long long data type in GCC and what's the meaning of 1LL

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.