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udp - Internet User Datagram Protocol
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
Options at the IP transport level may be used with UDP; see ip(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
[ENOBUFS] when the system runs out of memory for an internal
[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.
The udp protocol implements a number of variables in the net.inet branch
of the sysctl(3) MIB.
UDPCTL_CHECKSUM (udp.checksum) Enable udp checksums (enabled by
UDPCTL_MAXDGRAM (udp.maxdgram) Maximum outgoing UDP datagram size
UDPCTL_RECVSPACE (udp.recvspace) Maximum space for incoming UDP
udp.log_in_vain For all udp datagrams, to ports on which there is no
socket listening, log the connection attempt (disabled
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
getsockopt(2), recv(2), send(2), socket(2), blackhole(4), inet(4),
The udp protocol appeared in 4.2BSD.