alx-0029r6 - Restore the traditional realloc(3) specification

[Alejandro Colomar]

Hi H. Peter,

On Sun, Jun 29, 2025 at 09:25:13AM -0700, H. Peter Anvin wrote:
> On 2025-06-27 07:01, Alejandro Colomar wrote:
> > Hi!
> > 
> > Here's a new revision of the proposal, addressing some points raised by
> > Mark, plus clarifying that the paragraph about when size is zero refers
> > to the total size, as Florian was concerned that it might not be
> > symmetric.
> 
> I don't know if it would be useful, but proposing a new interface of the
> form:
> 
> 	reallocp(&ptr, size)
> 
> ... to separate the status return from the pointer might be a really good
> idea.  This would hopefully eliminate users doing the "obvious":
> 
> 	ptr = realloc(ptr, size)

This is resolved with the reallocf(3) wrapper that FreeBSD provides,
which I pretend to standardize *after* fixing realloc(3).  This is
provided in Linux (and other POSIX) systems by the libbsd library,
present in many distros.

reallocf(3) is a realloc(3) wrapper that unconditionally frees the
input pointer, even on error.  So, it is designed to be used exactly as

	p = reallocf(p, size);
	if (p == NULL)
		goto err;

As simple as it gets.

> Not that this resolves anything in the short or even medium term, but
> perhaps can help avoid problems in the more distant time frame.  It is also
> an interface which is mostly trivial to infill.

No, this would make static analysis a lot harder.  Currently, realloc(3)
is specified as ending the lifetime of the input object, and creating a
new one, and a static analyzer could easily understand that with
attributes like the existing [[gnu::malloc(free)]] --although for some
reason, there's no similar existing attribute for realloc(3)--.
However, if the new object is created in the parameter, you'll have a
very hard time tracking that.


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es/>
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