1SEND(2) Linux Programmer's Manual SEND(2)
2
6 send, sendto, sendmsg - send a message on a socket
7
9 #include <sys/socket.h>
10 ssize_t send(int sockfd, const void *buf, size_t len, int flags);
12 ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
13 const struct sockaddr *dest_addr, socklen_t addrlen);
14 ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);
15
17 The system calls send(), sendto(), and sendmsg() are used to transmit a
18 message to another socket.
19 The send() call may be used only when the socket is in a connected
21 state (so that the intended recipient is known). The only difference
22 between send() and write(2) is the presence of flags. With a zero
23 flags argument, send() is equivalent to write(2). Also, the following
24 call
25 send(sockfd, buf, len, flags);
27 is equivalent to
29 sendto(sockfd, buf, len, flags, NULL, 0);
31 The argument sockfd is the file descriptor of the sending socket.
33 If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
35 socket, the arguments dest_addr and addrlen are ignored (and the error
36 EISCONN may be returned when they are not NULL and 0), and the error
37 ENOTCONN is returned when the socket was not actually connected. Oth‐
38 erwise, the address of the target is given by dest_addr with addrlen
39 specifying its size. For sendmsg(), the address of the target is given
40 by msg.msg_name, with msg.msg_namelen specifying its size.
41 For send() and sendto(), the message is found in buf and has length
43 len. For sendmsg(), the message is pointed to by the elements of the
44 array msg.msg_iov. The sendmsg() call also allows sending ancillary
45 data (also known as control information).
46 If the message is too long to pass atomically through the underlying
48 protocol, the error EMSGSIZE is returned, and the message is not trans‐
49 mitted.
50 No indication of failure to deliver is implicit in a send(). Locally
52 detected errors are indicated by a return value of -1.
53 When the message does not fit into the send buffer of the socket,
55 send() normally blocks, unless the socket has been placed in nonblock‐
56 ing I/O mode. In nonblocking mode it would fail with the error EAGAIN
57 or EWOULDBLOCK in this case. The select(2) call may be used to deter‐
58 mine when it is possible to send more data.
59 The flags argument
61 The flags argument is the bitwise OR of zero or more of the following
62 flags.
63 MSG_CONFIRM (since Linux 2.3.15)
65 Tell the link layer that forward progress happened: you got a
66 successful reply from the other side. If the link layer doesn't
67 get this it will regularly reprobe the neighbor (e.g., via a
68 unicast ARP). Valid only on SOCK_DGRAM and SOCK_RAW sockets and
69 currently implemented only for IPv4 and IPv6. See arp(7) for
70 details.
71 MSG_DONTROUTE
73 Don't use a gateway to send out the packet, send to hosts only
74 on directly connected networks. This is usually used only by
75 diagnostic or routing programs. This is defined only for proto‐
76 col families that route; packet sockets don't.
77 MSG_DONTWAIT (since Linux 2.2)
79 Enables nonblocking operation; if the operation would block, EA‐
80 GAIN or EWOULDBLOCK is returned. This provides similar behavior
81 to setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL opera‐
82 tion), but differs in that MSG_DONTWAIT is a per-call option,
83 whereas O_NONBLOCK is a setting on the open file description
84 (see open(2)), which will affect all threads in the calling
85 process and as well as other processes that hold file descrip‐
86 tors referring to the same open file description.
87 MSG_EOR (since Linux 2.2)
89 Terminates a record (when this notion is supported, as for sock‐
90 ets of type SOCK_SEQPACKET).
91 MSG_MORE (since Linux 2.4.4)
93 The caller has more data to send. This flag is used with TCP
94 sockets to obtain the same effect as the TCP_CORK socket option
95 (see tcp(7)), with the difference that this flag can be set on a
96 per-call basis.
97 Since Linux 2.6, this flag is also supported for UDP sockets,
99 and informs the kernel to package all of the data sent in calls
100 with this flag set into a single datagram which is transmitted
101 only when a call is performed that does not specify this flag.
102 (See also the UDP_CORK socket option described in udp(7).)
103 MSG_NOSIGNAL (since Linux 2.2)
105 Don't generate a SIGPIPE signal if the peer on a stream-oriented
106 socket has closed the connection. The EPIPE error is still re‐
107 turned. This provides similar behavior to using sigaction(2) to
108 ignore SIGPIPE, but, whereas MSG_NOSIGNAL is a per-call feature,
109 ignoring SIGPIPE sets a process attribute that affects all
110 threads in the process.
111 MSG_OOB
113 Sends out-of-band data on sockets that support this notion
114 (e.g., of type SOCK_STREAM); the underlying protocol must also
115 support out-of-band data.
