Question

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
structure, write a routine that
returns a pointer to the structure.

Thank you!

Answer 1

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));