Messages in this thread | | | Subject | Re: Expose 'array_length()' macro in <sys/param.h> | From | Alejandro Colomar <> | Date | Tue, 22 Sep 2020 11:10:08 +0200 |
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[[ 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
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#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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