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Name
send, sendto, sendmsg — send a message on a
socket
Synopsis
ssize_t send( |
int |
s, |
| |
const void * |
buf, |
| |
size_t |
len, |
| |
int |
flags); |
ssize_t sendto( |
int |
s, |
| |
const void * |
buf, |
| |
size_t |
len, |
| |
int |
flags, |
| |
const struct sockaddr
* |
to, |
| |
socklen_t |
tolen); |
ssize_t sendmsg( |
int |
s, |
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const struct msghdr
* |
msg, |
| |
int |
flags); |
DESCRIPTION
The system calls send(),
sendto(), and sendmsg() are used to transmit a message to
another socket.
The send() call may be used
only when the socket is in a connected state (so that the
intended recipient is known). The only difference between
send() and write(2) is the presence of
flags. With zero
flags parameter,
send() is equivalent to
write(2). Also,
send(s,buf,len,flags) is equivalent to
sendto(s,buf,len,flags,NULL,0).
The parameter s is
the file descriptor of the sending socket.
If sendto() is used on a
connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the
parameters to and
tolen are ignored
(and the error EISCONN may be returned when they are not NULL
and 0), and the error ENOTCONN is returned when the socket
was not actually connected. Otherwise, the address of the
target is given by to
with tolen specifying
its size. For sendmsg(), the
address of the target is given by msg.msg_name, with msg.msg_namelen specifying
its size.
For send() and sendto(), the message is found in
buf and has length
len. For sendmsg(), the message is pointed to by the
elements of the array msg.msg_iov. The sendmsg() call also allows sending
ancillary data (also known as control information).
If the message is too long to pass atomically through the
underlying protocol, the error EMSGSIZE is returned, and the message is
not transmitted.
No indication of failure to deliver is implicit in a
send(). Locally detected errors
are indicated by a return value of −1.
When the message does not fit into the send buffer of the
socket, send() normally blocks,
unless the socket has been placed in non-blocking I/O mode.
In non-blocking mode it would return EAGAIN in this case. The select(2) call may be used
to determine when it is possible to send more data.
The flags
parameter is the bitwise OR of zero or more of the following
flags.
MSG_CONFIRM (Linux 2.3+
only)-
Tell the link layer that forward progress happened:
you got a successful reply from the other side. If the
link layer doesn't get this it will regularly reprobe
the neighbour (e.g. via a unicast ARP). Only valid on
SOCK_DGRAM and
SOCK_RAW sockets and
currently only implemented for IPv4 and IPv6. See
arp(7) for
details.
MSG_DONTROUTE-
Don't use a gateway to send out the packet, only
send to hosts on directly connected networks. This is
usually used only by diagnostic or routing programs.
This is only defined for protocol families that route;
packet sockets don't.
MSG_DONTWAIT-
Enables non-blocking operation; if the operation
would block, EAGAIN is
returned (this can also be enabled using the
O_NONBLOCK with the
F_SETFL fcntl(2)).
MSG_EOR-
Terminates a record (when this notion is supported,
as for sockets of type SOCK_SEQPACKET).
MSG_MORE (Since Linux
2.4.4)-
The caller has more data to send. This flag is used
with TCP sockets to obtain the same effect as the
TCP_CORK socket option (see tcp(7)), with the
difference that this flag can be set on a per-call
basis.
Since Linux 2.6, this flag is also supported for UDP
sockets, and informs the kernel to package all of the
data sent in calls with this flag set into a single
datagram which is only transmitted when a call is
performed that does not specify this flag. (See also
the UDP_CORK socket
option described in udp(7).)
MSG_NOSIGNAL-
Requests not to send SIGPIPE on errors on stream oriented
sockets when the other end breaks the connection. The
EPIPE error is still
returned.
MSG_OOB-
Sends out-of-band data on
sockets that support this notion (e.g. of type
SOCK_STREAM); the
underlying protocol must also support out-of-band data.
The definition of the msghdr structure follows. See
recv(2) and below for an
exact description of its fields.
| struct |
msghdr { |
| |
void |
* |
msg_name; |
/* optional address */ |
| |
socklen_t |
|
msg_namelen; |
/* size of address */ |
| |
struct iovec |
* |
msg_iov; |
/* scatter/gather array */ |
| |
size_t |
|
msg_iovlen; |
/* # elements in msg_iov */ |
| |
void |
* |
msg_control; |
/* ancillary data, see below */ |
| |
socklen_t |
|
msg_controllen; |
/* ancillary data buffer len */ |
| |
int |
|
msg_flags; |
/* flags on received message */ |
| }; |
You may send control information using the msg_control and msg_controllen members. The
maximum control buffer length the kernel can process is
limited per socket by the net.core.optmem_max sysctl;
see socket(7).
RETURN VALUE
On success, these calls return the number of characters
sent. On error, −1 is returned, and errno is set appropriately.
ERRORS
These are some standard errors generated by the socket
layer. Additional errors may be generated and returned from
the underlying protocol modules; see their respective manual
pages.
- EACCES
-
(For Unix domain sockets, which are identified by
pathname) Write permission is denied on the destination
socket file, or search permission is denied for one of
the directories the path prefix. (See path_resolution(7).)
- EAGAIN or EWOULDBLOCK
-
The socket is marked non-blocking and the requested
operation would block.
- EBADF
-
An invalid descriptor was specified.
- ECONNRESET
-
Connection reset by peer.
- EDESTADDRREQ
-
The socket is not connection-mode, and no peer
address is set.
- EFAULT
-
An invalid user space address was specified for a
parameter.
- EINTR
-
A signal occurred before any data was
transmitted.
- EINVAL
-
Invalid argument passed.
- EISCONN
-
The connection-mode socket was connected already but
a recipient was specified. (Now either this error is
returned, or the recipient specification is
ignored.)
- EMSGSIZE
-
The socket type requires that message be sent
atomically, and the size of the message to be sent made
this impossible.
- ENOBUFS
-
The output queue for a network interface was full.
This generally indicates that the interface has stopped
sending, but may be caused by transient congestion.
(Normally, this does not occur in Linux. Packets are
just silently dropped when a device queue
overflows.)
- ENOMEM
-
No memory available.
- ENOTCONN
-
The socket is not connected, and no target has been
given.
- ENOTSOCK
-
The argument s is not a socket.
- EOPNOTSUPP
-
Some bit in the flags argument is
inappropriate for the socket type.
- EPIPE
-
The local end has been shut down on a connection
oriented socket. In this case the process will also
receive a SIGPIPE unless
MSG_NOSIGNAL is set.
CONFORMING TO
4.4BSD, SVr4, POSIX.1-2001. These function calls appeared
in 4.2BSD.
POSIX.1-2001 only describes the MSG_OOB and MSG_EOR flags. The MSG_CONFIRM flag is a Linux extension.
NOTES
The prototypes given above follow the Single Unix
Specification, as glibc2 also does; the flags argument was `int' in 4.x
BSD, but `unsigned int' in libc4 and libc5; the len argument was `int' in 4.x
BSD and libc4, but `size_t' in libc5; the tolen argument was `int' in 4.x
BSD and libc4 and libc5. See also accept(2).
According to POSIX.1-2001, the msg_controllen field of the
msghdr structure
should be typed as socklen_t, but glibc
currently (2.4) types it as size_t.
BUGS
Linux may return EPIPE instead of ENOTCONN.
SEE ALSO
fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), shutdown(2), socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7)
Copyright (c) 1983, 1991 The Regents of the University of California.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
Modified Oct 1998 by Andi Kleen
Modified Oct 2003 by aeb
Modified 2004-07-01 by mtk
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