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[C] sizeof single struct member and relative length of struct

#define member_size(type, member) sizeof(((type *)0)->member)

(reference: https://stackoverflow.com/questions/3553296/c-sizeof-single-struct-member)

// member_length
#define member_length(type, member) ((((type *)0)->member)+member_size(type, member))

If you use padding, it's is possible that member_length has some errors.
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