1 .\" Copyright (c) 1983, 1991 The Regents of the University of California.
2 .\" All rights reserved.
4 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
5 .\" Redistribution and use in source and binary forms, with or without
6 .\" modification, are permitted provided that the following conditions
8 .\" 1. Redistributions of source code must retain the above copyright
9 .\" notice, this list of conditions and the following disclaimer.
10 .\" 2. Redistributions in binary form must reproduce the above copyright
11 .\" notice, this list of conditions and the following disclaimer in the
12 .\" documentation and/or other materials provided with the distribution.
13 .\" 3. All advertising materials mentioning features or use of this software
14 .\" must display the following acknowledgement:
15 .\" This product includes software developed by the University of
16 .\" California, Berkeley and its contributors.
17 .\" 4. Neither the name of the University nor the names of its contributors
18 .\" may be used to endorse or promote products derived from this software
19 .\" without specific prior written permission.
21 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
22 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
23 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
24 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
25 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
26 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
27 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
28 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
29 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
30 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
35 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
36 .\" Modified Oct 1998 by Andi Kleen
37 .\" Modified Oct 2003 by aeb
38 .\" Modified 2004-07-01 by mtk
40 .TH SEND 2 2017-03-13 "Linux" "Linux Programmer's Manual"
42 send, sendto, sendmsg \- send a message on a socket
45 .B #include <sys/types.h>
46 .B #include <sys/socket.h>
48 .BI "ssize_t send(int " sockfd ", const void *" buf ", size_t " len \
51 .BI "ssize_t sendto(int " sockfd ", const void *" buf ", size_t " len \
53 .BI " const struct sockaddr *" dest_addr ", socklen_t " addrlen );
55 .BI "ssize_t sendmsg(int " sockfd ", const struct msghdr *" msg \
64 are used to transmit a message to another socket.
68 call may be used only when the socket is in a
70 state (so that the intended recipient is known).
71 The only difference between
83 Also, the following call
85 send(sockfd, buf, len, flags);
89 sendto(sockfd, buf, len, flags, NULL, 0);
93 is the file descriptor of the sending socket.
97 is used on a connection-mode
100 socket, the arguments
104 are ignored (and the error
106 may be returned when they are
107 not NULL and 0), and the error
109 is returned when the socket was not actually connected.
110 Otherwise, the address of the target is given by
117 the address of the target is given by
127 the message is found in
133 the message is pointed to by the elements of the array
137 call also allows sending ancillary data (also known as control information).
139 If the message is too long to pass atomically through the
140 underlying protocol, the error
142 is returned, and the message is not transmitted.
144 No indication of failure to deliver is implicit in a
146 Locally detected errors are indicated by a return value of \-1.
148 When the message does not fit into the send buffer of the socket,
150 normally blocks, unless the socket has been placed in nonblocking I/O
152 In nonblocking mode it would fail with the error
159 call may be used to determine when it is possible to send more data.
160 .SS The flags argument
163 argument is the bitwise OR
164 of zero or more of the following flags.
165 .\" FIXME . ? document MSG_PROXY (which went away in 2.3.15)
167 .BR MSG_CONFIRM " (since Linux 2.3.15)"
168 Tell the link layer that forward progress happened: you got a successful
169 reply from the other side.
170 If the link layer doesn't get this
171 it will regularly reprobe the neighbor (e.g., via a unicast ARP).
176 sockets and currently implemented only for IPv4 and IPv6.
182 Don't use a gateway to send out the packet, send to hosts only on
183 directly connected networks.
184 This is usually used only
185 by diagnostic or routing programs.
186 This is defined only for protocol
187 families that route; packet sockets don't.
189 .BR MSG_DONTWAIT " (since Linux 2.2)"
190 Enables nonblocking operation; if the operation would block,
195 This provides similar behavior to setting the
200 operation), but differs in that
202 is a per-call option, whereas
204 is a setting on the open file description (see
206 which will affect all threads in the calling process
207 and as well as other processes that hold file descriptors
208 referring to the same open file description.
210 .BR MSG_EOR " (since Linux 2.2)"
211 Terminates a record (when this notion is supported, as for sockets of type
212 .BR SOCK_SEQPACKET ).
214 .BR MSG_MORE " (since Linux 2.4.4)"
215 The caller has more data to send.
216 This flag is used with TCP sockets to obtain the same effect
221 with the difference that this flag can be set on a per-call basis.
