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Message-ID: <2bbe4b2f-46c1-4caa-fbd9-6c8efbc0c7e5@gmail.com>
Date: Tue, 27 Oct 2020 12:38:25 +0100
From: Alejandro Colomar <colomar.6.4.3@...il.com>
To: libc-alpha@...rceware.org, libc-coord@...ts.openwall.com
Cc: libstdc++@....gnu.org, gcc@....gnu.org, linux-kernel@...r.kernel.org,
linux-man@...r.kernel.org, jwakely@...hat.com, fweimer@...hat.com,
ville.voutilainen@...il.com, enh@...gle.com, rusty@...tcorp.com.au
Subject: Ping(2): [PATCH v4] <sys/param.h>: Add nitems()
Ping
On 2020-09-28 21:12, Alejandro Colomar wrote:
> 'nitems()' calculates the length of an array in number of items.
> It is safe: if a pointer is passed to the macro (or function, in C++),
> the compilation is broken due to:
> - In >= C11: _Static_assert()
> - In C89, C99: Negative anonymous bitfield
> - In C++: The template requires an array
>
> Some BSDs already provide a macro nitems() in <sys/param.h>,
> although it usually doesn't provide safety against pointers.
>
> This patch uses the same name for compatibility reasons,
> and to be the least disruptive with existing code.
>
> This patch also adds some other macros, which are required by 'nitems()':
>
> __is_same_type(a, b):
> Returns non-zero if the two input arguments are of the same type.
>
> __is_array(arr):
> Returns non-zero if the input argument is of an array type.
>
> __must_be(expr, msg):
> Allows using _Static_assert() everywhere an expression can be used.
> It evaluates '(int)0' or breaks the compilation.
>
> __must_be_array(arr):
> It evaluates to '(int)0' if the argument is of an array type.
> Else, it breaks compilation.
>
> __nitems(arr):
> It implements the basic sizeof division needed to calculate the array length.
>
>
> P.S.: I'd like to put this patch in the public domain.
>
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@...il.com>
> ---
>
> A few changes since v3:
>
> - Macros don't need reserved names in their parameters,
> so I simplified those names.
>
> - I fixed some wrong indentation levels.
>
> - Renamed __array_len() to __nitems() for consistency.
>
>
> misc/sys/param.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 47 insertions(+)
>
> diff --git a/misc/sys/param.h b/misc/sys/param.h
> index d7c319b157..08d4093961 100644
> --- a/misc/sys/param.h
> +++ b/misc/sys/param.h
> @@ -102,5 +102,52 @@
> #define MIN(a,b) (((a)<(b))?(a):(b))
> #define MAX(a,b) (((a)>(b))?(a):(b))
>
> +/* Macros related to the types of variables */
> +#define __is_same_type(a, b) \
> + __builtin_types_compatible_p(__typeof__(a), __typeof__(b))
> +#define __is_array(arr) (!__is_same_type((arr), &(arr)[0]))
> +
> +/* Macros for embedding _Static_assert() in expressions */
> +#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!")
> +
> +/* Macros for array sizes */
> +#if defined(__cplusplus)
> +# if __cplusplus >= 201103L
> +template<typename _Tp, std::size_t _Len>
> + constexpr inline std::size_t
> + nitems(const _Tp(&)[_Len]) __THROW
> + {
> + return _Len;
> + }
> +# else /* __cplusplus < 201103L */
> +template<typename _Tp, std::size_t _Len>
> + char
> + (&__nitems_chararr(const _Tp(&)[_Len]))[_Len];
> +# define nitems(arr) (sizeof(__nitems_chararr(arr)))
> +# endif /* __cplusplus < 201103L */
> +#else /* !defined(__cplusplus) */
> +# define __nitems(arr) (sizeof((arr)) / sizeof((arr)[0]))
> +# define nitems(arr) (__nitems(arr) + __must_be_array(arr))
> +#endif /* !defined(__cplusplus) */
> +
>
> #endif /* sys/param.h */
>
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