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Message-ID: <87seex52qz.fsf@mid.deneb.enyo.de>
Date: Sat, 01 Nov 2025 13:57:56 +0100
From: Florian Weimer <fw@...eb.enyo.de>
To: Paul Eggert <eggert@...ucla.edu>
Cc: Alejandro Colomar <alx@...nel.org>, libc-alpha@...rceware.org,
musl@...ts.openwall.com, "A. Wilcox" <AWilcox@...cox-tech.com>,
Lénárd
Szolnoki <cpp@...ardszolnoki.com>, Collin Funk <collin.funk1@...il.com>,
Arthur O'Dwyer <arthur.j.odwyer@...il.com>, Jonathan Wakely
<jwakely@...hat.com>, "Paul E. McKenney" <paulmck@...nel.org>, Thiago
Macieira <thiago@...ieira.org>
Subject: Re: Re: realloci(): A realloc() variant that works in-place
* Paul Eggert:
> On 10/31/25 10:22, Alejandro Colomar wrote:
>> Paul, for context, this is a discussion for adding a function
>>
>> int realloci(void *p, size_t n);
>>
>> that changes the size of a memory block without moving it. (And thus,
>> fails rather often, for some implementations of allocators.)
>
> Reading the threads leading into this, the motivation for this seems to
> be C++ and similar memory allocators that want a cheap way to grow an
> object - if the object doesn't move they can skip some reinitialization
> work, otherwise they have more work to do.
>
> With that in mind, the proposed API is not the best way to go about the
> problem. What these users want is a function that acts just like
> R=realloc(P,N) EXCEPT that it lets you compare R==P, and if the two
> values are the same pointer you know the object did not move and you can
> skip some work. This is simpler than realloci because it means that you
> need only one call (not two) in the common case when realloci returns
> the null pointer.
This would not help C++ because C++ doesn't have this kind of in-place
adjust-all-pointers operation. There's just copy and move, and both
need non-overlapping old and new storage at the same time.
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