I've used that sort of construct in the past and have confirmed that it
solves the problem (in addition to producing cleaner-looking code).
Thank you!
Bill
On Mon, Nov 3, 2014 at 2:45 AM, Shmulik Ladkani <shmulik.ladk...@gmail.com>
wrote:
> Hi Bill,
>
> On Sun, 2 Nov 2014 21:50:18 -0600 Bill Fischofer <
> bill.fischo...@linaro.org> wrote:
> > I'm working on code to implement the new buffer APIs and have the
> following
> > internal function:
> >
> > static inline void *get_block(struct pool_entry_s *pool)
> > {
> > void volatile *oldhead;
> > void volatile *newhead;
> >
> > do {
> > oldhead = pool->blk_freelist;
> > if (oldhead == NULL) break;
> > newhead = ((odp_buf_blk_t *)oldhead)->next;
> > } while (odp_cs(pool->blk_freelist,oldhead,newhead) == 0);
> >
> > return (void *)oldhead;
> > }
> >
> > However when compiling this generates the following strange error
> message:
> >
> > ./include/odp_buffer_pool_internal.h: In function 'get_block':
> > ./include/odp_buffer_pool_internal.h:88:14: error: cast discards
> > '__attribute__((noreturn))' qualifier from pointer target type
> > [-Werror=cast-qual]
> > newhead = ((odp_buf_blk_t *)oldhead)->next;
> > ^
>
> On gcc version 4.9.1 it gives:
>
> warning: cast discards 'volatile' qualifier from pointer target type
> [-Wcast-qual]
> newhead = ((odp_buf_blk_t *)oldhead)->next;
> ^
>
> The warning is indeed correct and suitable for -Wcast-qual, since
> 'oldhead' is a volatile pointer while it is casted to a non volatile
> pointer type.
>
> In your gcc the warning output shown is incorrect.
>
> Using -Wcast-qual is a bit evil. It won't allow you to cast away
> properties of pointers such as const or volatile without some tricks.
>
> The easy option would be to disable -Wcast-qual.
>
> However this might work for you:
>
> #define VOLATILE_ACCESS_PTR(p) \
> ((typeof(p))(uintptr_t) *(volatile typeof(p) const *)&(p))
>
> static inline void *get_block(struct pool_entry_s *pool)
> {
> void *oldhead;
> void *newhead;
>
> do {
> oldhead = VOLATILE_ACCESS_PTR(pool->blk_freelist);
> if (oldhead == NULL) break;
> newhead = ((odp_buf_blk_t *)oldhead)->next;
> } while (odp_cs(pool->blk_freelist,oldhead,newhead) == 0);
>
>
> return oldhead;
> }
>
> Advantage is pool->blk_freelist is accessed once (which I beleive was
> your intention), and encapsulating all details of the ugly casts needed
> to solve -Wcast-qual warnings into a single place.
>
> Regards,
> Shmulik
>
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