C99 standard [1], §5.2.4.2.1 says otherwise. Int must acommodate at least 16 bits, long 32 bits, and long long 64 bits. C89 [2], §2.2.4.2 says the same thing but omits long long, so tweetnacl's u64 may not exist (this was the case with MSVC not too long ago).

Additionally, char is required to be at least 8 bits by the C standard, but tweetnacl assumes exactly 8. Some oddball architectures have larger character types, but POSIX mandates 8.

[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

[2] http://nepsweb.co.uk/langstand/isoC/gordon/ansi-c89w.txt