[dpdk-dev] rte_mbuf structure size in Windows

[Tal Shnaiderman]

Hi,

I've noticed that there is a difference between the size of rte_mbuf in a Unix 
build comparing to Windows.

The requirements is for rte_mbuf is to be RTE_CACHE_LINE_MIN_SIZE * 2 bytes 
however when I'm building it in Windows the size is RTE_CACHE_LINE_MIN_SIZE * 3.

Looks like the diff results from the usage of bit fields inside rte_mbuf, from 
my testing it looks to me like the usage of 2 different bit fielded types 
inside rte_mbuf causes additional padding in Windows.

For example from rte_mbuf, the following unions have the same size in Windows 
and Linux:

        union {
                uint32_t packet_type; 
                // bit fields of type uint32_t will follow
                ...
        };...

4 bytes both in Unix and Windows.

        union {
                uint64_t tx_offload;
                // bit fields of type uint64_t will follow
                ...
        };

8 bytes both in Unix and Windows.

However when creating a struct containing both unions I'm getting sizeof 16 
bytes in Unix and 24 bytes in Windows.

Did someone faced this issue before? Is this a result of different alignment 
between gcc and clang when bit fields are used? 

Thanks,

Tal