rust/library/core/src/ffi/c_ulonglong.md

Equivalent to C's unsigned long long type.

This type will almost always be [u64], but may differ on some systems. The C standard technically only requires that this type be an unsigned integer with the size of a long long, although in practice, no system would have a long long that is not a u64, as most systems do not have a standardised [u128] type.