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Message-ID: <CANv4PNnd6maoR6yMZyRZ7CWAqdKYXQN6QPQyEdu7Szmu+PeZVg@mail.gmail.com>
Date: Fri, 31 Oct 2025 19:46:23 -0400
From: Morten Welinder <mwelinder@...il.com>
To: musl@...ts.openwall.com
Cc: Alejandro Colomar <alx@...nel.org>, Thorsten Glaser <tg@...bsd.de>, libc-alpha@...rceware.org,
"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>
Subject: Re: Re: realloci(): A realloc() variant that works in-place
> The conclusion among C++ developers is that using the previous pointer in any
> way is UB. Therefore, you simply cannot know if the area was moved or not.
With a bit of effort you can travel from UB to implementation-defined.
The trick is to
use the pointer before the realloc. Something like
sprintf(buffer1, "%p", (void *)p);
q = realloc(p, newsize);
sprintf(buffer2, "%p", (void *)q);
int moved = strcmp(buffer1, buffer2) != 0;
Clearly not UB. I just can't convince myself that the standards
actually guarantee that a changed pointer doesn't have the same %p
representation.
I don't know of any platforms where a collision would actually happen, though.
M.
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