Originální popis anglicky:
udp - User Datagram Protocol for IPv4
Návod, kniha: Linux Programmer's Manual
#include <sys/socket.h>
#include <netinet/in.h>
udp_socket = socket(PF_INET, SOCK_DGRAM, 0);
This is an implemention of the User Datagram Protocol described in RFC768. 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 an
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 an 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 an local interface address and the packet is only sent to that
interface.
UDP fragments a packet when its total length exceeds the interface MTU (Maximum
Transmission Unit). A more network friendly alternative is to use path MTU
discovery as described in the
IP_MTU_DISCOVER section of
ip(7).
UDP uses the IPv4
sockaddr_in address format described in
ip(7).
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 behaviour differs from many other BSD socket implementations
which don't pass any errors unless the socket is connected. Linux's behaviour
is mandated by
RFC1122.
For compatibility with legacy code it is 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). It is better to fix
the code to handle errors properly than to enable this option. Locally
generated errors are always passed.
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.
These ioctls can be accessed using
ioctl(2). The correct syntax is:
int value;
error = ioctl(tcp_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.
IP_RECVERR is a new feature in Linux 2.2.
This man page was written by Andi Kleen.
ip(7),
raw(7),
socket(7)
RFC768 for the User Datagram Protocol.
RFC1122 for the host requirements.
RFC1191 for a description of path MTU discovery.
