Provided by: manpages_3.35-0.1ubuntu1_all bug

NAME

       udp - User Datagram Protocol for IPv4

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>

       udp_socket = socket(AF_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 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
       /proc/sys/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 /proc parameter 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) discovery.  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 /proc/sys/net/ipv4/ip_no_pmtu_disc file; 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 don't pass any errors  unless  the  socket  is
       connected.  Linux's 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
       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.

   /proc interfaces
       System-wide  UDP  parameter  settings  can  be  accessed  by  files   in   the   directory
       /proc/sys/net/ipv4/.

       udp_mem (since Linux 2.6.25)
              This  is  a  vector  of  three  integers  governing the number of pages allowed for
              queueing by all UDP sockets.

              min       Below this number  of  pages,  UDP  is  not  bothered  about  its  memory
                        appetite.   When  the  amount  of  memory  allocated  by UDP exceeds this
                        number, UDP starts to moderate memory usage.

              pressure  This value was introduced to follow the format of tcp_mem (see tcp(7)).

              max       Number of pages allowed for queueing by all UDP sockets.

              Defaults values for these three items are calculated at boot time from  the  amount
              of available memory.

       udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
              Minimal size, in bytes, of receive buffers used by UDP sockets in moderation.  Each
              UDP socket is able to use the size for receiving data, even if total pages  of  UDP
              sockets exceed udp_mem pressure.

       udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
              Minimal size, in bytes, of send buffer used by UDP sockets in moderation.  Each UDP
              socket is able to use the size for sending data, even if total pages of UDP sockets
              exceed udp_mem pressure.

   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.  Warning: Using
              FIONREAD, it is impossible to distinguish the case where  no  datagram  is  pending
              from  the  case where the next pending datagram contains zero bytes of data.  It is
              safer to use select(2), poll(2), or epoll(7) to distinguish these cases.

       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), udplite(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 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.