Understanding the container_of macro

When it comes to managing several data structures in the code, you'll almost always need to embed one structure into another and retrieve them at any moment without being asked questions about memory offset or boundaries. Let's say you have a struct person, as defined here:

 struct person { 
     int  age; 
     char *name; 
 } p;

By only having a pointer on age or name, you can retrieve the whole structure wrapping (containing) that pointer. As the name says, the container_of macro is used to find the container of the given field of a structure. The macro is defined in include/linux/kernel.h and looks like:

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

Don't be afraid of the pointers; just...