Hello, On OS-X, this works: (define-foreign-library libc (t (:default "libc"))) (use-foreign-library libc) But this doesn't: (define-foreign-library libc (t (:or "libc" "libc.so.6"))) (use-foreign-library libc) This seems to be because load-foreign-library doesn't attempt to guess the appropriate suffix for the library name when used with :or. This is the documented behavior but it could be a little confusing since the second example appears to the naive (like me) to be a super- set of the first example. Also: (define-foreign-library libc (:unix (:or "libc.so.6")) (t (:default "libc"))) (use-foreign-library libc) Doesn't work since OS-X is unix, but not linux. Maybe after failing on the specific case (i.e. unix), it could fall back to the general case? I could try and provide patches for one or both of these behaviors if people think that this would be an improvement. best, -Hazen