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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