1SEND(2)                    Linux Programmer's Manual                   SEND(2)
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NAME

6       send, sendto, sendmsg - send a message on a socket
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/socket.h>
11
12       ssize_t send(int sockfd, const void *buf, size_t len, int flags);
13
14       ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
15                      const struct sockaddr *dest_addr, socklen_t addrlen);
16
17       ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);
18

DESCRIPTION

20       The system calls send(), sendto(), and sendmsg() are used to transmit a
21       message to another socket.
22
23       The send() call may be used only when the  socket  is  in  a  connected
24       state  (so  that the intended recipient is known).  The only difference
25       between send() and write(2) is the presence  of  flags.   With  a  zero
26       flags  argument, send() is equivalent to write(2).  Also, the following
27       call
28
29           send(sockfd, buf, len, flags);
30
31       is equivalent to
32
33           sendto(sockfd, buf, len, flags, NULL, 0);
34
35       The argument sockfd is the file descriptor of the sending socket.
36
37       If sendto() is used on a connection-mode (SOCK_STREAM,  SOCK_SEQPACKET)
38       socket,  the arguments dest_addr and addrlen are ignored (and the error
39       EISCONN may be returned when they are not NULL and 0),  and  the  error
40       ENOTCONN  is returned when the socket was not actually connected.  Oth‐
41       erwise, the address of the target is given by  dest_addr  with  addrlen
42       specifying its size.  For sendmsg(), the address of the target is given
43       by msg.msg_name, with msg.msg_namelen specifying its size.
44
45       For send() and sendto(), the message is found in  buf  and  has  length
46       len.   For  sendmsg(), the message is pointed to by the elements of the
47       array msg.msg_iov.  The sendmsg() call also  allows  sending  ancillary
48       data (also known as control information).
49
50       If  the  message  is too long to pass atomically through the underlying
51       protocol, the error EMSGSIZE is returned, and the message is not trans‐
52       mitted.
53
54       No  indication  of failure to deliver is implicit in a send().  Locally
55       detected errors are indicated by a return value of -1.
56
57       When the message does not fit into  the  send  buffer  of  the  socket,
58       send()  normally blocks, unless the socket has been placed in nonblock‐
59       ing I/O mode.  In nonblocking mode it would fail with the error  EAGAIN
60       or  EWOULDBLOCK in this case.  The select(2) call may be used to deter‐
61       mine when it is possible to send more data.
62
63   The flags argument
64       The flags argument is the bitwise OR of zero or more of  the  following
65       flags.
66
67       MSG_CONFIRM (since Linux 2.3.15)
68              Tell  the  link  layer that forward progress happened: you got a
69              successful reply from the other side.  If the link layer doesn't
70              get  this  it  will  regularly reprobe the neighbor (e.g., via a
71              unicast ARP).  Valid only on SOCK_DGRAM and SOCK_RAW sockets and
72              currently  implemented  only  for IPv4 and IPv6.  See arp(7) for
73              details.
74
75       MSG_DONTROUTE
76              Don't use a gateway to send out the packet, send to  hosts  only
77              on  directly  connected  networks.  This is usually used only by
78              diagnostic or routing programs.  This is defined only for proto‐
79              col families that route; packet sockets don't.
80
81       MSG_DONTWAIT (since Linux 2.2)
82              Enables nonblocking operation; if the operation would block, EA‐
83              GAIN or EWOULDBLOCK is returned.  This provides similar behavior
84              to  setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL opera‐
85              tion), but differs in that MSG_DONTWAIT is  a  per-call  option,
86              whereas  O_NONBLOCK  is  a  setting on the open file description
87              (see open(2)), which will affect  all  threads  in  the  calling
88              process  and  as well as other processes that hold file descrip‐
89              tors referring to the same open file description.
90
91       MSG_EOR (since Linux 2.2)
92              Terminates a record (when this notion is supported, as for sock‐
93              ets of type SOCK_SEQPACKET).
94
95       MSG_MORE (since Linux 2.4.4)
96              The  caller  has  more data to send.  This flag is used with TCP
97              sockets to obtain the same effect as the TCP_CORK socket  option
98              (see tcp(7)), with the difference that this flag can be set on a
99              per-call basis.
100
101              Since Linux 2.6, this flag is also supported  for  UDP  sockets,
102              and  informs the kernel to package all of the data sent in calls
103              with this flag set into a single datagram which  is  transmitted
104              only  when  a call is performed that does not specify this flag.
105              (See also the UDP_CORK socket option described in udp(7).)
106
107       MSG_NOSIGNAL (since Linux 2.2)
108              Don't generate a SIGPIPE signal if the peer on a stream-oriented
109              socket  has closed the connection.  The EPIPE error is still re‐
110              turned.  This provides similar behavior to using sigaction(2) to
111              ignore SIGPIPE, but, whereas MSG_NOSIGNAL is a per-call feature,
112              ignoring SIGPIPE sets  a  process  attribute  that  affects  all
113              threads in the process.
114
115       MSG_OOB
116              Sends  out-of-band  data  on  sockets  that  support this notion
117              (e.g., of type SOCK_STREAM); the underlying protocol  must  also
118              support out-of-band data.
119
120   sendmsg()
121       The definition of the msghdr structure employed by sendmsg() is as fol‐
122       lows:
123
124           struct msghdr {
125               void         *msg_name;       /* Optional address */
126               socklen_t     msg_namelen;    /* Size of address */
127               struct iovec *msg_iov;        /* Scatter/gather array */
128               size_t        msg_iovlen;     /* # elements in msg_iov */
129               void         *msg_control;    /* Ancillary data, see below */
130               size_t        msg_controllen; /* Ancillary data buffer len */
131               int           msg_flags;      /* Flags (unused) */
132           };
133
134       The msg_name field is used on an unconnected socket to specify the tar‐
135       get  address  for a datagram.  It points to a buffer containing the ad‐
136       dress; the msg_namelen field should be set to the size of the  address.
137       For a connected socket, these fields should be specified as NULL and 0,
138       respectively.
139
140       The msg_iov and msg_iovlen fields specify scatter-gather locations,  as
141       for writev(2).
142
143       You may send control information (ancillary data) using the msg_control
144       and msg_controllen members.  The maximum control buffer length the ker‐
145       nel   can   process   is   limited   per   socket   by   the  value  in
146       /proc/sys/net/core/optmem_max; see socket(7).  For further  information
147       on the use of ancillary data in various socket domains, see unix(7) and
148       ip(7).
149
150       The msg_flags field is ignored.
151

