1SOCKET(7) Linux Programmer's Manual SOCKET(7)
2
6 socket - Linux socket interface
7
9 #include <sys/socket.h>
10 sockfd = socket(int socket_family, int socket_type, int protocol);
12
14 This manual page describes the Linux networking socket layer user
15 interface. The BSD compatible sockets are the uniform interface
16 between the user process and the network protocol stacks in the kernel.
17 The protocol modules are grouped into protocol families like AF_INET,
18 AF_IPX, AF_PACKET and socket types like SOCK_STREAM or SOCK_DGRAM. See
19 socket(2) for more information on families and types.
20 Socket Layer Functions
22 These functions are used by the user process to send or receive packets
23 and to do other socket operations. For more information see their
24 respective manual pages.
25 socket(2) creates a socket, connect(2) connects a socket to a remote
27 socket address, the bind(2) function binds a socket to a local socket
28 address, listen(2) tells the socket that new connections shall be
29 accepted, and accept(2) is used to get a new socket with a new incoming
30 connection. socketpair(2) returns two connected anonymous sockets
31 (only implemented for a few local families like AF_UNIX)
32 send(2), sendto(2), and sendmsg(2) send data over a socket, and
34 recv(2), recvfrom(2), recvmsg(2) receive data from a socket. poll(2)
35 and select(2) wait for arriving data or a readiness to send data. In
36 addition, the standard I/O operations like write(2), writev(2), send‐
37 file(2), read(2), and readv(2) can be used to read and write data.
38 getsockname(2) returns the local socket address and getpeername(2)
40 returns the remote socket address. getsockopt(2) and setsockopt(2) are
41 used to set or get socket layer or protocol options. ioctl(2) can be
42 used to set or read some other options.
43 close(2) is used to close a socket. shutdown(2) closes parts of a
45 full-duplex socket connection.
46 Seeking, or calling pread(2) or pwrite(2) with a nonzero position is
48 not supported on sockets.
49 It is possible to do nonblocking I/O on sockets by setting the O_NON‐
51 BLOCK flag on a socket file descriptor using fcntl(2). Then all opera‐
52 tions that would block will (usually) return with EAGAIN (operation
53 should be retried later); connect(2) will return EINPROGRESS error.
54 The user can then wait for various events via poll(2) or select(2).
55 ┌────────────────────────────────────────────────────────────────────┐
57 │ I/O events │
58 ├───────────┬───────────┬────────────────────────────────────────────┤
59 │Event │ Poll flag │ Occurrence │
60 ├───────────┼───────────┼────────────────────────────────────────────┤
61 │Read │ POLLIN │ New data arrived. │
62 ├───────────┼───────────┼────────────────────────────────────────────┤
63 │Read │ POLLIN │ A connection setup has been completed (for │
64 │ │ │ connection-oriented sockets) │
65 ├───────────┼───────────┼────────────────────────────────────────────┤
66 │Read │ POLLHUP │ A disconnection request has been initiated │
67 │ │ │ by the other end. │
68 ├───────────┼───────────┼────────────────────────────────────────────┤
69 │Read │ POLLHUP │ A connection is broken (only for connec‐ │
70 │ │ │ tion-oriented protocols). When the socket │
71 │ │ │ is written SIGPIPE is also sent. │
72 ├───────────┼───────────┼────────────────────────────────────────────┤
73 │Write │ POLLOUT │ Socket has enough send buffer space for │
74 │ │ │ writing new data. │
75 ├───────────┼───────────┼────────────────────────────────────────────┤
76 │Read/Write │ POLLIN| │ An outgoing connect(2) finished. │
77 │ │ POLLOUT │ │
78 ├───────────┼───────────┼────────────────────────────────────────────┤
79 │Read/Write │ POLLERR │ An asynchronous error occurred. │
80 ├───────────┼───────────┼────────────────────────────────────────────┤
81 │Read/Write │ POLLHUP │ The other end has shut down one direction. │
82 ├───────────┼───────────┼────────────────────────────────────────────┤
83 │Exception │ POLLPRI │ Urgent data arrived. SIGURG is sent then. │
84 └───────────┴───────────┴────────────────────────────────────────────┘
85 An alternative to poll(2) and select(2) is to let the kernel inform the
87 application about events via a SIGIO signal. For that the O_ASYNC flag
88 must be set on a socket file descriptor via fcntl(2) and a valid signal
89 handler for SIGIO must be installed via sigaction(2). See the Signals
90 discussion below.
