### 5.8 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.