Re: [dpdk-dev] [PATCH 1/2] eal: detect endianness

["Qiu, Michael" <michael.qiu@intel.com>]

On 12/4/2014 5:01 PM, Thomas Monjalon wrote:
> 2014-12-04 02:28, Qiu, Michael:
>> On 12/4/2014 5:26 AM, Thomas Monjalon wrote:
>>> There is no standard to check endianness.
>>> So we need to try different checks.
>>> Previous trials were done in testpmd (see commits
>>> 51f694dd40f56 and 64741f237cf29) without full success.
>>> This one is not guaranteed to work everywhere so it could
>>> evolve when exceptions are found.
> [...]
>>>  #include <stdint.h>
>>> +#ifdef RTE_EXEC_ENV_BSDAPP
>>> +#include <sys/endian.h>
>>> +#else
>>> +#include <endian.h>
>>> +#endif
>>> +
>>> +/*
>>> + * Compile-time endianness detection
>>> + */
>>> +#define RTE_BIG_ENDIAN    1
>>> +#define RTE_LITTLE_ENDIAN 2
>>> +#if defined __BYTE_ORDER
>>> +#if __BYTE_ORDER == __BIG_ENDIAN
>>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>>> +#elif __BYTE_ORDER == __LITTLE_ENDIAN
>>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>>> +#endif /* __BYTE_ORDER */
>>> +#elif defined __BYTE_ORDER__
>>> +#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>>> +#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
>>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>>> +#endif /* __BYTE_ORDER__ */
>>> +#elif defined __BIG_ENDIAN__
>>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>>> +#elif defined __LITTLE_ENDIAN__
>>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>>> +#endif
>> What do you think about :
>>
>> +/*
>> +  * Compile-time endianness detection
>> + */
>> +#define RTE_BIG_ENDIAN 1
>> +#define RTE_LITTLE_ENDIAN 2
>> +if defined __BYTE_ORDER__    /* Prefer gcc build-in macros */
>> +#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>> +#elif __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>> +#endif /* __BYTE_ORDER__ */
>> +#else
>> +#if defined RTE_EXEC_ENV_BSDAPP
>> +#include <sys/endian.h>
>> +#else
>> +#include <endian.h>
>> +#endif
>> +#if defined __BYTE_ORDER
>> +#if __BYTE_ORDER == __BIG_ENDIAN
>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>> +#elif __BYTE_ORDER == __LITTLE_ENDIAN
>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>> +#endif /* __BYTE_ORDER */
>> +#elif defined __BIG_ENDIAN__
>> +#define RTE_BYTE_ORDER RTE_BIG_ENDIAN
>> +#elif defined __LITTLE_ENDIAN__
>> +#define RTE_BYTE_ORDER RTE_LITTLE_ENDIAN
>> +#endif
>> +#endif
> Please, could you give more explanations about your proposal?
> Why not always try to include endian.h?

I assume that if gcc can handler why we need include that file?

Also it seems that only old version could have this issue, newer
versions has build in this marcos.

So that's why I prefer  "__BYTE_ORDER__" for high priority.

Thanks,
Michael
> Why giving high priority to __BYTE_ORDER__?
>