What is "long long" type in c++?

long long is not the same as long (although they can have the same size, e.g. in most 64-bit POSIX system). It is just guaranteed that a long long is at least as long as a long. In most platforms, a long long represents a 64-bit signed integer type.

You could use long long to store the 8-byte value safely in most conventional platforms, but it's better to use int64_t/int_least64_t from <stdint.h>/<cstdint> to clarify that you want an integer type having ≥64-bit.