91 Socket Options
93 These socket options can be set by using setsockopt(2) and read with
94 getsockopt(2) with the socket level set to SOL_SOCKET for all sockets:
95 SO_ACCEPTCONN
97 Returns a value indicating whether or not this socket has been
98 marked to accept connections with listen(2). The value 0 indi‐
99 cates that this is not a listening socket, the value 1 indicates
100 that this is a listening socket. This socket option is read-
101 only.
102 SO_BINDTODEVICE
104 Bind this socket to a particular device like “eth0”, as speci‐
105 fied in the passed interface name. If the name is an empty
106 string or the option length is zero, the socket device binding
107 is removed. The passed option is a variable-length null-termi‐
108 nated interface name string with the maximum size of IFNAMSIZ.
109 If a socket is bound to an interface, only packets received from
110 that particular interface are processed by the socket. Note
111 that this only works for some socket types, particularly AF_INET
112 sockets. It is not supported for packet sockets (use normal
113 bind(8) there).
114 SO_BROADCAST
116 Set or get the broadcast flag. When enabled, datagram sockets
117 receive packets sent to a broadcast address and they are allowed
118 to send packets to a broadcast address. This option has no
119 effect on stream-oriented sockets.
120 SO_BSDCOMPAT
122 Enable BSD bug-to-bug compatibility. This is used by the UDP
123 protocol module in Linux 2.0 and 2.2. If enabled ICMP errors
124 received for a UDP socket will not be passed to the user pro‐
125 gram. In later kernel versions, support for this option has
126 been phased out: Linux 2.4 silently ignores it, and Linux 2.6
127 generates a kernel warning (printk()) if a program uses this
128 option. Linux 2.0 also enabled BSD bug-to-bug compatibility
129 options (random header changing, skipping of the broadcast flag)
130 for raw sockets with this option, but that was removed in Linux
131 2.2.
132 SO_DEBUG
134 Enable socket debugging. Only allowed for processes with the
135 CAP_NET_ADMIN capability or an effective user ID of 0.
136 SO_DOMAIN (since Linux 2.6.32)
138 Retrieves the socket domain as an integer, returning a value
139 such as AF_INET6. See socket(2) for details. This socket
140 option is read-only.
141 SO_ERROR
143 Get and clear the pending socket error. This socket option is
144 read-only. Expects an integer.
145 SO_DONTROUTE
147 Don't send via a gateway, only send to directly connected hosts.
148 The same effect can be achieved by setting the MSG_DONTROUTE
149 flag on a socket send(2) operation. Expects an integer boolean
150 flag.
151 SO_KEEPALIVE
153 Enable sending of keep-alive messages on connection-oriented
154 sockets. Expects an integer boolean flag.
155 SO_LINGER
157 Sets or gets the SO_LINGER option. The argument is a linger
158 structure.
159 struct linger {
161 int l_onoff; /* linger active */
162 int l_linger; /* how many seconds to linger for */
163 };
164 When enabled, a close(2) or shutdown(2) will not return until
166 all queued messages for the socket have been successfully sent
167 or the linger timeout has been reached. Otherwise, the call
168 returns immediately and the closing is done in the background.
169 When the socket is closed as part of exit(2), it always lingers
170 in the background.
171 SO_OOBINLINE
173 If this option is enabled, out-of-band data is directly placed
174 into the receive data stream. Otherwise out-of-band data is
175 only passed when the MSG_OOB flag is set during receiving.
176 SO_PASSCRED
178 Enable or disable the receiving of the SCM_CREDENTIALS control
179 message. For more information see unix(7).
180 SO_PEERCRED
182 Return the credentials of the foreign process connected to this
183 socket. This is only possible for connected AF_UNIX stream
184 sockets and AF_UNIX stream and datagram socket pairs created
185 using socketpair(2); see unix(7). The returned credentials are
186 those that were in effect at the time of the call to connect(2)
187 or socketpair(2). Argument is a ucred structure. This socket
188 option is read-only.
189 SO_PRIORITY
191 Set the protocol-defined priority for all packets to be sent on
192 this socket. Linux uses this value to order the networking
193 queues: packets with a higher priority may be processed first
194 depending on the selected device queueing discipline. For
195 ip(7), this also sets the IP type-of-service (TOS) field for
196 outgoing packets. Setting a priority outside the range 0 to 6
197 requires the CAP_NET_ADMIN capability.
