There are some invariants that are being violated by pthread_kill(id, SIGINT) and I'm having difficulty understanding them. I get the following errors: cd ./concurrent/should_run && '/home/ezyang/Dev/ghc/inplace/bin/ghc-stage2' -fforce-recomp -dcore-lint -dcmm-lint -no-user-package-conf -rtsopts -dno-debug-output -o foreignInterruptible foreignInterruptible.hs -threaded -debug >foreignInterruptible.comp.stderr 2>&1 cd ./concurrent/should_run && ./foreignInterruptible foreignInterruptible.run.stdout 2>foreignInterruptible.run.stderr Wrong exit code (expected 0 , actual 134 ) Stdout: Stderr: foreignInterruptible: internal error: WEAK object entered! (GHC version 6.13.20100908 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted (core dumped) *** unexpected failure for foreignInterruptible(threaded1) Even more curiously, though, if I hand-run the command I don't get any problems. Edward