Re: [dpdk-dev] [PATCH v2 1/8] eal: define container_of macro

[Shreyansh Jain <shreyansh.jain@nxp.com>]

Hello Jan,

Need your help in resolving the checkpatch issue for this patch.
I have used this in EAL Bus series [1].

[1] http://dpdk.org/ml/archives/dev/2016-December/051350.html

On Monday 21 November 2016 10:25 PM, Jan Blunck wrote:
> This macro is based on Jan Viktorin's original patch but also checks the
> type of the passed pointer against the type of the member.
>
> Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
> Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
> [jblunck@infradead.org: add type checking and __extension__]
> Signed-off-by: Jan Blunck <jblunck@infradead.org>
> ---
>  lib/librte_eal/common/include/rte_common.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
> index db5ac91..8dda3e2 100644
> --- a/lib/librte_eal/common/include/rte_common.h
> +++ b/lib/librte_eal/common/include/rte_common.h
> @@ -331,6 +331,26 @@ rte_bsf32(uint32_t v)
>  #define offsetof(TYPE, MEMBER)  __builtin_offsetof (TYPE, MEMBER)
>  #endif
>
> +/**
> + * Return pointer to the wrapping struct instance.
> + *
> + * Example:
> + *
> + *  struct wrapper {
> + *      ...
> + *      struct child c;
> + *      ...
> + *  };
> + *
> + *  struct child *x = obtain(...);
> + *  struct wrapper *w = container_of(x, struct wrapper, c);
> + */
> +#ifndef container_of
> +#define container_of(ptr, type, member)	__extension__ ({		\
> +			typeof(((type *)0)->member) *_ptr = (ptr);	\
> +			(type *)(((char *)_ptr) - offsetof(type, member)); })
> +#endif

(I think there was discussion in ML about this but where, I couldn't 
find it).

Above code snippet doesn't go down well with the checkpatch script. It 
reports:

---->8----
ERROR:COMPLEX_MACRO: Macros with complex values should be enclosed in 
parentheses
#40: FILE: lib/librte_eal/common/include/rte_common.h:349:
+#define container_of(ptr, type, member)        __extension__ ({ 
        \
+                       typeof(((type *)0)->member) *_ptr = (ptr);      \
+                       (type *)(((char *)_ptr) - offsetof(type, 
member)); })

ERROR:SPACING: need consistent spacing around '*' (ctx:WxV)
#41: FILE: lib/librte_eal/common/include/rte_common.h:350:
+                       typeof(((type *)0)->member) *_ptr = (ptr);      \
---->8----

Second error is primarily because of '*<space>_ptr' rather than '*_ptr'.

Is do{ ... }while(0) the fix for the first one?

This:

---->8----
#ifndef container_of
#define container_of(ptr, type, member) do {                 \
            __extension__                                     \
            typeof(((type *)0)->member) * _ptr = (ptr);       \
            (type *)(((char *)_ptr) - offsetof(type, member));\
            } while (0)
#endif
---->8----

Seems to be ok with checkpatch. Do you see any technical/compiler issue 
with this?

> +
>  #define _RTE_STR(x) #x
>  /** Take a macro value and get a string version of it */
>  #define RTE_STR(x) _RTE_STR(x)
>

-
Shreyansh