The following reproducibly fails: $ darcs get http://cryp.to/hsdns [*] $ cd hsdns/ $ hsc2hs ADNS.hsc $ ghc -threaded -Wall -O --make test.hs -o test -ladns | Chasing modules from: test.hs | Compiling ADNS ( ./ADNS.hs, ./ADNS.o ) | /tmp/ghc2613.hc:9:23: ADNS_stub.h: No such file or directory | /tmp/ghc2613.hc: In function `s8Xa_ret': | /tmp/ghc2613.hc:6340: error: `ADNS_d7eN' undeclared (...) If you build the program without optimization, $ ghc -threaded --make test -ladns it works just fine. This seems to happen only when the threaded RTS in involved, -O without -threaded works. Peter