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Message-ID: <20251103002857.GW1827@brightrain.aerifal.cx>
Date: Sun, 2 Nov 2025 19:28:57 -0500
From: Rich Felker <dalias@...c.org>
To: Alejandro Colomar <alx@...nel.org>
Cc: Thiago Macieira <thiago@...ieira.org>,
Florian Weimer <fw@...eb.enyo.de>, libc-alpha@...rceware.org,
musl@...ts.openwall.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
On Mon, Nov 03, 2025 at 12:58:39AM +0100, Alejandro Colomar wrote:
> > All this will need fine-tuning once implementations exist.
> >
> > > So, why not require the caller to not ask too much? We could go back to
> > > reporting an error if there's not enough memory.
> > >
> > > Of course, it would still guarantee no errors when shrinking, but
> > > I think we could error out when growing.
> >
> > I'd prefer no errors either way. If there isn't memory to grow the underlying
> > space (a brk() system call returns ENOMEM), then realloci() returns as much as
> > it could get but not more.
>
> The problem is that this is asking the implementation to speculate.
>
> Consider the case that a realloci() implementation knows that the
> requested size fails. Let's put some arbitrary numbers:
>
> old_size = 10000;
> requested_size = 30000;
>
> It knows the block can grow to somewhere between 10000 (which it
> currently has) and 30000 (the system reported ENOMEM), but now it has
> the task of allocating as much as it can get. Should it do a binary
> search of the size? Try 20000, then if it fails try 15000, etc.?
> That's speculation, and it would make this function too slow.
I don't see any plausible implementation in which this involved a
binary search. Either you have fixed-size slots in which case you just
look at the size of the slot to see what the max obtainable is, or you
have a dlmalloc-like situation where you check the size of the
adjacent free block (if any) to determine the max obtainable. These
are O(1) operations.
Rich
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