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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 2012-04-23 "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.
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,
194 is returned (this can also be enabled using the
200 .BR MSG_EOR " (since Linux 2.2)"
201 Terminates a record (when this notion is supported, as for sockets of type
202 .BR SOCK_SEQPACKET ).
204 .BR MSG_MORE " (Since Linux 2.4.4)"
205 The caller has more data to send.
206 This flag is used with TCP sockets to obtain the same effect
211 with the difference that this flag can be set on a per-call basis.
213 Since Linux 2.6, this flag is also supported for UDP sockets, and informs
214 the kernel to package all of the data sent in calls with this flag set
215 into a single datagram which is transmitted only when a call is performed
216 that does not specify this flag.
219 socket option described in
222 .BR MSG_NOSIGNAL " (since Linux 2.2)"
225 on errors on stream oriented sockets when the other end breaks the
229 error is still returned.
234 data on sockets that support this notion (e.g., of type
236 the underlying protocol must also support
240 The definition of the
245 and below for an exact description of its fields.
250 void *msg_name; /* optional address */
251 socklen_t msg_namelen; /* size of address */
252 struct iovec *msg_iov; /* scatter/gather array */
253 size_t msg_iovlen; /* # elements in msg_iov */
254 void *msg_control; /* ancillary data, see below */
255 size_t msg_controllen; /* ancillary data buffer len */
256 int msg_flags; /* flags on received message */
261 You may send control information using the
266 The maximum control buffer length the kernel can process is limited
267 per socket by the value in
268 .IR /proc/sys/net/core/optmem_max ;
271 .\" Still to be documented:
272 .\" Send file descriptors and user credentials using the
273 .\" msg_control* fields.
274 .\" The flags returned in msg_flags.
276 On success, these calls return the number of characters sent.
277 On error, \-1 is returned, and
279 is set appropriately.
281 These are some standard errors generated by the socket layer.
283 may be generated and returned from the underlying protocol modules;
284 see their respective manual pages.
287 (For UNIX domain sockets, which are identified by pathname)
288 Write permission is denied on the destination socket file,
289 or search permission is denied for one of the directories
292 .BR path_resolution (7).)
294 (For UDP sockets) An attempt was made to send to a
295 network/broadcast address as though it was a unicast address.
297 .BR EAGAIN " or " EWOULDBLOCK
298 .\" Actually EAGAIN on Linux
299 The socket is marked nonblocking and the requested operation
301 POSIX.1-2001 allows either error to be returned for this case,
302 and does not require these constants to have the same value,
303 so a portable application should check for both possibilities.
306 An invalid descriptor was specified.
309 Connection reset by peer.
312 The socket is not connection-mode, and no peer address is set.
315 An invalid user space address was specified for an argument.
318 A signal occurred before any data was transmitted; see
322 Invalid argument passed.
325 The connection-mode socket was connected already but a
326 recipient was specified.
327 (Now either this error is returned, or the recipient specification
332 .\" (e.g., SOCK_DGRAM )
333 requires that message be sent atomically, and the size
334 of the message to be sent made this impossible.
337 The output queue for a network interface was full.
338 This generally indicates that the interface has stopped sending,
339 but may be caused by transient congestion.
340 (Normally, this does not occur in Linux.
341 Packets are just silently dropped
342 when a device queue overflows.)
348 The socket is not connected, and no target has been given.
358 argument is inappropriate for the socket type.
361 The local end has been shut down on a connection oriented socket.
362 In this case the process
369 4.4BSD, SVr4, POSIX.1-2001.
370 These function calls appeared in 4.2BSD.
372 POSIX.1-2001 describes only the
377 POSIX.1-2008 adds a specification of
381 flag is a Linux extension.
383 The prototypes given above follow the Single UNIX Specification,
384 as glibc2 also does; the
386 argument was \fIint\fP in 4.x BSD, but \fIunsigned int\fP in libc4 and libc5;
389 argument was \fIint\fP in 4.x BSD and libc4, but \fIsize_t\fP in libc5;
392 argument was \fIint\fP in 4.x BSD and libc4 and libc5.
396 According to POSIX.1-2001, the
400 structure should be typed as
402 but glibc currently types it as
404 .\" glibc bug raised 12 Mar 2006
405 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
406 .\" The problem is an underlying kernel issue: the size of the
407 .\" __kernel_size_t type used to type this field varies
408 .\" across architectures, but socklen_t is always 32 bits.
412 for information about a Linux-specific system call
413 that can be used to transmit multiple datagrams in a single call.
420 An example of the use of