UDP(7)			   Linux Programmers Manual		       UDP(7)



NAME
       udp - User Datagram Protocol for IPv4

SYNOPSIS
       #include 
       #include 

       udp_socket = socket(PF_INET, SOCK_DGRAM, 0);

DESCRIPTION
       This  is  an  implementation of the User Datagram Protocol described in
       RFC 768.  It implements a connectionless,  unreliable  datagram	packet
       service.   Packets  may	be reordered or duplicated before they arrive.
       UDP generates and checks checksums to catch transmission errors.

       When a UDP socket is  created,  its  local  and	remote	addresses  are
       unspecified.   Datagrams  can  be  sent	immediately using sendto(2) or
       sendmsg(2) with a valid destination address as an argument.  When  con
       nect(2)	is called on the socket the default destination address is set
       and datagrams can now be sent using send(2) or write(2) without	speci
       fying  a  destination  address.	 It is still possible to send to other
       destinations by passing an address  to  sendto(2)  or  sendmsg(2).   In
       order  to  receive  packets  the socket can be bound to a local address
       first by using bind(2).	Otherwise the socket layer will  automatically
       assign	a   free   local   port   out	of   the   range   defined  by
       net.ipv4.ip_local_port_range and bind the socket to INADDR_ANY.

       All receive operations return only one  packet.	 When  the  packet  is
       smaller than the passed buffer only that much data is returned, when it
       is bigger the packet is	truncated  and	the  MSG_TRUNC	flag  is  set.
       MSG_WAITALL is not supported.

       IP  options  may be sent or received using the socket options described
       in ip(7).  They are only processed by the kernel when  the  appropriate
       sysctl  is enabled (but still passed to the user even when it is turned
       off).  See ip(7).

       When the MSG_DONTROUTE flag is set on sending the  destination  address
       must  refer to a local interface address and the packet is only sent to
       that interface.

       By default Linux UDP does path MTU (Maximum Transmission Unit)  discov
       ery.   This  means  the kernel will keep track of the MTU to a specific
       target IP address and return EMSGSIZE when a UDP packet	write  exceeds
       it.  When this happens the application should decrease the packet size.
       Path MTU discovery can be also turned  off  using  the  IP_MTU_DISCOVER
       socket  option  or  the	ip_no_pmtu_disc sysctl, see ip(7) for details.
       When turned off UDP will fragment outgoing UDP packets that exceed  the
       interface MTU.  However disabling it is not recommended for performance
       and reliability reasons.

   Address Format
       UDP uses the IPv4 sockaddr_in address format described in ip(7).

   Error Handling
       All fatal errors will be passed to the user as  an  error  return  even
       when  the  socket  is not connected.  This includes asynchronous errors
       received from the network.  You may get an error for an earlier	packet
       that  was  sent	on  the  same socket.  This behavior differs from many
       other BSD socket implementations which dont pass any errors unless the
       socket is connected.  Linuxs behavior is mandated by RFC 1122.

       For compatibility with legacy code in Linux 2.0 and 2.2 it was possible
       to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote errors only
       when  the  socket  has been connected (except for EPROTO and EMSGSIZE).
       Locally generated errors are always passed.  Support  for  this	socket
       option was removed in later kernels; see socket(7) for further informa
       tion.

       When the IP_RECVERR option is enabled all  errors  are  stored  in  the
       socket  error  queue  and  can  be  received  by  recvmsg(2)  with  the
       MSG_ERRQUEUE flag set.

   Socket Options
       To set or get a UDP socket option, call getsockopt(2) to read  or  set
       sockopt(2)  to  write  the option with the option level argument set to
       IPPROTO_UDP.

       UDP_CORK (since Linux 2.5.44)
	      If this option is enabled, then all data output on  this	socket
	      is  accumulated  into a single datagram that is transmitted when
	      the option is disabled.  This option should not be used in  code
	      intended to be portable.

   Ioctls
       These ioctls can be accessed using ioctl(2).  The correct syntax is:

	      int value;
	      error = ioctl(udp_socket, ioctl_type, &value);

       FIONREAD (SIOCINQ)
	      Gets  a  pointer to an integer as argument.  Returns the size of
	      the next pending datagram in the integer in bytes, or 0 when  no
	      datagram is pending.

       TIOCOUTQ (SIOCOUTQ)
	      Returns  the number of data bytes in the local send queue.  Only
	      supported with Linux 2.4 and above.

       In addition all ioctls documented in ip(7) and socket(7) are supported.

ERRORS
       All  errors documented for socket(7) or ip(7) may be returned by a send
       or receive on a UDP socket.

       ECONNREFUSED No receiver was associated with the  destination  address.
       This might be caused by a previous packet sent over the socket.

VERSIONS
       IP_RECVERR is a new feature in Linux 2.2.

SEE ALSO
       ip(7), raw(7), socket(7)

       RFC 768 for the User Datagram Protocol.
       RFC 1122 for the host requirements.
       RFC 1191 for a description of path MTU discovery.

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  1998-10-02				UDP(7)