linux-kernel - Re: [RFC 1/4] hashtable: introduce a small and naive hashtable

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Date:	Fri, 3 Aug 2012 10:59:11 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, paul.gortmaker@...driver.com
Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable

On Thu, Aug 02, 2012 at 11:47:01PM +0200, Sasha Levin wrote:
> On 08/02/2012 10:41 PM, Josh Triplett wrote:
> > On Thu, Aug 02, 2012 at 07:54:42PM +0200, Sasha Levin wrote:
> >> /* I've "preprocessed" the DEFINE macro below */
> >> union {
> >> 	struct hash_table table;
> >> 	struct {
> >> 		size_t bits;
> >> 		struct hlist_head buckets[32];
> >> 	}
> >> } my_hashtable;
> > 
> > That expansion doesn't match the macros.  Using the most recent
> > definitions of DEFINE_HASHTABLE and DEFINE_STATIC_HASHTABLE from above,
> > the definition would look something like this:
> > 
> > static union {
> > 	struct hash_table my_hashtable;
> > 	struct {
> > 		size_t bits;
> > 		struct hlist_head buckets[1 << 5];
> > 	} __my_hashtable;
> > } = { .my_hashtable.bits = 5 };
> 
> It's different because I don't think you can do what you did above with global variables.
> 
> You won't be defining any instances of that anonymous struct, so my_hashtable won't exist anywhere.

...how strange.  The above syntax ought to work, and many other
compilers document it as legal syntax (and I thought that C1x's
anonymous structs and unions allowed it), but indeed GCC doesn't accept
it.

Fair enough; looks like consolidating the macro implementations won't
actually work.

- Josh Triplett
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