udp - User Datagram Protocol for IPv4


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

   udp_socket = socket(AF_INET, SOCK_DGRAM, 0);


   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

   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 processed by the kernel only 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 sent only  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

          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

   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.  Unless otherwise noted, optval is a pointer to an int.

   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.

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

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

          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.

          Returns the number of  data  bytes  in  the  local  send  queue.
          Supported only with Linux 2.4 and above.

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


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

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


   IP_RECVERR is a new feature in Linux 2.2.


   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.


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   description  of  the project, information about reporting bugs, and the
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