Copyright	(c) Lars Petersen 2015
License	MIT
Maintainer	info@lars-petersen.net
Stability	experimental
Safe Haskell	None
Language	Haskell2010

System.Socket

Contents

Operations
Sockets
MsgFlags
getSockOpt / setSockOpt
Generic socket options
- SO_ACCEPTCONN
SocketException

Description

{-# LANGUAGE OverloadedStrings #-}
module Main where

import System.Socket
import Data.ByteString
import Control.Monad
import Control.Concurrent

main :: IO ()
main = do
  s <- socket :: IO (Socket SockAddrIn STREAM TCP)
  bind s (SockAddrIn 8080 (pack [127,0,0,1]))
  listen s 5
  forever $ do
    (peer,addr) <- accept s
    forkIO $ do
      send peer "Hello world!"
      close peer

Synopsis

Operations

socket

socket :: (Address a, Type t, Protocol p) => IO (Socket a t p) Source

Creates a new socket.

Whereas the underlying POSIX socket function takes 3 parameters, this library encodes this information in the type variables. This rules out several kinds of errors and escpecially simplifies the handling of addresses (by using associated type families). Examples:

-- create a IPv4-UDP-datagram socket
sock <- socket :: IO (Socket SockAddrIn DGRAM UDP)
-- create a IPv6-TCP-streaming socket
sock6 <- socket :: IO (Socket SockAddrIn6 STREAM TCP)

This operation sets up a finalizer that automatically closes the socket when the garbage collection decides to collect it. This is just a fail-safe. You might still run out of file descriptors as there's no guarantee about when the finalizer is run. You're advised to manually close the socket when it's no longer needed.
This operation configures the socket non-blocking to work seamlessly with the runtime system's event notification mechanism.
This operation can safely deal with asynchronous exceptions without leaking file descriptors.
This operation throws SocketExceptions:

EAFNOSUPPORT: The socket domain is not supported.
EMFILE: The process is out file descriptors.
ENFILE: The system is out file descriptors.
EPROTONOSUPPORT: The socket protocol is not supported (for this socket domain).
EPROTOTYPE: The socket type is not supported by the protocol.
EACCES: The process is lacking necessary privileges.
ENOMEM: Insufficient memory.

bind

bind :: (Address a, Type t, Protocol p) => Socket a t p -> a -> IO () Source

Bind a socket to an address.

Calling bind on a closed socket throws EBADF even if the former file descriptor has been reassigned.
It is assumed that c_bind never blocks and therefore EINPROGRESS, EALREADY and EINTR don't occur. This assumption is supported by the fact that the Linux manpage doesn't mention any of these errors, the Posix manpage doesn't mention the last one and even MacOS' implementation will never fail with any of these when the socket is configured non-blocking as argued here.
The following SocketExceptions are relevant and might be thrown (see man bind for more exceptions regarding SockAddrUn sockets):

EADDRINUSE: The address is in use.
EADDRNOTAVAIL: The address is not available.
EBADF: Not a valid file descriptor.
EINVAL: Socket is already bound and cannot be re-bound or the socket has been shut down.
ENOBUFS: Insufficient resources.
EOPNOTSUPP: The socket type does not support binding.
EACCES: The address is protected and the process is lacking permission.
EISCONN: The socket is already connected.
ELOOP: More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in address.
ENAMETOOLONG: The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EAFNOSUPPORT: The address family is invalid.
ENOTSOCK: The file descriptor is not a socket.
EINVAL: Address length does not match address family.

listen

listen :: (Address a, Type t, Protocol p) => Socket a t p -> Int -> IO () Source

Accept connections on a connection-mode socket.

Calling listen on a closed socket throws EBADF even if the former file descriptor has been reassigned.
The second parameter is called backlog and sets a limit on how many unaccepted connections the socket implementation shall queue. A value of 0 leaves the decision to the implementation.
This operation throws SocketExceptions:

EBADF: Not a valid file descriptor (only after socket has been closed).
EDESTADDRREQ: The socket is not bound and the protocol does not support listening on an unbound socket.
EINVAL: The socket is already connected or has been shut down.
ENOTSOCK: The file descriptor is not a socket (should be impossible).
EOPNOTSUPP: The protocol does not support listening.
EACCES: The process is lacking privileges.
ENOBUFS: Insufficient resources.

accept

accept :: (Address a, Type t, Protocol p) => Socket a t p -> IO (Socket a t p, a) Source

Accept a new connection.

