udp — User Datagram Protocol for IPv4
#include <sys/socket.h> #include <netinet/in.h>
udp_socket = socket( |
PF_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 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 sysctl 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 ip_no_pmtu_disc sysctl, 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.
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.
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.
These ioctls can be accessed using ioctl(2). The correct syntax is:
intvalue;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.
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.
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.
RFC 768 for the User Datagram Protocol.
RFC 1122 for the host requirements.
RFC 1191 for a description of path MTU discovery.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $ |