Are there platforms where rlim_t is signed? Posix defines it as unsigned: <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_resource.h.html>.

Also there are subtle differences between the PyLong_AsUnsigned[. . .] and the current signed versions. The unsigned version does not call __int__() so you would no longer be able to pass a float().

Also this could cause compatibility problems with code that was written for earlier documentation, which said to use -1 to mean unlimited. Perhaps that is why the code has rlim_* & RLIM_INFINITY masking.