198 SO_PROTOCOL (since Linux 2.6.32)
200 Retrieves the socket protocol as an integer, returning a value
201 such as IPPROTO_SCTP. See socket(2) for details. This socket
202 option is read-only.
203 SO_RCVBUF
205 Sets or gets the maximum socket receive buffer in bytes. The
206 kernel doubles this value (to allow space for bookkeeping over‐
207 head) when it is set using setsockopt(2), and this doubled value
208 is returned by getsockopt(2). The default value is set by the
209 /proc/sys/net/core/rmem_default file, and the maximum allowed
210 value is set by the /proc/sys/net/core/rmem_max file. The mini‐
211 mum (doubled) value for this option is 256.
212 SO_RCVBUFFORCE (since Linux 2.6.14)
214 Using this socket option, a privileged (CAP_NET_ADMIN) process
215 can perform the same task as SO_RCVBUF, but the rmem_max limit
216 can be overridden.
217 SO_RCVLOWAT and SO_SNDLOWAT
219 Specify the minimum number of bytes in the buffer until the
220 socket layer will pass the data to the protocol (SO_SNDLOWAT) or
221 the user on receiving (SO_RCVLOWAT). These two values are ini‐
222 tialized to 1. SO_SNDLOWAT is not changeable on Linux (setsock‐
223 opt(2) fails with the error ENOPROTOOPT). SO_RCVLOWAT is
224 changeable only since Linux 2.4. The select(2) and poll(2) sys‐
225 tem calls currently do not respect the SO_RCVLOWAT setting on
226 Linux, and mark a socket readable when even a single byte of
227 data is available. A subsequent read from the socket will block
228 until SO_RCVLOWAT bytes are available.
229 SO_RCVTIMEO and SO_SNDTIMEO
231 Specify the receiving or sending timeouts until reporting an
232 error. The argument is a struct timeval. If an input or output
233 function blocks for this period of time, and data has been sent
234 or received, the return value of that function will be the
235 amount of data transferred; if no data has been transferred and
236 the timeout has been reached then -1 is returned with errno set
237 to EAGAIN or EWOULDBLOCK just as if the socket was specified to
238 be nonblocking. If the timeout is set to zero (the default)
239 then the operation will never timeout. Timeouts only have
240 effect for system calls that perform socket I/O (e.g., read(2),
241 recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for
242 select(2), poll(2), epoll_wait(2), etc.
243 SO_REUSEADDR
245 Indicates that the rules used in validating addresses supplied
246 in a bind(2) call should allow reuse of local addresses. For
247 AF_INET sockets this means that a socket may bind, except when
248 there is an active listening socket bound to the address. When
249 the listening socket is bound to INADDR_ANY with a specific port
250 then it is not possible to bind to this port for any local
251 address. Argument is an integer boolean flag.
252 SO_SNDBUF
254 Sets or gets the maximum socket send buffer in bytes. The ker‐
255 nel doubles this value (to allow space for bookkeeping overhead)
256 when it is set using setsockopt(2), and this doubled value is
257 returned by getsockopt(2). The default value is set by the
258 /proc/sys/net/core/wmem_default file and the maximum allowed
259 value is set by the /proc/sys/net/core/wmem_max file. The mini‐
260 mum (doubled) value for this option is 2048.
261 SO_SNDBUFFORCE (since Linux 2.6.14)
263 Using this socket option, a privileged (CAP_NET_ADMIN) process
264 can perform the same task as SO_SNDBUF, but the wmem_max limit
265 can be overridden.
266 SO_TIMESTAMP
268 Enable or disable the receiving of the SO_TIMESTAMP control mes‐
269 sage. The timestamp control message is sent with level
270 SOL_SOCKET and the cmsg_data field is a struct timeval indicat‐
271 ing the reception time of the last packet passed to the user in
272 this call. See cmsg(3) for details on control messages.
273 SO_TYPE
275 Gets the socket type as an integer (e.g., SOCK_STREAM). This
276 socket option is read-only.
277 Signals
279 When writing onto a connection-oriented socket that has been shut down
280 (by the local or the remote end) SIGPIPE is sent to the writing process
281 and EPIPE is returned. The signal is not sent when the write call
282 specified the MSG_NOSIGNAL flag.
283 When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
285 is sent when an I/O event occurs. It is possible to use poll(2) or
286 select(2) in the signal handler to find out which socket the event
287 occurred on. An alternative (in Linux 2.2) is to set a real-time sig‐
288 nal using the F_SETSIG fcntl(2); the handler of the real time signal
289 will be called with the file descriptor in the si_fd field of its sig‐
290 info_t. See fcntl(2) for more information.
