What is the meaning of "1ULL"in c programming language [duplicate]

Question

for (x = 0; x < 1ULL<<(2*length); ++x){

this above line is a c program code which I downloaded. It has a part "1ULL". First I thought it was a mistake. but the code compiles and works well. can anyone please explain me what is that thing mean.

thank you....

Answer 1

Suffix ULL to an integer represents type specifier. It means

unsigned long long

You may also like to read this and this one for more detail.

Answer 2

Those letters modify the literal 1 and make it of type unsigned long long.

This is covered in C99 (ISO/IEC 9899) §6.4.4.1 Integer constants:

integer-suffix:
unsigned-suffix long-suffixopt
unsigned-suffix long-long-suffix
long-suffix unsigned-suffixopt
long-long-suffix unsigned-suffixopt
unsigned-suffix: one of
u U
long-suffix: one of
l L
long-long-suffix: one of
ll LL