A friend of mine's showed me this in a code he used for programming an educational operating system. If you have a pointer to a member of a struct, with the macro container_of you can retrieve a pointer to the enclosing struct.

  /* Return the offset of 'member' relative to the beginning of a struct type */
  #define offsetof(type, member)  ((size_t) (&((type*)0)->member))
  
  #define container_of(ptr, type, member) \
   ((type *)((char *)(ptr) - offsetof(type, member)))

wow that is cool, do you have any pointers (no pun intended) for usage of this technique in practical terms?

The way it's used is that Ganesha supports defining of alternate filesystem backends and serving them as NFS shares. Handles to objects (e.g. files) would exist as pointers which live inside the struct of the backend's handle struct. i.e.:

  struct my_file_data {
    struct ganesha_file_data {
      // generic data
    };

    // data specific to my module
  };