291 Under some circumstances (e.g., multiple processes accessing a single
293 socket), the condition that caused the SIGIO may have already disap‐
294 peared when the process reacts to the signal. If this happens, the
295 process should wait again because Linux will resend the signal later.
296 /proc interfaces
298 The core socket networking parameters can be accessed via files in the
299 directory /proc/sys/net/core/.
300 rmem_default
302 contains the default setting in bytes of the socket receive buf‐
303 fer.
304 rmem_max
306 contains the maximum socket receive buffer size in bytes which a
307 user may set by using the SO_RCVBUF socket option.
308 wmem_default
310 contains the default setting in bytes of the socket send buffer.
311 wmem_max
313 contains the maximum socket send buffer size in bytes which a
314 user may set by using the SO_SNDBUF socket option.
315 message_cost and message_burst
317 configure the token bucket filter used to load limit warning
318 messages caused by external network events.
319 netdev_max_backlog
321 Maximum number of packets in the global input queue.
322 optmem_max
324 Maximum length of ancillary data and user control data like the
325 iovecs per socket.
326 Ioctls
328 These operations can be accessed using ioctl(2):
329 error = ioctl(ip_socket, ioctl_type, &value_result);
331 SIOCGSTAMP
333 Return a struct timeval with the receive timestamp of the last
334 packet passed to the user. This is useful for accurate round
335 trip time measurements. See setitimer(2) for a description of
336 struct timeval. This ioctl should only be used if the socket
337 option SO_TIMESTAMP is not set on the socket. Otherwise, it
338 returns the timestamp of the last packet that was received while
339 SO_TIMESTAMP was not set, or it fails if no such packet has been
340 received, (i.e., ioctl(2) returns -1 with errno set to ENOENT).
341 SIOCSPGRP
343 Set the process or process group to send SIGIO or SIGURG signals
344 to when an asynchronous I/O operation has finished or urgent
345 data is available. The argument is a pointer to a pid_t. If
346 the argument is positive, send the signals to that process. If
347 the argument is negative, send the signals to the process group
348 with the ID of the absolute value of the argument. The process
349 may only choose itself or its own process group to receive sig‐
350 nals unless it has the CAP_KILL capability or an effective UID
351 of 0.
352 FIOASYNC
354 Change the O_ASYNC flag to enable or disable asynchronous I/O
355 mode of the socket. Asynchronous I/O mode means that the SIGIO
356 signal or the signal set with F_SETSIG is raised when a new I/O
357 event occurs.
358 Argument is an integer boolean flag. (This operation is synony‐
360 mous with the use of fcntl(2) to set the O_ASYNC flag.)
361 SIOCGPGRP
363 Get the current process or process group that receives SIGIO or
364 SIGURG signals, or 0 when none is set.
365 Valid fcntl(2) operations:
367 FIOGETOWN
369 The same as the SIOCGPGRP ioctl(2).
370 FIOSETOWN
372 The same as the SIOCSPGRP ioctl(2).
373
375 SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASSCRED is new in
376 Linux 2.2. The /proc interfaces was introduced in Linux 2.2. SO_RCV‐
377 TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41. Earlier, time‐
378 outs were fixed to a protocol-specific setting, and could not be read
379 or written.
380
382 Linux assumes that half of the send/receive buffer is used for internal
383 kernel structures; thus the values in the corresponding /proc files are
384 twice what can be observed on the wire.
385 Linux will only allow port reuse with the SO_REUSEADDR option when this
387 option was set both in the previous program that performed a bind(2) to
388 the port and in the program that wants to reuse the port. This differs
389 from some implementations (e.g., FreeBSD) where only the later program
390 needs to set the SO_REUSEADDR option. Typically this difference is
391 invisible, since, for example, a server program is designed to always
392 set this option.
393
395 The CONFIG_FILTER socket options SO_ATTACH_FILTER and SO_DETACH_FILTER
396 are not documented. The suggested interface to use them is via the
397 libpcap library.
398
400 getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7),
401 ip(7), packet(7), tcp(7), udp(7), unix(7)
402
404 This page is part of release 3.25 of the Linux man-pages project. A
405 description of the project, and information about reporting bugs, can
406 be found at http://www.kernel.org/doc/man-pages/.
407Linux 2010-06-13 SOCKET(7)