container_of() implementation in linux kernel

Question

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Rationale behind the container_of macro in linux/list.h

#define container_of(ptr, type, member) ({                      \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})\

Why do we need to construct mptr here instead of casting ptr directly to a char* ?

score 1 · Answer 1 · answered Dec 16 '12 at 11:15

1

Type safety, it assures that mptr is of the same type as pointer to member instead of just casting. If it's not you will get a warning.

answered Dec 16 '12 at 11:15

iabdalkader

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you probably mean that `ptr` has the type pointer to the type of `member`, don't you? – Jens Gustedt Dec 16 '12 at 11:17
@JensGustedt yup, fixed thanks. – iabdalkader Dec 16 '12 at 11:19

score 0 · Answer 2 · answered Dec 16 '12 at 11:17

The macro as given in the kernel has a type check, namely it ensures that ptr has a type that is assignment compatible to the type "pointer to the type of member".

E.g if by accident ptr happens to be an integer, a cast would be perfectly happy to interpret this as a char*.

container_of() implementation in linux kernel

2 Answers2