.TH UDP 4P "25 March 1982"
.UC
.SH NAME
udp \- Internet User Datagram Protocol
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <netinet/in.h>
.PP
.B s = socket(AF_INET, SOCK_DGRAM, 0);
.SH DESCRIPTION
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
.I sendto 
and
.IR recvfrom 
calls, though the
.IR connect (2)
call may also be used to fix the destination for future
packets (in which case the 
.IR recv (2)
or
.IR read (2)
and 
.IR send (2)
or
.IR write(2)
system calls may be used).
.PP
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 \*(lqconnected\*(rq to a TCP port).  In addition broadcast
packets may be sent (assuming the underlying network supports
this) by using a reserved \*(lqbroadcast address\*(rq; this address
is network interface dependent.
.SH DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.TP 15
[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;
.TP 15
[ENOTCONN]
when trying to send a datagram, but
no destination address is specified, and the socket hasn't been
connected;
.TP 15
[ENOBUFS]
when the system runs out of memory for
an internal data structure;
.TP 15
[EADDRINUSE]
when an attempt
is made to create a socket with a port which has already been
allocated;
.TP 15
[EADDRNOTAVAIL]
when an attempt is made to create a 
socket with a network address for which no network interface
exists.
.SH SEE ALSO
send(2),
recv(2),
intro(4N),
inet(4F)