1UDP(7) Linux Programmer's Manual UDP(7)
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6 udp - User Datagram Protocol for IPv4
7
9 #include <sys/socket.h>
10 #include <netinet/in.h>
11 udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
13
15 This is an implementation of the User Datagram Protocol described in
16 RFC 768. It implements a connectionless, unreliable datagram packet
17 service. Packets may be reordered or duplicated before they arrive.
18 UDP generates and checks checksums to catch transmission errors.
19 When a UDP socket is created, its local and remote addresses are
21 unspecified. Datagrams can be sent immediately using sendto(2) or
22 sendmsg(2) with a valid destination address as an argument. When con‐
23 nect(2) is called on the socket, the default destination address is set
24 and datagrams can now be sent using send(2) or write(2) without speci‐
25 fying a destination address. It is still possible to send to other
26 destinations by passing an address to sendto(2) or sendmsg(2). In
27 order to receive packets, the socket can be bound to a local address
28 first by using bind(2). Otherwise the socket layer will automatically
29 assign a free local port out of the range defined by
30 net.ipv4.ip_local_port_range and bind the socket to INADDR_ANY.
31 All receive operations return only one packet. When the packet is
33 smaller than the passed buffer, only that much data is returned; when
34 it is bigger, the packet is truncated and the MSG_TRUNC flag is set.
35 MSG_WAITALL is not supported.
36 IP options may be sent or received using the socket options described
38 in ip(7). They are only processed by the kernel when the appropriate
39 /proc parameter is enabled (but still passed to the user even when it
40 is turned off). See ip(7).
41 When the MSG_DONTROUTE flag is set on sending, the destination address
43 must refer to a local interface address and the packet is only sent to
44 that interface.
45 By default, Linux UDP does path MTU (Maximum Transmission Unit) discov‐
47 ery. This means the kernel will keep track of the MTU to a specific
48 target IP address and return EMSGSIZE when a UDP packet write exceeds
49 it. When this happens, the application should decrease the packet
50 size. Path MTU discovery can be also turned off using the IP_MTU_DIS‐
51 COVER socket option or the /proc/sys/net/ipv4/ip_no_pmtu_disc file; see
52 ip(7) for details. When turned off, UDP will fragment outgoing UDP
53 packets that exceed the interface MTU. However, disabling it is not
54 recommended for performance and reliability reasons.
55 Address Format
57 UDP uses the IPv4 sockaddr_in address format described in ip(7).
58 Error Handling
60 All fatal errors will be passed to the user as an error return even
61 when the socket is not connected. This includes asynchronous errors
62 received from the network. You may get an error for an earlier packet
63 that was sent on the same socket. This behavior differs from many
64 other BSD socket implementations which don't pass any errors unless the
65 socket is connected. Linux's behavior is mandated by RFC 1122.
66 For compatibility with legacy code, in Linux 2.0 and 2.2 it was possi‐
68 ble to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote errors
69 only when the socket has been connected (except for EPROTO and EMSG‐
70 SIZE). Locally generated errors are always passed. Support for this
71 socket option was removed in later kernels; see socket(7) for further
72 information.
73 When the IP_RECVERR option is enabled, all errors are stored in the
75 socket error queue, and can be received by recvmsg(2) with the
76 MSG_ERRQUEUE flag set.
77 /proc interfaces
79 System-wide UDP parameter settings can be accessed by files in the
80 directory /proc/sys/net/ipv4/.
81 udp_mem (since Linux 2.6.25)
83 This is a vector of three integers governing the number of pages
84 allowed for queueing by all UDP sockets.
85 min Below this number of pages, UDP is not bothered about
87 its memory appetite. When the amount of memory allo‐
88 cated by UDP exceeds this number, UDP starts to moder‐
89 ate memory usage.
90 pressure This value was introduced to follow the format of
92 tcp_mem (see tcp(7)).
93 max Number of pages allowed for queueing by all UDP sock‐
95 ets.
96 Defaults values for these three items are calculated at boot
98 time from the amount of available memory.
99 udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
101 Minimal size, in bites, of receive buffer used by UDP sockets in
102 moderation. Each UDP socket is able to use the size for receiv‐
103 ing data, even if total pages of UDP sockets exceed udp_mem
104 pressure.
105 udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
107 Minimal size, in bytes, of send buffer used by UDP sockets in
108 moderation. Each UDP socket is able to use the size for sending
109 data, even if total pages of UDP sockets exceed udp_mem pres‐
110 sure.
111 Socket Options
113 To set or get a UDP socket option, call getsockopt(2) to read or set‐
114 sockopt(2) to write the option with the option level argument set to
115 IPPROTO_UDP.
116 UDP_CORK (since Linux 2.5.44)
118 If this option is enabled, then all data output on this socket
119 is accumulated into a single datagram that is transmitted when
120 the option is disabled. This option should not be used in code
121 intended to be portable.
122 Ioctls
124 These ioctls can be accessed using ioctl(2). The correct syntax is:
125 int value;
127 error = ioctl(udp_socket, ioctl_type, &value);
128 FIONREAD (SIOCINQ)
130 Gets a pointer to an integer as argument. Returns the size of
131 the next pending datagram in the integer in bytes, or 0 when no
132 datagram is pending.
133 TIOCOUTQ (SIOCOUTQ)
135 Returns the number of data bytes in the local send queue. Only
136 supported with Linux 2.4 and above.
137 In addition all ioctls documented in ip(7) and socket(7) are supported.
139
141 All errors documented for socket(7) or ip(7) may be returned by a send
142 or receive on a UDP socket.
143 ECONNREFUSED
145 No receiver was associated with the destination address. This
146 might be caused by a previous packet sent over the socket.
147
149 IP_RECVERR is a new feature in Linux 2.2.
150
152 ip(7), raw(7), socket(7), udplite(7)
153 RFC 768 for the User Datagram Protocol.
155 RFC 1122 for the host requirements.
156 RFC 1191 for a description of path MTU discovery.
157
159 This page is part of release 3.22 of the Linux man-pages project. A
160 description of the project, and information about reporting bugs, can
161 be found at http://www.kernel.org/doc/man-pages/.
162Linux 2008-11-21 UDP(7)