# 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 C89 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 fullword-to-doubleword a 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, unless you declare function prototypes. If a function
expects type `int` for its argument, and you pass a value of type
`long long int`, confusion will result because the caller and the
subroutine will 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.