Re: [PATCH v2] <sys/param.h>: Add nitems() and snitems() macros

[Jonathan Wakely]

On 25/09/20 16:10 +0200, Alejandro Colomar wrote:
> On 2020-09-25 15:20, 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
> >
> > 'snitems()' is equivalent to nitems(),
> > but it returns a 'ptrdiff_t' instead of a 'size_t'.
> > It is useful for comparison with signed integer values.
> >
> > 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.
> >
> > __array_len(_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@xxxxxxxxx>
> > ---
>
> I patched my own system's <sys/param.h> with this,
> and while 'nitems()' works fine,
> I had to include <stddef.h> in my main.c to be able to use 'snitems()',
> because I didn't have 'ptrdiff_t',
> eventhough <sys/param.h> already includes <stddef.h>.
>
> I completely ignore the mechanisms behind system headers including
> other system headers.
>
> Moreover, I didn't find 'ptrdiff_t' defined in any of my systems headers
> I used 'user@debian:/usr/include$ grep -rn ptrdiff_t'.  Does GCC do magic?
>
> What's the problem with that?  How should I fix the patch?

Do you really need to provide snitems?

Users can use (ptrdiff_t)nitems if needed, can't they?

C++ provides std::ssize because there are reasons to want it in
generic contexts when using the function on arbitrary container-like
objects. But for an array size you know that ptrdiff_t is wide enough
to represent the size of any array.

Do you have a use case that requries snitems, or can you assume YAGNI?