http://uninformed.org/index.cgi?v=1&a=1&p=16 http://lists.apple.com/archives/cocoa-dev/2005/Oct/msg00836.html

Mac OS X has an undocumented behavior concerning 'execve(2)' inside a threaded process. If a process tries to call 'execve(2)' and has more than one native thread, the kernel returns 'EOPNOTSUPP'. This prevents 'executeFile' from working when the threaded runtime is used.

% ghci
GHCi, version 7.0.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> System.Posix.Process.executeFile "/bin/sh" False [] Nothing
Loading package unix-2.4.2.0 ... linking ... done.
*** Exception: /bin/sh: executeFile: failed (Operation not supported)
Prelude> 
Leaving GHCi.
% uname
Darwin

To work around this, we have to 'fork(2)' before calling 'execve(2)', to make sure there is only a single active thread.