can use its __THROW
macro instead.


># endif
>{
># if __cplusplus >= 201703L
>	return	std::size(arr);
># else
>	return	N;
># endif
>}

I don't see much point in using std::size here. If you're going to
provide the alternative implementation for when std::size isn't
defined, why not just use it always?

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

This only requires <cstddef>, not <iterator>.

>
>template<typename T, std::ptrdiff_t N>

This declaration is wrong, the array extent has type std::size_t. The
type you return from the function doesn't change that.

The __array_slength definition should be identical to __array_length
except for its name and return type.

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


>
>
>#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(e, msg)	(					\
>	0 * (int)sizeof(						\
>		struct {						\
>			_Static_assert((e), msg);			\
>			char ISO_C_forbids_a_struct_with_no_members__;	\
>		}							\
>	)								\
>)
># else
>#  define __must_be(e, msg)	(					\
>	0 * (int)sizeof(						\
>		struct {						\
>			int	: (-!(e));				\
>			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
>
>
>int main(void)
>{
>	int a[5];
>	const int x = 6;
>	int v[x];
>	int y = 7;
>	int w[y];
>	int *p;
>	(void)p;
>	(void)v;
>	(void)w;
>
>	return	__array_slength(a)
>		+ __array_slength(v)
>		+ __array_slength(w)
>//		+ __array_length(p)
>		;
>}
>

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