unsigned long long if you want at least a 64-bit unsigned. uint64_t if you want a fixed-length 64-bit unsigned (and your compiler is at least C99, but I think

unsigned long long if you want at least a 64-bit unsigned. uint64_t if you want a fixed-length 64-bit unsigned (and your compiler is at least C99, but I think long long came with C99 too). Note that unsigned long is at least 32-bit, so it could be 64-bit too, there is no guarantee it is exactly 32-bit.