Date:   Tue, 22 Sep 2020 11:10:08 +0200
From:   Alejandro Colomar <colomar.6.4.3@...il.com>
To:     Ville Voutilainen <ville.voutilainen@...il.com>,
        Jonathan Wakely <jwakely@...hat.com>,
        Florian Weimer <fweimer@...hat.com>
Cc:     gcc@....gnu.org, libstdc++ <libstdc++@....gnu.org>,
        Libc-alpha <libc-alpha@...rceware.org>,
        libc-coord@...ts.openwall.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Expose 'array_length()' macro in <sys/param.h>

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

template<typename T, std::ptrdiff_t N>
# if __cplusplus >= 201103L
constexpr
# endif
inline std::ptrdiff_t
# if __cplusplus >= 202002L
__array_slength(const T(&arr)[N])
# else
__array_slength(const T(&)[N])
# endif
# if __cplusplus >= 201103L
noexcept
# endif
{
# if __cplusplus >= 202002L
	return	std::ssize(arr);
# else
	return	N;
# endif
}


#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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