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 /proc/sys/net/ipv4/ip_local_port_range and bind the socket to
31 INADDR_ANY.
32 All receive operations return only one packet. When the packet is
34 smaller than the passed buffer, only that much data is returned; when
35 it is bigger, the packet is truncated and the MSG_TRUNC flag is set.
36 MSG_WAITALL is not supported.
37 IP options may be sent or received using the socket options described
39 in ip(7). They are only processed by the kernel when the appropriate
40 /proc parameter is enabled (but still passed to the user even when it
41 is turned off). See ip(7).
42 When the MSG_DONTROUTE flag is set on sending, the destination address
44 must refer to a local interface address and the packet is only sent to
45 that interface.
46 By default, Linux UDP does path MTU (Maximum Transmission Unit) discov‐
48 ery. This means the kernel will keep track of the MTU to a specific
49 target IP address and return EMSGSIZE when a UDP packet write exceeds
50 it. When this happens, the application should decrease the packet
51 size. Path MTU discovery can be also turned off using the IP_MTU_DIS‐
52 COVER socket option or the /proc/sys/net/ipv4/ip_no_pmtu_disc file; see
53 ip(7) for details. When turned off, UDP will fragment outgoing UDP
54 packets that exceed the interface MTU. However, disabling it is not
55 recommended for performance and reliability reasons.
56 Address Format
58 UDP uses the IPv4 sockaddr_in address format described in ip(7).
59 Error Handling
61 All fatal errors will be passed to the user as an error return even
62 when the socket is not connected. This includes asynchronous errors
63 received from the network. You may get an error for an earlier packet
64 that was sent on the same socket. This behavior differs from many
65 other BSD socket implementations which don't pass any errors unless the
66 socket is connected. Linux's behavior is mandated by RFC 1122.
67 For compatibility with legacy code, in Linux 2.0 and 2.2 it was possi‐
69 ble to set the SO_BSDCOMPAT SOL_SOCKET option to receive remote errors
70 only when the socket has been connected (except for EPROTO and EMSG‐
71 SIZE). Locally generated errors are always passed. Support for this
72 socket option was removed in later kernels; see socket(7) for further
73 information.
74 When the IP_RECVERR option is enabled, all errors are stored in the
76 socket error queue, and can be received by recvmsg(2) with the
77 MSG_ERRQUEUE flag set.
78 /proc interfaces
80 System-wide UDP parameter settings can be accessed by files in the
81 directory /proc/sys/net/ipv4/.
82 udp_mem (since Linux 2.6.25)
84 This is a vector of three integers governing the number of pages
85 allowed for queueing by all UDP sockets.
86 min Below this number of pages, UDP is not bothered about
88 its memory appetite. When the amount of memory allo‐
89 cated by UDP exceeds this number, UDP starts to moder‐
90 ate memory usage.
91 pressure This value was introduced to follow the format of
93 tcp_mem (see tcp(7)).
94 max Number of pages allowed for queueing by all UDP sock‐
96 ets.
97 Defaults values for these three items are calculated at boot
99 time from the amount of available memory.
100 udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
102 Minimal size, in bytes, of receive buffers used by UDP sockets
103 in moderation. Each UDP socket is able to use the size for
104 receiving data, even if total pages of UDP sockets exceed
105 udp_mem pressure.
106 udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
108 Minimal size, in bytes, of send buffer used by UDP sockets in
109 moderation. Each UDP socket is able to use the size for sending
110 data, even if total pages of UDP sockets exceed udp_mem pres‐
111 sure.
112 Socket Options
114 To set or get a UDP socket option, call getsockopt(2) to read or set‐
115 sockopt(2) to write the option with the option level argument set to
116 IPPROTO_UDP.
117 UDP_CORK (since Linux 2.5.44)
119 If this option is enabled, then all data output on this socket
120 is accumulated into a single datagram that is transmitted when
121 the option is disabled. This option should not be used in code
122 intended to be portable.
123 Ioctls
125 These ioctls can be accessed using ioctl(2). The correct syntax is:
126 int value;
128 error = ioctl(udp_socket, ioctl_type, &value);
129 FIONREAD (SIOCINQ)
131 Gets a pointer to an integer as argument. Returns the size of
132 the next pending datagram in the integer in bytes, or 0 when no
133 datagram is pending. Warning: Using FIONREAD, it is impossible
134 to distinguish the case where no datagram is pending from the
135 case where the next pending datagram contains zero bytes of
136 data. It is safer to use select(2), poll(2), or epoll(7) to
137 distinguish these cases.
138 TIOCOUTQ (SIOCOUTQ)
140 Returns the number of data bytes in the local send queue. Only
141 supported with Linux 2.4 and above.
142 In addition all ioctls documented in ip(7) and socket(7) are supported.
144
146 All errors documented for socket(7) or ip(7) may be returned by a send
147 or receive on a UDP socket.
148 ECONNREFUSED
150 No receiver was associated with the destination address. This
151 might be caused by a previous packet sent over the socket.
152
154 IP_RECVERR is a new feature in Linux 2.2.
155
157 ip(7), raw(7), socket(7), udplite(7)
158 RFC 768 for the User Datagram Protocol.
160 RFC 1122 for the host requirements.
161 RFC 1191 for a description of path MTU discovery.
162
164 This page is part of release 3.25 of the Linux man-pages project. A
165 description of the project, and information about reporting bugs, can
166 be found at http://www.kernel.org/doc/man-pages/.
167Linux 2010-06-13 UDP(7)