Hi Yang Zhang, My guess (purely speculative, not authoritative) is that on your platform, a long int is 64-bit, and a long long is 64-bit. Assuming that guess is correct, since long int is 64-bit *AND* long int is part of the C99 standard the stdint.h used long int to be the alias for int64_t. Whereas long long is a compiler extension*, so the long int was preferential to long long. Sincerely, --Eljay * Is long long still a compiler extension for C99? I don't use C99.