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Message-ID: <e4986448-ab01-a8cd-96ab-905a0e5ab701@gmail.com> Date: Sun, 11 Oct 2020 18:43:47 +0200 From: Alejandro Colomar <colomar.6.4.3@...il.com> To: libc-alpha@...rceware.org Cc: libc-coord@...ts.openwall.com, 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: Re: [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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