< UNIXSocket < BasicSocket < IO

Class Socket provides access to the underlying operating system socket implementations. It can be used to provide more operating system specific functionality than the protocol-specific socket classes but at the expense of greater complexity. In particular, the class handles addresses using +struct sockaddr+ structures packed into Ruby strings, which can be a joy to manipulate.

Exception Handling

Ruby‘s implementation of Socket causes an exception to be raised based on the error generated by the system dependent implementation. This is why the methods are documented in a way that isolate Unix-based system exceptions from Windows based exceptions. If more information on particular exception is needed please refer to the Unix manual pages or the Windows WinSock reference.

Documentation by

  • Zach Dennis
  • Sam Roberts
  • Programming Ruby from The Pragmatic Bookshelf.

Much material in this documentation is taken with permission from Programming Ruby from The Pragmatic Bookshelf.



Visibility Signature
public accept_nonblock ()
public listen (p1)

Instance Method Detail

unixserver.accept_nonblock => unixsocket

Accepts an incoming connection using accept(2) after O_NONBLOCK is set for the underlying file descriptor. It returns an accepted UNIXSocket for the incoming connection.


     require 'socket'
     serv ="/tmp/sock")
       sock = serv.accept_nonblock
     rescue Errno::EAGAIN, Errno::EWOULDBLOCK, Errno::ECONNABORTED, Errno::EPROTO, Errno::EINTR[serv])
     # sock is an accepted socket.

Refer to Socket#accept for the exceptions that may be thrown if the call to UNIXServer#accept_nonblock fails.

UNIXServer#accept_nonblock may raise any error corresponding to accept(2) failure, including Errno::EAGAIN.


socket.listen( int ) => 0

Listens for connections, using the specified int as the backlog. A call to listen only applies if the socket is of type SOCK_STREAM or SOCK_SEQPACKET.


  • backlog - the maximum length of the queue for pending connections.

Example 1

     require 'socket'
     include Socket::Constants
     socket = AF_INET, SOCK_STREAM, 0 )
     sockaddr = Socket.pack_sockaddr_in( 2200, 'localhost' )
     socket.bind( sockaddr )
     socket.listen( 5 )

Example 2 (listening on an arbitary port, unix-based systems only):

     require 'socket'
     include Socket::Constants
     socket = AF_INET, SOCK_STREAM, 0 )
     socket.listen( 1 )

Unix-based Exceptions

On unix based systems the above will work because a new sockaddr struct is created on the address ADDR_ANY, for an arbitrary port number as handed off by the kernel. It will not work on Windows, because Windows requires that the socket is bound by calling bind before it can listen.

If the backlog amount exceeds the implementation-dependent maximum queue length, the implementation‘s maximum queue length will be used.

On unix-based based systems the following system exceptions may be raised if the call to listen fails:

  • Errno::EBADF - the socket argument is not a valid file descriptor
  • Errno::EDESTADDRREQ - the socket is not bound to a local address, and the protocol does not support listening on an unbound socket
  • Errno::EINVAL - the socket is already connected
  • Errno::ENOTSOCK - the socket argument does not refer to a socket
  • Errno::EOPNOTSUPP - the socket protocol does not support listen
  • Errno::EACCES - the calling process does not have approriate privileges
  • Errno::EINVAL - the socket has been shut down
  • Errno::ENOBUFS - insufficient resources are available in the system to complete the call

Windows Exceptions

On Windows systems the following system exceptions may be raised if the call to listen fails:

  • Errno::ENETDOWN - the network is down
  • Errno::EADDRINUSE - the socket‘s local address is already in use. This usually occurs during the execution of bind but could be delayed if the call to bind was to a partially wildcard address (involving ADDR_ANY) and if a specific address needs to be commmitted at the time of the call to listen
  • Errno::EINPROGRESS - a Windows Sockets 1.1 call is in progress or the service provider is still processing a callback function
  • Errno::EINVAL - the socket has not been bound with a call to bind.
  • Errno::EISCONN - the socket is already connected
  • Errno::EMFILE - no more socket descriptors are available
  • Errno::ENOBUFS - no buffer space is available
  • Errno::ENOTSOC - socket is not a socket
  • Errno::EOPNOTSUPP - the referenced socket is not a type that supports the listen method


  • listen manual pages on unix-based systems
  • listen function in Microsoft‘s Winsock functions reference