1SEND(2) Linux Programmer's Manual SEND(2)
2
6 send, sendto, sendmsg - send a message on a socket
7
9 #include <sys/types.h>
10 #include <sys/socket.h>
11 ssize_t send(int sockfd, const void *buf, size_t len, int flags);
13 ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
15 const struct sockaddr *dest_addr, socklen_t addrlen);
16 ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);
18
20 The system calls send(), sendto(), and sendmsg() are used to transmit a
21 message to another socket.
22 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 zero flags
26 argument, send() is equivalent to write(2). Also, the following call
27 send(sockfd, buf, len, flags);
29 is equivalent to
31 sendto(sockfd, buf, len, flags, NULL, 0);
33 The argument sockfd is the file descriptor of the sending socket.
35 If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
37 socket, the arguments dest_addr and addrlen are ignored (and the error
38 EISCONN may be returned when they are not NULL and 0), and the error
39 ENOTCONN is returned when the socket was not actually connected. Oth‐
40 erwise, the address of the target is given by dest_addr with addrlen
41 specifying its size. For sendmsg(), the address of the target is given
42 by msg.msg_name, with msg.msg_namelen specifying its size.
43 For send() and sendto(), the message is found in buf and has length
45 len. For sendmsg(), the message is pointed to by the elements of the
46 array msg.msg_iov. The sendmsg() call also allows sending ancillary
47 data (also known as control information).
48 If the message is too long to pass atomically through the underlying
50 protocol, the error EMSGSIZE is returned, and the message is not trans‐
51 mitted.
52 No indication of failure to deliver is implicit in a send(). Locally
54 detected errors are indicated by a return value of -1.
55 When the message does not fit into the send buffer of the socket,
57 send() normally blocks, unless the socket has been placed in non-block‐
58 ing I/O mode. In non-blocking mode it would fail with the error EAGAIN
59 or EWOULDBLOCK in this case. The select(2) call may be used to deter‐
60 mine when it is possible to send more data.
61 The flags argument is the bitwise OR of zero or more of the following
63 flags.
64 MSG_CONFIRM (Since Linux 2.3.15)
66 Tell the link layer that forward progress happened: you got a
67 successful reply from the other side. If the link layer doesn't
68 get this it will regularly reprobe the neighbor (e.g., via a
69 unicast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and
70 currently only implemented for IPv4 and IPv6. See arp(7) for
71 details.
72 MSG_DONTROUTE
74 Don't use a gateway to send out the packet, only send to hosts
75 on directly connected networks. This is usually used only by
76 diagnostic or routing programs. This is only defined for proto‐
77 col families that route; packet sockets don't.
78 MSG_DONTWAIT (since Linux 2.2)
80 Enables non-blocking operation; if the operation would block,
81 EAGAIN or EWOULDBLOCK is returned (this can also be enabled
82 using the O_NONBLOCK flag with the F_SETFL fcntl(2)).
83 MSG_EOR (since Linux 2.2)
85 Terminates a record (when this notion is supported, as for sock‐
86 ets of type SOCK_SEQPACKET).
87 MSG_MORE (Since Linux 2.4.4)
89 The caller has more data to send. This flag is used with TCP
90 sockets to obtain the same effect as the TCP_CORK socket option
91 (see tcp(7)), with the difference that this flag can be set on a
92 per-call basis.
93 Since Linux 2.6, this flag is also supported for UDP sockets,
95 and informs the kernel to package all of the data sent in calls
96 with this flag set into a single datagram which is only trans‐
97 mitted when a call is performed that does not specify this flag.
98 (See also the UDP_CORK socket option described in udp(7).)
99 MSG_NOSIGNAL (since Linux 2.2)
101 Requests not to send SIGPIPE on errors on stream oriented sock‐
102 ets when the other end breaks the connection. The EPIPE error
103 is still returned.
104 MSG_OOB
106 Sends out-of-band data on sockets that support this notion
107 (e.g., of type SOCK_STREAM); the underlying protocol must also
108 support out-of-band data.
