the (char *) cast in list_entry() of the Linux Kernel linked list implementation

Question

This is the macro definition:

/**
 * list_entry - get the struct for this entry
 * @ptr:    the &struct list_head pointer.
 * @type:   the type of the struct this is embedded in.
 * @member: the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
    ((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member)))

I don't understand why ptr is casted to (char *) . Can't I just subtract the offset of member from ptr ? Like this:

#define list_entry(ptr, type, member) \
        ((type *)((ptr)-(unsigned long)(&((type *)0)->member)))

Thanks!

Answer 1

No. Pointer arithmetic is equivalent to:

ptr[addend]

not

(ptr_type *)((unsigned long)&ptr + addend)

The latter requires an explicit cast to char * (as that is the unit of memory) to manipulate a pointer's value directly.