Calling accept on a closed socket throws EBADF even if the former file descriptor has been reassigned.
This operation configures the new socket non-blocking (TODO: use accept4 if available).
This operation sets up a finalizer for the new socket that automatically closes the socket when the garbage collection decides to collect it. This is just a fail-safe. You might still run out of file descriptors as there's no guarantee about when the finalizer is run. You're advised to manually close the socket when it's no longer needed.
This operation catches EAGAIN, EWOULDBLOCK and EINTR internally and retries automatically.
This operation throws SocketExceptions:

EBADF: Not a valid file descriptor (only after the socket has been closed).
ECONNABORTED: A connection has been aborted.
EINVAL: The socket is not accepting/listening.
EMFILE: The process is out file descriptors.
ENFILE: The system is out file descriptors.
ENOBUFS: No buffer space available.
ENOMEM: Out of memory.
ENOSOCK: Not a valid socket descriptor (should be impossible).
EOPNOTSUPP: The socket type does not support accepting connections.
EPROTO: Generic protocol error.

connect

connect :: (Address a, Type t, Protocol p) => Socket a t p -> a -> IO () Source

Connects to an remote address.

Calling connect on a closed socket throws EBADF even if the former file descriptor has been reassigned.
This function returns as soon as a connection has either been established or refused. A failed connection attempt does not throw an exception if EINTR or EINPROGRESS were caught internally. The operation just unblocks and returns in this case. The approach is to just try to read or write the socket and eventually fail there instead. Also see these considerations for an explanation. EINTR and EINPROGRESS are handled internally and won't be thrown.
The following SocketExceptions are relevant and might be thrown if the OS was able to decide the connection request synchronously:

EADDRNOTAVAIL: The address is not available.
EBADF: The file descriptor is invalid.
ECONNREFUSED: The target was not listening or refused the connection.
EISCONN: The socket is already connected.
ENETUNREACH: The network is unreachable.
ETIMEDOUT: The connect timed out before a connection was established.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EAFNOTSUPPORT: Address family does not match the socket.
ENOTSOCK: The descriptor is not a socket.
EPROTOTYPE: The address type does not match the socket.

send

send :: (Address a, Type t, Protocol p) => Socket a t p -> ByteString -> MsgFlags -> IO Int Source

Send a message on a connected socket.

Calling send on a closed socket throws EBADF even if the former file descriptor has been reassigned.
The operation returns the number of bytes sent.
EAGAIN, EWOULDBLOCK and EINTR and handled internally and won't be thrown.
The flag MSG_NOSIGNAL is set to supress signals which are pointless.
The following SocketExceptions are relevant and might be thrown:

EBADF: The file descriptor is invalid.
ECONNRESET: The peer forcibly closed the connection.
EDESTADDREQ: Remote address has not been set, but is required.
EMSGSIZE: The message is too large to be sent all at once, but the protocol requires this.
ENOTCONN: The socket is not connected.
EPIPE: The socket is shut down for writing or the socket is not connected anymore.
EACCESS: The process is lacking permissions.
EIO: An I/O error occured while writing to the filesystem.
ENETDOWN: The local network interface is down.
ENETUNREACH: No route to network.
ENOBUFS: Insufficient resources to fulfill the request.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EOPNOTSUPP: The specified flags are not supported.
ENOTSOCK: The descriptor does not refer to a socket.

sendTo

sendTo :: (Address a, Type t, Protocol p) => Socket a t p -> ByteString -> MsgFlags -> a -> IO Int Source

Send a message on a socket with a specific destination address.

Calling sendTo on a closed socket throws EBADF even if the former file descriptor has been reassigned.
The operation returns the number of bytes sent.
EAGAIN, EWOULDBLOCK and EINTR and handled internally and won't be thrown.
The flag MSG_NOSIGNAL is set to supress signals which are pointless.
The following SocketExceptions are relevant and might be thrown:

