The most useful user-made C-macros (in GCC, also C99)? [closed]

Answer 1

This one is from linux kernel (gcc specific):

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

Another missing from other answers:

#define LSB(x) ((x) ^ ((x) - 1) & (x))   // least significant bit