109 The definition of the msghdr structure follows. See recv(2) and below
111 for an exact description of its fields.
112 struct msghdr {
114 void *msg_name; /* optional address */
115 socklen_t msg_namelen; /* size of address */
116 struct iovec *msg_iov; /* scatter/gather array */
117 size_t msg_iovlen; /* # elements in msg_iov */
118 void *msg_control; /* ancillary data, see below */
119 socklen_t msg_controllen; /* ancillary data buffer len */
120 int msg_flags; /* flags on received message */
121 };
122 You may send control information using the msg_control and msg_con‐
124 trollen members. The maximum control buffer length the kernel can
125 process is limited per socket by the value in /proc/sys/net/core/opt‐
126 mem_max; see socket(7).
127
129 On success, these calls return the number of characters sent. On
130 error, -1 is returned, and errno is set appropriately.
131
133 These are some standard errors generated by the socket layer. Addi‐
134 tional errors may be generated and returned from the underlying proto‐
135 col modules; see their respective manual pages.
136 EACCES (For Unix domain sockets, which are identified by pathname)
138 Write permission is denied on the destination socket file, or
139 search permission is denied for one of the directories the path
140 prefix. (See path_resolution(7).)
141 EAGAIN or EWOULDBLOCK
143 The socket is marked non-blocking and the requested operation
144 would block. POSIX.1-2001 allows either error to be returned
145 for this case, and does not require these constants to have the
146 same value, so a portable application should check for both pos‐
147 sibilities.
148 EBADF An invalid descriptor was specified.
150 ECONNRESET
152 Connection reset by peer.
153 EDESTADDRREQ
155 The socket is not connection-mode, and no peer address is set.
156 EFAULT An invalid user space address was specified for an argument.
158 EINTR A signal occurred before any data was transmitted; see sig‐
160 nal(7).
161 EINVAL Invalid argument passed.
163 EISCONN
165 The connection-mode socket was connected already but a recipient
166 was specified. (Now either this error is returned, or the
167 recipient specification is ignored.)
168 EMSGSIZE
170 The socket type requires that message be sent atomically, and
171 the size of the message to be sent made this impossible.
172 ENOBUFS
174 The output queue for a network interface was full. This gener‐
175 ally indicates that the interface has stopped sending, but may
176 be caused by transient congestion. (Normally, this does not
177 occur in Linux. Packets are just silently dropped when a device
178 queue overflows.)
179 ENOMEM No memory available.
181 ENOTCONN
183 The socket is not connected, and no target has been given.
184 ENOTSOCK
186 The argument sockfd is not a socket.
187 EOPNOTSUPP
189 Some bit in the flags argument is inappropriate for the socket
190 type.
191 EPIPE The local end has been shut down on a connection oriented
193 socket. In this case the process will also receive a SIGPIPE
194 unless MSG_NOSIGNAL is set.
195
197 4.4BSD, SVr4, POSIX.1-2001. These function calls appeared in 4.2BSD.
198 POSIX.1-2001 only describes the MSG_OOB and MSG_EOR flags. The
200 MSG_CONFIRM flag is a Linux extension.
201
203 The prototypes given above follow the Single Unix Specification, as
204 glibc2 also does; the flags argument was int in 4.x BSD, but unsigned
205 int in libc4 and libc5; the len argument was int in 4.x BSD and libc4,
206 but size_t in libc5; the addrlen argument was int in 4.x BSD and libc4
207 and libc5. See also accept(2).
208 According to POSIX.1-2001, the msg_controllen field of the msghdr
210 structure should be typed as socklen_t, but glibc currently (2.4) types
211 it as size_t.
212
214 Linux may return EPIPE instead of ENOTCONN.
215
217 An example of the use of sendto() is shown in getaddrinfo(3).
218
220 fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), shutdown(2),
221 socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7)
222
224 This page is part of release 3.22 of the Linux man-pages project. A
225 description of the project, and information about reporting bugs, can
226 be found at http://www.kernel.org/doc/man-pages/.
227Linux 2009-02-23 SEND(2)