116 sendmsg()
118 The definition of the msghdr structure employed by sendmsg() is as fol‐
119 lows:
120 struct msghdr {
122 void *msg_name; /* Optional address */
123 socklen_t msg_namelen; /* Size of address */
124 struct iovec *msg_iov; /* Scatter/gather array */
125 size_t msg_iovlen; /* # elements in msg_iov */
126 void *msg_control; /* Ancillary data, see below */
127 size_t msg_controllen; /* Ancillary data buffer len */
128 int msg_flags; /* Flags (unused) */
129 };
130 The msg_name field is used on an unconnected socket to specify the tar‐
132 get address for a datagram. It points to a buffer containing the ad‐
133 dress; the msg_namelen field should be set to the size of the address.
134 For a connected socket, these fields should be specified as NULL and 0,
135 respectively.
136 The msg_iov and msg_iovlen fields specify scatter-gather locations, as
138 for writev(2).
139 You may send control information (ancillary data) using the msg_control
141 and msg_controllen members. The maximum control buffer length the ker‐
142 nel can process is limited per socket by the value in
143 /proc/sys/net/core/optmem_max; see socket(7). For further information
144 on the use of ancillary data in various socket domains, see unix(7) and
145 ip(7).
146 The msg_flags field is ignored.
148
150 On success, these calls return the number of bytes sent. On error, -1
151 is returned, and errno is set to indicate the error.
152
154 These are some standard errors generated by the socket layer. Addi‐
155 tional errors may be generated and returned from the underlying proto‐
156 col modules; see their respective manual pages.
157 EACCES (For UNIX domain sockets, which are identified by pathname)
159 Write permission is denied on the destination socket file, or
160 search permission is denied for one of the directories the path
161 prefix. (See path_resolution(7).)
162 (For UDP sockets) An attempt was made to send to a net‐
164 work/broadcast address as though it was a unicast address.
165 EAGAIN or EWOULDBLOCK
167 The socket is marked nonblocking and the requested operation
168 would block. POSIX.1-2001 allows either error to be returned
169 for this case, and does not require these constants to have the
170 same value, so a portable application should check for both pos‐
171 sibilities.
172 EAGAIN (Internet domain datagram sockets) The socket referred to by
174 sockfd had not previously been bound to an address and, upon at‐
175 tempting to bind it to an ephemeral port, it was determined that
176 all port numbers in the ephemeral port range are currently in
177 use. See the discussion of /proc/sys/net/ipv4/ip_lo‐
178 cal_port_range in ip(7).
179 EALREADY
181 Another Fast Open is in progress.
182 EBADF sockfd is not a valid open file descriptor.
184 ECONNRESET
186 Connection reset by peer.
187 EDESTADDRREQ
189 The socket is not connection-mode, and no peer address is set.
190 EFAULT An invalid user space address was specified for an argument.
192 EINTR A signal occurred before any data was transmitted; see sig‐
194 nal(7).
195 EINVAL Invalid argument passed.
197 EISCONN
199 The connection-mode socket was connected already but a recipient
200 was specified. (Now either this error is returned, or the re‐
201 cipient specification is ignored.)
202 EMSGSIZE
204 The socket type requires that message be sent atomically, and
205 the size of the message to be sent made this impossible.
206 ENOBUFS
208 The output queue for a network interface was full. This gener‐
209 ally indicates that the interface has stopped sending, but may
210 be caused by transient congestion. (Normally, this does not oc‐
211 cur in Linux. Packets are just silently dropped when a device
212 queue overflows.)
213 ENOMEM No memory available.
215 ENOTCONN
217 The socket is not connected, and no target has been given.
218 ENOTSOCK
220 The file descriptor sockfd does not refer to a socket.
221 EOPNOTSUPP
223 Some bit in the flags argument is inappropriate for the socket
224 type.
225 EPIPE The local end has been shut down on a connection oriented
227 socket. In this case, the process will also receive a SIGPIPE
228 unless MSG_NOSIGNAL is set.
229
231 4.4BSD, SVr4, POSIX.1-2001. These interfaces first appeared in 4.2BSD.
232 POSIX.1-2001 describes only the MSG_OOB and MSG_EOR flags.
234 POSIX.1-2008 adds a specification of MSG_NOSIGNAL. The MSG_CONFIRM
235 flag is a Linux extension.
236
238 According to POSIX.1-2001, the msg_controllen field of the msghdr
239 structure should be typed as socklen_t, and the msg_iovlen field should
240 be typed as int, but glibc currently types both as size_t.
241 See sendmmsg(2) for information about a Linux-specific system call that
243 can be used to transmit multiple datagrams in a single call.
244
246 Linux may return EPIPE instead of ENOTCONN.
247
249 An example of the use of sendto() is shown in getaddrinfo(3).
250
252 fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), sendmmsg(2),
253 shutdown(2), socket(2), write(2), cmsg(3), ip(7), ipv6(7), socket(7),
254 tcp(7), udp(7), unix(7)
255
257 This page is part of release 5.13 of the Linux man-pages project. A
258 description of the project, information about reporting bugs, and the
259 latest version of this page, can be found at
260 https://www.kernel.org/doc/man-pages/.
261Linux 2021-03-22 SEND(2)