Given a pointer to a member within a struct, how to return a pointer to this struct?

This is an interview question. I have no idea how to solve it. could anybody help me:?

Given a pointer to member a within a struct开发者_开发知识库ure, write a routine that returns a pointer to the structure.

Thank you!

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Firstly, in order to pull that off you need to know the type of the enclosing struct StructType and the name of the member member. That must be a given. Without it, the problem has no solution.

Secondly, I don't know why other answers insist on reinventing the wheel (and engage into undefined and non-portable hacks on top of that) instead of using the standard offsetof macro. With offsetof the answer is

StructType *pstruct = 
  (StructType *) ((char *) pmember - offsetof(StructType, member));

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The Linux kernel code (GPLv2 licensed) includes a convenient container_of() macro:

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

It is used like this:

static inline struct ext3_inode_info *EXT3_I(struct inode *inode)
{
    return container_of(inode, struct ext3_inode_info, vfs_inode);
}

@Richard's answer is excellent; the main difference is this code is parameterized to accept any struct and any member, and as a result is correspondingly more complicated.

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Given a structure named s and a member named m

struct s * BaseS(byte *p)
{
     return (struct s *) (p - ((byte*) &(((struct s *)0)->m)));
}

The idea is to get the offset of the member and subtract that from the pointer.