Expose 'array_length()' macro in <sys/cdefs.h> or <sys/param.h>

[Alejandro Colomar]

[[ CC += libc-coord at lists.openwall.com ]]

On 2020-09-21 12:33, Florian Weimer wrote:
> * Alejandro Colomar:
> 
>> [[
>> CC += libc-coord at sourceware.org
>> CC += gcc at gcc.gnu.org
>> CC += libstdc++ at gcc.gnu.org
>> ]]
>>
>> Hi Florian,
>>
>> On 2020-09-21 10:38, Florian Weimer wrote:
>>> * Alejandro Colomar via Libc-alpha:
>>>
>>>> I'd like to propose exposing the macro 'array_length()' as defined in
>>>> 'include/array_length.h' to the user.
>>>
>>> It would need a good C++ port, probably one for C++98 and another one
>>> for C++14 or later.
>>
>> For C++, I use the following definition:
>>
>>
>> 	#include <cassert>
>> 	#include <sys/cdefs.h>
>> 	#include <type_traits>
>>
>>
>> 	#define is_array__(a)	(std::is_array <__typeof__(a)>::value)
> 
> Should be decltype.

Thanks.

> 
>> However, there are a few problems:
>>
>> 1) This doesn't work for VLAs (GNU extension).
>>     I couldn't find a way to do it.  Maybe I should file a bug in GCC.
> 
> I do not think VLA support is critical.  C++ programmers will be used to
> limited support in utility functions.
> 
>> 2) Also, this requires C++11; I don't know how to do it for older C++.
>>     Again, support from the compiler would be great.
> 
> I think limited C++98 support is possible using a function template,
> where the array length N is a template parameter.  To enable use in
> constant expressions, you can return a type of char[N], and the macro
> wrapper should then apply sizeof to the function result.

Sorry, I don't know much C++, and I don't know how to do this.

> 
>> 3) The macro can't be used in the same places as the C version,
>>     because of the `({})`.
>>     The `0 * sizeof(struct{...})` trick doesn't work in C++ due to:
>> 	error: types may not be defined in 'sizeof' expressions
> 
> For C++11, you can use a constexpr function instead of a macro.
> 
> array_length should not be a macro in current C++ modes, so that we
> retain compatibility if a future C++ standard adds array_length (or
> nitems) on its own.  This is not a concern for legacy C++98 mode.

See above.

> 
>>> Maybe also ask on the libc-coord list.
>>
>> Ok.  Added CCs.
> 
> libc-coord is not hosted on sourceware:
> 
>    <https://www.openwall.com/lists/libc-coord/2020/01/30/1>
> 
> The discussion here veered off into C++ territory anyway.

I added the correct list now.

> 
> Thanks,
> Florian
> 

Thanks,

Alex.