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

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Message-ID: <20200925173952.GN6061@redhat.com>
Date:   Fri, 25 Sep 2020 18:39:52 +0100
From:   Jonathan Wakely <jwakely@...hat.com>
To:     Alejandro Colomar <colomar.6.4.3@...il.com>
Cc:     fweimer@...hat.com, linux-man@...r.kernel.org,
        libc-alpha@...rceware.org, gcc@....gnu.org, rusty@...tcorp.com.au,
        linux-kernel@...r.kernel.org, libstdc++@....gnu.org,
        libc-coord@...ts.openwall.com, enh@...gle.com
Subject: Re: [PATCH v2] <sys/param.h>: Add nitems() and snitems() macros

On 25/09/20 18:30 +0200, Alejandro Colomar via Libstdc++ wrote:
>Hello Jonathan,
>
>On 2020-09-25 16:48, Jonathan Wakely wrote:
>> Do you really need to provide snitems?
>>
>> Users can use (ptrdiff_t)nitems if needed, can't they?
>
>They can, but that adds casts in the code,
>which makes longer lines that are somewhat harder to read.
>To avoid that, users may sometimes omit the cast with possible UB.
>BTW, I use
>
>IMO, array indices should be declared as 'ptrdiff_t' always,
>and not 'size_t'.  More generically, I use unsigned integer types for two
>reasons:  bitwise operations, and library functions that require me to 
>do so.
>
>I don't intend to force anyone with my opinion, of course,
>but if I were to choose a type for 'nitems()', it would be 'ptrdiff_t'.
>
>However, for legacy reasons people will expect that macro to be unsigned,
>so I'd have 'nitems()' unsigned, and then a signed version prefixed 
>with an 's'.
>
>Some very interesting links about this topic:
>
>Bjarne Stroustrup (and others) about signed and unsigned integers:
>https://www.youtube.com/watch?v=Puio5dly9N8&t=12m56s
>https://www.youtube.com/watch?v=Puio5dly9N8&t=42m41s
>
>The two links above are two interesting moments of the same video.
>
>I guess that might be the reason they added std::ssize, BTW.

Yes, I'm aware of all the rationale. I already said that it makes
sense in C++ where you have generic code. I am not convinced that it's
necessary to add to <sys/param.h> when all it does is a cast from
size_t to ptrdiff_t.