Date: Fri, 13 Mar 2026 06:58:00 -0700 From: Cy Schubert <Cy.Schubert@cschubert.com> To: Robert Clausecker <fuz@fuz.su> Cc: Aymeric Wibo <obiwac@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 4da237aee328 - main - alloca.3: Add entry about defining VLAs in same block as alloca() to BUGS Message-ID: <20260313135800.7355B1A5@slippy.cwsent.com> In-Reply-To: <abQNdjoZGXJ5caXo@fuz.su> References: <69b40cd1.27eba.de1cc3f@gitrepo.freebsd.org> <abQNdjoZGXJ5caXo@fuz.su>
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In message <abQNdjoZGXJ5caXo@fuz.su>, Robert Clausecker writes:
> Hi Aymeric,
>
> Am Fri, Mar 13, 2026 at 01:10:41PM +0000 schrieb Aymeric Wibo:
> > Avoid
> > .Fn alloca
> > with large unbounded allocations.
> > +.Pp
> > +The use of C99 variable-length arrays and
> > +.Fn alloca
> > +in the same function will cause the lifetime of
> > +.Fn alloca Ns 's
> > +storage to be limited to the block containing the
> > +.Fn alloca .
> > +For example, in the following snippet,
> > +.Va p Ns 's
> > +lifetime does not extend outside of the block, whereas it would've if
> > +.Va vla
> > +hadn't been defined or had been defined as a fixed-length array:
> > +.Bd -literal -offset indent
> > +char *p;
> > +{
> > + const int n = 100;
> > + int vla[n];
> > + p = alloca(32);
> > + strcpy(p, "Hello, world!");
> > + printf("Inside: %s\\n", p); /* Valid. */
> > +}
> > +printf("Outside: %s\\n", p); /* Undefined. */
> > +.Ed
>
> I am unsure if we should document the behaviour of mixing VLAs and
> alloca() in the same function as being defined, as that binds us to
> support it in the future. I would be a lot more comfortable just
> documenting that behaviour is undefined if the two are combined in
> one function.
Agreed.
>
> Yours,
> Robert Clausecker
>
> --
> () ascii ribbon campaign - for an encoding-agnostic world
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>
--
Cheers,
Cy Schubert <Cy.Schubert@cschubert.com>
FreeBSD UNIX: <cy@FreeBSD.org> Web: https://FreeBSD.org
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