Re: [PATCH 1/9] xdiff: use ssize_t for dstart/dend, make them last in xdfile_t

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 21, 2025, 17:18 UTC
Message-ID: <xmqqecqww4u7.fsf@gitster.g>
In-Reply-To: <9eafee4d-ea94-4382-ada0-58000d229d2e@gmail.com>

Phillip Wood <phillip.wood123@gmail.com> writes:

Show 15 quoted lines

> On 15/10/2025 22:18, Ezekiel Newren via GitGitGadget wrote:
>> From: Ezekiel Newren <ezekielnewren@gmail.com>
>> 
>> ssize_t is appropriate for dstart and dend because they both describe
>> positive or negative offsets relative to a pointer.
>
> Isn't ptrdiff_t the appropriate type for an offset to a pointer? ssize_t 
> is not guaranteed to be the same width as size_t (this has caused 
> problems in the past[1]) and is only defined by POSIX, not the C standard.
>
> Thanks
>
> Phillip
>
> [1] https://lore.kernel.org/git/loom.20150207T174514-727@post.gmane.org/

Thanks for bringing up a very good point.

We often consider that a function that yields what we would normally put in a size_t variable, when we _know_ that the return value would not be so big to exceed half the range of size_t, can instead return ssize_t and use the negative half of the range to signal error conditions, but as the cited incident shows that it is an easy mistake to make.