2STUPID4C

I have smoked a significant amount of 420 but still can't understand how this macro function works:
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})

https://github.com/torvalds/linux/blob/master/include/linux/kernel.h

>>2
What's the point of the typeof stuff on the first line? It's cast to char * for byte pointer arithmetic anyway, so wouldn't char *__mptr = (char *)(ptr); work fine?