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NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | SEE ALSO | COLOPHON |
UDP(7) Linux Programmer's Manual UDP(7)
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
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 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
sockets.
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 pressure.
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.
Ioctls
These ioctls can be accessed using ioctl(2). The correct syntax is:
int value;
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. 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.
TIOCOUTQ (SIOCOUTQ)
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
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.
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.
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2013-07-31 UDP(7)
Pages that refer to this page: getsockopt(2), recv(2), send(2), socket(2), services(5), ip(7), socket(7), udplite(7), unix(7)