UDPSocket represents a UDP/IP socket.

Class Methods

Creates a new UDPSocket object.

address_family should be an integer, a string or a symbol: Socket::AF_INET, “AF_INET”, :INET, etc.

require 'socket'                   #=> #<UDPSocket:fd 3> #=> #<UDPSocket:fd 4>
Instance Methods

Binds udpsocket to host:port.

u1 =
u1.bind("", 4913)
u1.send "message-to-self", 0, "", 4913
p u1.recvfrom(10) #=> ["message-to", ["AF_INET", 4913, "localhost", ""]]

Connects udpsocket to host:port.

This makes possible to send without destination address.

u1 =
u1.bind("", 4913)
u2 =
u2.connect("", 4913)
u2.send "uuuu", 0
p u1.recvfrom(10) #=> ["uuuu", ["AF_INET", 33230, "localhost", ""]]

Receives up to maxlen bytes from udpsocket using recvfrom(2) after O_NONBLOCK is set for the underlying file descriptor. flags is zero or more of the MSG_ options. The first element of the results, mesg, is the data received. The second element, sender_inet_addr, is an array to represent the sender address.

When recvfrom(2) returns 0, Socket#recvfrom_nonblock returns an empty string as data. It means an empty packet.


  • maxlen - the number of bytes to receive from the socket

  • flags - zero or more of the MSG_ options

  • outbuf - destination String buffer

  • options - keyword hash, supporting ‘exception: false`


require 'socket'
s1 =
s1.bind("", 0)
s2 =
s2.bind("", 0)
s1.send "aaa", 0
begin # emulate blocking recvfrom
  p s2.recvfrom_nonblock(10)  #=> ["aaa", ["AF_INET", 33302, "localhost.localdomain", ""]]
rescue IO::WaitReadable[s2])

Refer to Socket#recvfrom for the exceptions that may be thrown if the call to recvfrom_nonblock fails.

UDPSocket#recvfrom_nonblock may raise any error corresponding to recvfrom(2) failure, including Errno::EWOULDBLOCK.

If the exception is Errno::EWOULDBLOCK or Errno::EAGAIN, it is extended by IO::WaitReadable. So IO::WaitReadable can be used to rescue the exceptions for retrying recvfrom_nonblock.

By specifying a keyword argument exception to false, you can indicate that recvfrom_nonblock should not raise an IO::WaitReadable exception, but return the symbol :wait_readable instead.


Sends mesg via udpsocket.

flags should be a bitwise OR of Socket::MSG_* constants.

u1 =
u1.bind("", 4913)

u2 =
u2.send "hi", 0, "", 4913

mesg, addr = u1.recvfrom(10)
u1.send mesg, 0, addr[3], addr[1]

p u2.recv(100) #=> "hi"