The Quad Integer Library is full set of functions to perform operations (arithmetic, logical, etc) on integer that are twice the size of the compiler's native long integer. The size of a quad integer is generally 64-bits, but this library does not depend on it.
The library contains a set of functions and a set of macros that wrap those functions. The macros should be used when created a software product that is to remain portable. If the macros are used on a compiler that has a long long integer type then the operations built into the compiler are used, if not the macros calls the respective library call to complete the operation.
It is generally suggested that you always call the macros instead of the calling the library calls directly.