No it isn't. The only guarantees you have about the size of long are that it is at least as large as (greater than or equal) a normal int. And a normal int is greater than or equal as big as a short.
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.
This code is attempting to be portable to C89, or why not just use C99's stdint.h? I don't believe your statement "C89 says the same thing," do you have a source to point to?
Do you have the section number in the latest freely available C99 working draft? The "Types" section (which 6.something in the draft) simply says what I said earlier about scalar rank. And 5.4.4.2.1 doesn't seem to exist in the draft.
Sorry, I mistyped the section number. Updated the parent comment with corrections and links.
I assume the intent of tweetnacl is to be C89-compatible, but due to the long long (and char, although that one's pretty pedantic) issue there's little guarantee of success.
