See http://is.gd/57Ptu (Network.Socket.setSocketOption). `setSocketOption' uses FFI binding to setsockopt(2), named `c_setsockopt'.
foreign import CALLCONV unsafe "setsockopt" c_setsockopt :: CInt -> CInt -> CInt -> Ptr CInt -> CInt -> IO CInt
The binding treats OPTVAL (4th argument) as a pointer to `int'. This is mostly fine but not when `SO_LINGER' option is being set. In the latter case OPTVAL must be a pointer to `struct linger'. [1] [1] http://www.gnu.org/s/libc/manual/html_node/Socket_002dLevel-Options.html Haskell programs trying to set SO_LINGER socket option will successfully compile but fail in runtime. $ runhaskell socket-linger.hs || echo X $ runhaskell -DLINGER socket-linger.hs || echo X socket-linger.hs: setSocketOption: invalid argument (Invalid argument) X $ gcc -g -Wall -W -o /tmp/1 socket-linger.c && /tmp/1 || echo X $ gcc -g -DLINGER_AS_INT -Wall -W -o /tmp/1 socket-linger.c && /tmp/1 || echo X setsockopt: Invalid argument X (See attached files `socket-linger.c', `socket-linger.hs'.) The necessity of `SO_LINGER' option is controversial (see [2]). I think it is better to comment the `Linger' constructor out of Network/Socket.hsc. [2] http://www.developerweb.net/forum/archive/index.php/t-2982.html Cheers. -- vvv