EBADF: The file descriptor is invalid.
ECONNRESET: The peer forcibly closed the connection.
EDESTADDREQ: Remote address has not been set, but is required.
EMSGSIZE: The message is too large to be sent all at once, but the protocol requires this.
ENOTCONN: The socket is not connected.
EPIPE: The socket is shut down for writing or the socket is not connected anymore.
EACCESS: The process is lacking permissions.
EDESTADDRREQ: The destination address is required.
EHOSTUNREACH: The destination host cannot be reached.
EIO: An I/O error occured.
EISCONN: The socket is already connected.
ENETDOWN: The local network is down.
ENETUNREACH: No route to the network.
ENUBUFS: Insufficient resources to fulfill the request.
ENOMEM: Insufficient memory to fulfill the request.
ELOOP: AF_UNIX only.
ENAMETOOLONG: AF_UNIX only.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EAFNOTSUPP: The address family does not match.
EOPNOTSUPP: The specified flags are not supported.
ENOTSOCK: The descriptor does not refer to a socket.
EINVAL: The address len does not match.

recv

recv :: (Address a, Type t, Protocol p) => Socket a t p -> Int -> MsgFlags -> IO ByteString Source

Receive a message on a connected socket.

Calling recv on a closed socket throws EBADF even if the former file descriptor has been reassigned.
The operation takes a buffer size in bytes a first parameter which limits the maximum length of the returned ByteString.
EAGAIN, EWOULDBLOCK and EINTR and handled internally and won't be thrown.
The following SocketExceptions are relevant and might be thrown:

EBADF: The file descriptor is invalid.
ECONNRESET: The peer forcibly closed the connection.
ENOTCONN: The socket is not connected.
ETIMEDOUT: The connection timed out.
EIO: An I/O error occured while writing to the filesystem.
ENOBUFS: Insufficient resources to fulfill the request.
ENONMEM: Insufficient memory to fulfill the request.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EOPNOTSUPP: The specified flags are not supported.
ENOTSOCK: The descriptor does not refer to a socket.

recvFrom

recvFrom :: forall a t p. (Address a, Type t, Protocol p) => Socket a t p -> Int -> MsgFlags -> IO (ByteString, a) Source

Receive a message on a socket and additionally yield the peer address.

Calling recvFrom on a closed socket throws EBADF even if the former file descriptor has been reassigned.
The operation takes a buffer size in bytes a first parameter which limits the maximum length of the returned ByteString.
EAGAIN, EWOULDBLOCK and EINTR and handled internally and won't be thrown.
The following SocketExceptions are relevant and might be thrown:

EBADF: The file descriptor is invalid.
ECONNRESET: The peer forcibly closed the connection.
ENOTCONN: The socket is not connected.
ETIMEDOUT: The connection timed out.
EIO: An I/O error occured while writing to the filesystem.
ENOBUFS: Insufficient resources to fulfill the request.
ENONMEM: Insufficient memory to fulfill the request.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EOPNOTSUPP: The specified flags are not supported.
ENOTSOCK: The descriptor does not refer to a socket.

close

close :: (Address a, Type t, Protocol p) => Socket a t p -> IO () Source

Closes a socket.

This operation is idempotent and thus can be performed more than once without throwing an exception. If it throws an exception it is presumably a not recoverable situation and the process should exit.
This operation does not block.
This operation wakes up all threads that are currently blocking on this socket. All other threads are guaranteed not to block on operations on this socket in the future. Threads that perform operations other than close on this socket will fail with EBADF after the socket has been closed (close replaces the Fd in the MVar with -1 to reliably avoid use-after-free situations).
The following SocketExceptions are relevant and might be thrown:

EIO: An I/O error occured while writing to the filesystem.

The following SocketExceptions are theoretically possible, but should not occur if the library is correct:

EBADF: The file descriptor is invalid.

Sockets

data Socket d t p Source

A generic socket type. Also see socket for details.

The socket is just an MVar-wrapped file descriptor. It is exposed in order to make this library easily extensible, but it is usually not necessary nor advised to work directly on the file descriptor. If you do, the following rules must be obeyed:

Make sure not to deadlock. Use withMVar or similar.
The lock must not be held during a blocking call. This would make it impossible to send and receive simultaneously or to close the socket.
The lock must be held when calling operations that use the file descriptor. Otherwise the socket might get closed or even reused by another thread/capability which might result in reading from or writing totally different connection. This is a security nightmare!
The socket is non-blocking and all the code relies on that assumption. You need to use GHC's eventing mechanism primitives to block until something happens. The former rules forbid to use threadWaitRead as it does not seperate between registering the file descriptor (for which the lock must be held) and the actual waiting (for which you must not hold the lock). Also see this thread and read the library code to see how the problem is currently circumvented.