7:udp

From Linux Man Pages

      udp - User Datagram Protocol for IPv4
      

Contents 1 SYNOPSIS 2 DESCRIPTION 3 ADDRESS FORMAT 4 ERROR HANDLING 5 SOCKET OPTIONS 6 IOCTLS 7 ERRORS 8 VERSIONS 9 CREDITS 10 RELATED 11 CATEGORY

SYNOPSIS

      #include <sys/socket.h>
      #include <netinet/in.h>
      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 connect(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
      specifying an 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 an 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 an local interface address
      and the packet is only sent to that interface.
 
      UDP fragments a packet when its total length exceeds the interface MTU (Maximum Transmission Unit).  A more  net-
      work friendly alternative is to use path MTU discovery as described in the IP_MTU_DISCOVER section of ip(7).

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  behaviour  differs from many other BSD socket implementations which don't pass any
      errors unless the socket is connected.  Linux's behaviour 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 gen-
      erated errors are always passed.  Support for this socket option was removed in later kernels; see socket(7)  for
      further information.
 
      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 setsockopt(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.

CREDITS

      This man page was written by Andi Kleen.

RELATED

      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.

CATEGORY