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     udp — Internet User Datagram Protocol


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

     socket(AF_INET, SOCK_DGRAM, 0);


     UDP is a simple, unreliable datagram protocol which is used to support the SOCK_DGRAM
     abstraction for the Internet protocol family.  UDP sockets are connectionless, and are
     normally used with the sendto(2) and recvfrom(2) calls, though the connect(2) call may also
     be used to fix the destination for future packets (in which case the recv(2) or read(2) and
     send(2) or write(2) system calls may be used).

     UDP address formats are identical to those used by TCP.  In particular UDP provides a port
     identifier in addition to the normal Internet address format.  Note that the UDP port space
     is separate from the TCP port space (i.e., a UDP port may not be “connected” to a TCP port).
     In addition broadcast packets may be sent (assuming the underlying network supports this) by
     using a reserved “broadcast address”; this address is network interface dependent.

     Options at the IP transport level may be used with UDP; see ip(4).  UDP_ENCAP socket option
     may be used at the IPPROTO_UDP level to encapsulate ESP packets in UDP.  Only one value is
     supported for this option: UDP_ENCAP_ESPINUDP from RFC 3948, defined in <netinet/udp.h>.


     The udp protocol implements a number of variables in the net.inet branch of the sysctl(3)

     UDPCTL_CHECKSUM    (udp.checksum) Enable udp checksums (enabled by default).

     UDPCTL_MAXDGRAM    (udp.maxdgram) Maximum outgoing UDP datagram size

     UDPCTL_RECVSPACE   (udp.recvspace) Maximum space for incoming UDP datagrams

     udp.log_in_vain    For all udp datagrams, to ports on which there is no socket listening,
                        log the connection attempt (disabled by default).

     udp.blackhole      When a datagram is received on a port where there is no socket listening,
                        do not return an ICMP port unreachable message.  (Disabled by default.
                        See blackhole(4).)


     A socket operation may fail with one of the following errors returned:

     [EISCONN]          when trying to establish a connection on a socket which already has one,
                        or when trying to send a datagram with the destination address specified
                        and the socket is already connected;

     [ENOTCONN]         when trying to send a datagram, but no destination address is specified,
                        and the socket has not been connected;

     [ENOBUFS]          when the system runs out of memory for an internal data structure;

     [EADDRINUSE]       when an attempt is made to create a socket with a port which has already
                        been allocated;

     [EADDRNOTAVAIL]    when an attempt is made to create a socket with a network address for
                        which no network interface exists.


     getsockopt(2), recv(2), send(2), socket(2), blackhole(4), inet(4), intro(4), ip(4),


     The udp protocol appeared in 4.2BSD.