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SubjectRe: [RFC 1/4] hashtable: introduce a small and naive hashtable
On Thu, Aug 02, 2012 at 03:04:19PM +0200, Sasha Levin wrote:
> On 08/02/2012 01:23 PM, Sasha Levin wrote:
> >> #define DEFINE_HASH_TABLE(name, length) struct hash_table name = { .count = length, .buckets = { [0 ... (length - 1)] = HLIST_HEAD_INIT } }
> > The limitation of this approach is that the struct hash_table variable must be 'static', which is a bit limiting - see for example the use of hashtable in 'struct user_namespace'.
> >
>
> What if we just use two possible decelerations? One of static structs and one for regular ones.
>
> struct hash_table {
> size_t bits;
> struct hlist_head buckets[];
> };
>
> #define DEFINE_HASHTABLE(name, bits) \
> union { \
> struct hash_table name; \
> struct { \
> size_t bits; \

This shouldn't use "bits", since it'll get expanded to the macro
argument.

> struct hlist_head buckets[1 << bits]; \
> } __name; \

__##name

> }
>
> #define DEFINE_STATIC_HASHTABLE(name, bit) \
> static struct hash_table name = { .bits = bit, \
> .buckets = { [0 ... (bit - 1)] = HLIST_HEAD_INIT } }

You probably wanted to change that to [0 ... ((1 << bit) - 1)] , to
match DEFINE_HASHTABLE.

Since your definition of DEFINE_HASHTABLE would also work fine when used
statically, why not just always use that?

#define DEFINE_STATIC_HASHTABLE(name, bits) static DEFINE_HASHTABLE(name, bits) = { .name.bits = bits }
One downside: you can't use this to define a global non-static hash
table, because you can't have a global non-static anonymous union.
Using the non-union form would actually allow a global non-static hash
table:

#define DEFINE_HASHTABLE_INIT(name, bits) struct hash_table name = { .bits = bits, .buckets = { [0 ... ((1 << bits) - 1)] = HLIST_HEAD_INIT } }
/* elsewhere */
extern struct hash_table name;
I don't know if that seems like a good idea or not.

- Josh Triplett


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