network- Low-level networking interface

Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/network/LICENSE)
Safe HaskellNone




This module is kept for backwards-compatibility. New users are encouraged to use Network.Socket instead.

Network was intended as a "higher-level" interface to networking facilities, and only supports TCP.


Basic data types

data Socket Source

Represents a socket. The fields are, respectively:

  • File descriptor
  • Socket family
  • Socket type
  • Protocol number
  • Status flag

If you are calling the MkSocket constructor directly you should ensure you have called withSocketsDo.

type HostName = String Source

Either a host name e.g., "" or a numeric host address string consisting of a dotted decimal IPv4 address or an IPv6 address e.g., "".

data PortNumber Source

Use the Num instance (i.e. use a literal) to create a PortNumber value with the correct network-byte-ordering. You should not use the PortNum constructor. It will be removed in the next release.


withSocketsDo :: IO a -> IO a Source

With older versions of the network library on Windows operating systems, the networking subsystem must be initialised using withSocketsDo before any networking operations can be used. eg.

main = withSocketsDo $ do {...}

It is fine to nest calls to withSocketsDo, and to perform networking operations after withSocketsDo has returned.

In newer versions of the network library it is only necessary to call withSocketsDo if you are calling the MkSocket constructor directly. However, for compatibility with older versions on Windows, it is good practice to always call withSocketsDo (it's very cheap).

Server-side connections

listenOn Source


:: PortID

Port Identifier

-> IO Socket

Listening Socket

Creates the server side socket which has been bound to the specified port.

maxListenQueue (typically 128) is specified to the listen queue. This is good enough for normal network servers but is too small for high performance servers.

To avoid the "Address already in use" problems, the ReuseAddr socket option is set on the listening socket.

If available, the IPv6Only socket option is set to 0 so that both IPv4 and IPv6 can be accepted with this socket.

If you don't like the behavior above, please use the lower level listen instead.

accept Source


:: Socket

Listening Socket

-> IO (Handle, HostName, PortNumber)

Triple of: read/write Handle for communicating with the client, the HostName of the peer socket, and the PortNumber of the remote connection.

Accept a connection on a socket created by listenOn. Normal I/O operations (see System.IO) can be used on the Handle returned to communicate with the client. Notice that although you can pass any Socket to Network.accept, only sockets of either AF_UNIX, AF_INET, or AF_INET6 will work (this shouldn't be a problem, though). When using AF_UNIX, HostName will be set to the path of the socket and PortNumber to -1.

sClose :: Socket -> IO () Source

Close the socket. All future operations on the socket object will fail. The remote end will receive no more data (after queued data is flushed).

Client-side connections

connectTo :: HostName -> PortID -> IO Handle Source

Calling connectTo creates a client side socket which is connected to the given host and port. The Protocol and socket type is derived from the given port identifier. If a port number is given then the result is always an internet family Stream socket.

Simple sending and receiving

Send and receive data from/to the given host and port number. These should normally only be used where the socket will not be required for further calls. Also, note that due to the use of hGetContents in recvFrom the socket will remain open (i.e. not available) even if the function already returned. Their use is strongly discouraged except for small test-applications or invocations from the command line.


socketPort :: Socket -> IO PortID Source

Returns the PortID associated with a given socket.

Networking Issues


The Handle returned by connectTo and accept is block-buffered by default. For an interactive application you may want to set the buffering mode on the Handle to LineBuffering or NoBuffering, like so:

h <- connectTo host port
hSetBuffering h LineBuffering

Improving I/O Performance over sockets

For really fast I/O, it might be worth looking at the hGetBuf and hPutBuf family of functions in System.IO.


On Unix, when writing to a socket and the reading end is closed by the remote client, the program is normally sent a SIGPIPE signal by the operating system. The default behaviour when a SIGPIPE is received is to terminate the program silently, which can be somewhat confusing if you haven't encountered this before. The solution is to specify that SIGPIPE is to be ignored, using the POSIX library:

 import Posix
 main = do installHandler sigPIPE Ignore Nothing; ...