Re: [PATCH] range-diff: fix some 'hdr-check' and sparse warnings

From: Jeff King <peff@peff.net>
Date: Jul 15, 2019, 14:46 UTC
Message-ID: <20190715144602.GA26636@sigill.intra.peff.net>
In-Reply-To: <54c2ee44-ee99-ea4a-3154-f642e0060877@kdbg.org>

On Sun, Jul 14, 2019 at 10:30:27AM +0200, Johannes Sixt wrote:

> Why would you re-order members? There's nothing wrong when a pointer is
> initialized by 0.

To appease tooling like "sparse" without having to remember to do anything specific at the point-of-use sites. I'm open to the idea that it is not worth appeasing sparse, but this may be a simple and practical compromise.

Show 6 quoted lines

> > But it does fall down
> > when the first element _has_ to be a struct (like, say, any user of our
> > hashmap.[ch] interface).
> 
> No, it does not. It is not necessary to spell out nested structs in the
> initializer.

Ah, that is news to me. I know that this compiles OK with "gcc -Wall", but is it guaranteed by the standard?

-- >8 -- #include <stdio.h>

struct outer {
	struct {
		int x;
		char *y;
	} inner;
};

int main(void)
{
	struct outer foo = { 0 };
	printf("%d %p\n", foo.inner.x, foo.inner.y);
	return 0;
}
-- 8< --

If so, then I agree that "0" is a pretty good solution, tooling like "sparse" aside. I do agree with your sentiment that "0" can be read as "please zero-initialize this struct" and not really as an attempt to set an initial member pointer to zero.

-Peff