2013/9/11 nobu (Nobuyoshi Nakada) <nobu / ruby-lang.org>:
> Issue #8890 has been updated by nobu (Nobuyoshi Nakada).
>
> Currently we don't assume C99, but C89.

According to K&R 2nd edition, size_t is unsigned.

I guess you are confusing with older Unix such as 4.3BSD.

4.3BSD (1986) uses (signed) long as size_t.
(4.3BSD Reno (1990) uses unsigned int, though.)
-- 
Tanaka Akira