Re: [PATCH] Properly align memory allocations and temporary buffers

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 7, 2022, 00:10 UTC
Message-ID: <xmqqfsq09ziq.fsf@gitster.g>
In-Reply-To: <97D2AB55-0F8F-4C38-A5C6-1AAA43EA064A@jrtc27.com>

Jessica Clarke <jrtc27@jrtc27.com> writes:

> This is also true of uint128_t, it doesn’t fit in a uintmax_t either.

uintmax_t is supposed to be an unsigned integer type capable of representing any value of any unsigned integer type, so if you have 128-bit unsigned integer, your uintmax_t should be at last that wide, or your uintmax_t is not uintmax_t as far as C standard is concerned, no?

uintptr_t is an unsigned integer type that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer. So the value of uintptr_t cannot be represented by uintmax_t, there is something wrong.

> uintmax_t was a mistake as it becomes part of the ABI and can never be
> revised even when new integer types come along. uintmax_t can hold any
> valid address, but will strip the metadata.

It is a flaw in the implementation of uintmax_t on the architecture that needs "the metadata", no? If the implementation supports a notion of uintptr_t (i.e. there exists an unsigned integer type that can safely go back and forth from pointer to void), an unsigned integer type that is at least as wide as any unsigned integer type should certainly be able to hold what would fit in uintptr_t, no?

Puzzled.