container_of macro

kernel container_of macro

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

When you use the cointainer_of macro, you want to retrieve the structure that contains the pointer of a given field. For example:

struct numbers {  
   int one; 
   int two; 
   int three; 
} n;  

int *ptr = &n.two; 
struct numbers *n_ptr; 
n_ptr = container_of(ptr, struct numbers, two);

You have a pointer that points in the middle of a structure (and you know that is a pointer totwo
 [the field name in the structure]), but you want to retrieve the entire structure (numbers). So, you calculate the offset of the filed two in the structure:

offsetof(type,member)

and subtract this offset from the given pointer. The result is the pointer to the start of the structure. Finally, you cast this pointer to the structure type to have a valid variable.