6.9 Double-Word Integers

ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C90 mode and in C++. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer. To make an integer constant of type long long int , add the suffix ‘ LL ’ to the integer. To make an integer constant of type unsigned long long int , add the suffix ‘ ULL ’ to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports a fullword-to-doubleword widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.

There may be pitfalls when you use long long types for function arguments without function prototypes. If a function expects type int for its argument, and you pass a value of type long long int , confusion results because the caller and the subroutine disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int . The best way to avoid such problems is to use prototypes.