 |
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.
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, as well as shifts with a constant
value. Multiplication, division and shifts are not open-coded and use
special library routines.
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.