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

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Jan 7, 2022, 00:31 UTC
Message-ID: <YdeJ8Ub2Q0/c9kO2@camp.crustytoothpaste.net>
In-Reply-To: <xmqqfsq09ziq.fsf@gitster.g>

On 2022-01-07 at 00:10:21, Junio C Hamano wrote:

> Jessica Clarke <jrtc27@jrtc27.com> writes:
> 
> > This is also true of uint128_t, it doesn’t fit in a uintmax_t either.

I don't have this type from either GCC or Clang on my Debian amd64/sid system. I know those compilers support 128-bit values because Rust uses them, but they are not exposed in standard C and therefore those compilers appear to be compliant when run in standards mode.

Show 11 quoted lines

> 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.

Yes, this is the case.  The C standard mandates this behavior.

Now, as far as I'm aware, the standard does not mandate that that uintmax_t have the strictest alignment requirements of any integral type. It could be that other types have stricter requirements, for example, on some systems. But it is required that conversion from void * to uintptr_t to uintmax_t to uintptr_t to void * preserve the functionality of the pointer.

-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA