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Message-ID: <5qn7xueobgqgqvuuaa7s4k64slj6lzpswwf5suupopf6m7hrhw@lkkmqe5mpia6>
Date: Fri, 31 Oct 2025 16:14:44 +0100
From: Alejandro Colomar <alx@...nel.org>
To: libc-alpha@...rceware.org, musl@...ts.openwall.com
Cc: "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>,
Thiago Macieira <thiago@...ieira.org>, Alejandro Colomar <alx@...nel.org>
Subject: Re: Re: realloci(): A realloc() variant that works in-place
Hi Thorsten,
On Fri, Oct 31, 2025 at 03:36:17PM +0100, Thorsten Glaser wrote:
> (no need to Cc me)
>
> On Fri, 31 Oct 2025, Laurent Bercot wrote:
>
> > As usual, and this applies to realloci() too, the question is: what
> > exact problem are you trying to solve?
>
> Yes. This.
I don't know much C++; I'll let C++ committee members speak about it.
In C, I've sometimes seen programmers trying to check if realloc(3)
moved or not, to skip some work. That's a micro-optimization that I've
never written myself, so I won't defend it. But for some reason, some
programmers keep wanting to do it.
Have a lovely day!
Alex
> > > > How is that supposed to work if you want to grow the
> > > > allocation?
> > > >
> > > > This seems like increasing burden on the implementation
> > > > for everyone, just for niche corner use cases.
> > >
> > > realloc(3) already does this sometimes. If the memory has some empty
>
> > In most cases, it will be very difficult for an implementation to
> > increase the size of an allocated block without relocating the block.
> > So you can expect realloci() to fail often. What should users do in
> > that case?
>
> Exactly. This makes me doubt its usefulness as a generic function.
>
> Even were you to limit its use to shrinking allocations only, that
> would constrain implementations, e.g. those that spread the allocations
> based on their size and thus could not free anything.
>
> So, in short:
>
> When called with a larger size, it’ll just fail most of the time.
>
> When called with a smaller size, it’ll just be a no-op in many cases.
>
> bye,
> //mirabilos
> --
> 22:20⎜<asarch> The crazy that persists in his craziness becomes a master
> 22:21⎜<asarch> And the distance between the craziness and geniality is
> only measured by the success 18:35⎜<asarch> "Psychotics are consistently
> inconsistent. The essence of sanity is to be inconsistently inconsistent
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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