Re: [RFC v3 5/6] alx-0081r2 - array parameters of 0 elements

[Alejandro Colomar <alx@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 1757 bytes --]

Hi Daniel,

On 2026-01-28T10:14:23+0000, Daniel Thompson wrote:
> On Mon, Jan 26, 2026 at 01:49:16PM +0100, Alejandro Colomar wrote:
> > Proposed wording
> > 	Based on N3685.
> >
> >     6.7.7.3  Array declarators
> > 	@@ Constraints, p1
> > 	 In addition to optional type qualifiers and the keyword static,
> > 	 the [ and ] can delimit an expression or *.
> > 	 If they delimit an expression,
> > 	 called the array length expression,
> > 	 the expression shall have an integer type.
> > 	 If the expression is a constant expression,
> > 	-it shall have a value greater than zero.
> > 	+it shall have a nonnegative value.
> > 	+An array length expression
> > 	+that is a constant expression with value zero
> > 	+shall appear only in
> > 	+a declaration of a function parameter with an array type,
> > 	+and then only in the outermost array type derivation.
> 
> This change did not have any motivating example within the discussion.
> Providing a motivational example showing why it is useful to allow array
> parameters whose constant length is zero would be good.

Makes sense; I'll add that.

Here's a function that I use in shadow-utils:

	char *
	stpecpy(char dst[], const char end[];
	    char dst[dst?end-dst:0], const char end[0], const char *restrict src)
	{
		ssize_t  dlen;

		if (dst == NULL)
			return NULL;

		dlen = strtcpy(dst, src, end - dst);
		if (dlen == -1)
			return NULL;

		return dst + dlen;
	}

It's essentially the same as Plan9's strecpy(2), except that Plan9's
API has an important bug.

Here's a GCC bug that shows such a use case:
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108036>


Have a lovely day!
Alex

> Daniel.

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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]