The macro NULL is defined in several system header files, including stddef.h and stdio.h.
It is good style to set every pointer that is not currently holding a valid address to NULL.

Odd Test

The & operator provides a simple way to check if an integral value is odd: that is, whether its least significant bit is 1.

 odd  = value & 1

odd has the value 1 if value is odd, 0 if value is even.