RETURN VALUE

153       On success, these calls return the number of bytes sent.  On error,  -1
154       is returned, and errno is set appropriately.
155

ERRORS

157       These  are  some  standard errors generated by the socket layer.  Addi‐
158       tional errors may be generated and returned from the underlying  proto‐
159       col modules; see their respective manual pages.
160
161       EACCES (For  UNIX  domain  sockets,  which  are identified by pathname)
162              Write permission is denied on the destination  socket  file,  or
163              search  permission is denied for one of the directories the path
164              prefix.  (See path_resolution(7).)
165
166              (For UDP sockets)  An  attempt  was  made  to  send  to  a  net‐
167              work/broadcast address as though it was a unicast address.
168
169       EAGAIN or EWOULDBLOCK
170              The  socket  is  marked  nonblocking and the requested operation
171              would block.  POSIX.1-2001 allows either error  to  be  returned
172              for  this case, and does not require these constants to have the
173              same value, so a portable application should check for both pos‐
174              sibilities.
175
176       EAGAIN (Internet  domain  datagram  sockets)  The socket referred to by
177              sockfd had not previously been bound to an address and, upon at‐
178              tempting to bind it to an ephemeral port, it was determined that
179              all port numbers in the ephemeral port range  are  currently  in
180              use.     See   the   discussion   of   /proc/sys/net/ipv4/ip_lo‐
181              cal_port_range in ip(7).
182
183       EALREADY
184              Another Fast Open is in progress.
185
186       EBADF  sockfd is not a valid open file descriptor.
187
188       ECONNRESET
189              Connection reset by peer.
190
191       EDESTADDRREQ
192              The socket is not connection-mode, and no peer address is set.
193
194       EFAULT An invalid user space address was specified for an argument.
195
196       EINTR  A signal occurred before any  data  was  transmitted;  see  sig‐
197              nal(7).
198
199       EINVAL Invalid argument passed.
200
201       EISCONN
202              The connection-mode socket was connected already but a recipient
203              was specified.  (Now either this error is returned, or  the  re‐
204              cipient specification is ignored.)
205
206       EMSGSIZE
207              The  socket  type  requires that message be sent atomically, and
208              the size of the message to be sent made this impossible.
209
210       ENOBUFS
211              The output queue for a network interface was full.  This  gener‐
212              ally  indicates  that the interface has stopped sending, but may
213              be caused by transient congestion.  (Normally, this does not oc‐
214              cur  in  Linux.  Packets are just silently dropped when a device
215              queue overflows.)
216
217       ENOMEM No memory available.
218
219       ENOTCONN
220              The socket is not connected, and no target has been given.
221
222       ENOTSOCK
223              The file descriptor sockfd does not refer to a socket.
224
225       EOPNOTSUPP
226              Some bit in the flags argument is inappropriate for  the  socket
227              type.
228
229       EPIPE  The  local  end  has  been  shut  down  on a connection oriented
230              socket.  In this case, the process will also receive  a  SIGPIPE
231              unless MSG_NOSIGNAL is set.
232

CONFORMING TO

234       4.4BSD, SVr4, POSIX.1-2001.  These interfaces first appeared in 4.2BSD.
235
236       POSIX.1-2001   describes   only   the   MSG_OOB   and   MSG_EOR  flags.
237       POSIX.1-2008 adds a specification  of  MSG_NOSIGNAL.   The  MSG_CONFIRM
238       flag is a Linux extension.
239

NOTES

241       According  to  POSIX.1-2001,  the  msg_controllen  field  of the msghdr
242       structure should be typed as socklen_t, and the msg_iovlen field should
243       be typed as int, but glibc currently types both as size_t.
244
245       See sendmmsg(2) for information about a Linux-specific system call that
246       can be used to transmit multiple datagrams in a single call.
247

BUGS

249       Linux may return EPIPE instead of ENOTCONN.
250

EXAMPLES

252       An example of the use of sendto() is shown in getaddrinfo(3).
253

SEE ALSO

255       fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2),  sendmmsg(2),
256       shutdown(2),  socket(2),  write(2), cmsg(3), ip(7), ipv6(7), socket(7),
257       tcp(7), udp(7), unix(7)
258

COLOPHON

260       This page is part of release 5.10 of the Linux  man-pages  project.   A
261       description  of  the project, information about reporting bugs, and the
262       latest    version    of    this    page,    can     be     found     at
263       https://www.kernel.org/doc/man-pages/.
264
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267Linux                             2020-11-01                           SEND(2)
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