223 Since Linux 2.6, this flag is also supported for UDP sockets, and informs
224 the kernel to package all of the data sent in calls with this flag set
225 into a single datagram which is transmitted only when a call is performed
226 that does not specify this flag.
229 socket option described in
232 .BR MSG_NOSIGNAL " (since Linux 2.2)"
235 signal if the peer on a stream-oriented socket has closed the connection.
238 error is still returned.
239 This provides similar behavior to using
245 is a per-call feature,
248 sets a process attribute that affects all threads in the process.
253 data on sockets that support this notion (e.g., of type
255 the underlying protocol must also support
259 The definition of the
261 structure employed by
268 void *msg_name; /* optional address */
269 socklen_t msg_namelen; /* size of address */
270 struct iovec *msg_iov; /* scatter/gather array */
271 size_t msg_iovlen; /* # elements in msg_iov */
272 void *msg_control; /* ancillary data, see below */
273 size_t msg_controllen; /* ancillary data buffer len */
274 int msg_flags; /* flags (unused) */
281 field is used on an unconnected socket to specify the target
282 address for a datagram.
283 It points to a buffer containing the address; the
285 field should be set to the size of the address.
286 For a connected socket, these fields should be specified as NULL and 0,
293 fields specify scatter-gather locations, as for
296 You may send control information using the
301 The maximum control buffer length the kernel can process is limited
302 per socket by the value in
303 .IR /proc/sys/net/core/optmem_max ;
310 .\" Still to be documented:
311 .\" Send file descriptors and user credentials using the
312 .\" msg_control* fields.
314 On success, these calls return the number of bytes sent.
315 On error, \-1 is returned, and
317 is set appropriately.
319 These are some standard errors generated by the socket layer.
321 may be generated and returned from the underlying protocol modules;
322 see their respective manual pages.
325 (For UNIX domain sockets, which are identified by pathname)
326 Write permission is denied on the destination socket file,
327 or search permission is denied for one of the directories
330 .BR path_resolution (7).)
332 (For UDP sockets) An attempt was made to send to a
333 network/broadcast address as though it was a unicast address.
335 .BR EAGAIN " or " EWOULDBLOCK
336 .\" Actually EAGAIN on Linux
337 The socket is marked nonblocking and the requested operation
339 POSIX.1-2001 allows either error to be returned for this case,
340 and does not require these constants to have the same value,
341 so a portable application should check for both possibilities.
344 (Internet domain datagram sockets)
345 The socket referred to by
347 had not previously been bound to an address and,
348 upon attempting to bind it to an ephemeral port,
349 it was determined that all port numbers in the ephemeral port range
350 are currently in use.
351 See the discussion of
352 .I /proc/sys/net/ipv4/ip_local_port_range
358 is not a valid open file descriptor.
361 Connection reset by peer.
364 The socket is not connection-mode, and no peer address is set.
367 An invalid user space address was specified for an argument.
370 A signal occurred before any data was transmitted; see
374 Invalid argument passed.
377 The connection-mode socket was connected already but a
378 recipient was specified.
379 (Now either this error is returned, or the recipient specification
384 .\" (e.g., SOCK_DGRAM )
385 requires that message be sent atomically, and the size
386 of the message to be sent made this impossible.
389 The output queue for a network interface was full.
390 This generally indicates that the interface has stopped sending,
391 but may be caused by transient congestion.
392 (Normally, this does not occur in Linux.
393 Packets are just silently dropped
394 when a device queue overflows.)
400 The socket is not connected, and no target has been given.
405 does not refer to a socket.
410 argument is inappropriate for the socket type.
413 The local end has been shut down on a connection oriented socket.
414 In this case, the process
421 4.4BSD, SVr4, POSIX.1-2001.
422 These interfaces first appeared in 4.2BSD.
424 POSIX.1-2001 describes only the
429 POSIX.1-2008 adds a specification of
433 flag is a Linux extension.
435 According to POSIX.1-2001, the
439 structure should be typed as
441 but glibc currently types it as
443 .\" glibc bug raised 12 Mar 2006
444 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
445 .\" The problem is an underlying kernel issue: the size of the
446 .\" __kernel_size_t type used to type this field varies
447 .\" across architectures, but socklen_t is always 32 bits.
451 for information about a Linux-specific system call
452 that can be used to transmit multiple datagrams in a single call.
459 An example of the use of