SENDMSG
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
sendmsg
--- send a message on a socket using a message structure
SYNOPSIS
#include <sys/socket.h>
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
DESCRIPTION
The
sendmsg()
function shall send a message through a connection-mode or
connectionless-mode socket. If the socket is a connectionless-mode
socket, the message shall be sent to the address specified by
msghdr
if no pre-specified peer address has been set. If a peer address has
been pre-specified, either the message shall be sent to the address
specified in
msghdr
(overriding the pre-specified peer address), or the function shall
return -1 and set
errno
to
[EISCONN].
If the socket is connection-mode, the destination address in
msghdr
shall be ignored.
The
sendmsg()
function takes the following arguments:
- socket
-
Specifies the socket file descriptor.
- message
-
Points to a
msghdr
structure, containing both the destination address and the buffers for
the outgoing message. The length and format of the address depend on
the address family of the socket. The
msg_flags
member is ignored.
- flags
-
Specifies the type of message transmission. The application may
specify 0 or the following flag:
-
- MSG_EOR
-
Terminates a record (if supported by the protocol).
- MSG_OOB
-
Sends out-of-band data on sockets that support out-of-bound data. The
significance and semantics of out-of-band data are protocol-specific.
- MSG_NOSIGNAL
-
Requests not to send the SIGPIPE signal if an attempt to send is made
on a stream-oriented socket that is no longer connected. The
[EPIPE]
error shall still be returned.
The
msg_iov
and
msg_iovlen
fields of
message
specify zero or more buffers containing the data to be sent.
msg_iov
points to an array of
iovec
structures;
msg_iovlen
shall be set to the dimension of this array. In each
iovec
structure, the
iov_base
field specifies a storage area and the
iov_len
field gives its size in bytes. Some of these sizes can be zero. The
data from each storage area indicated by
msg_iov
is sent in turn.
Successful completion of a call to
sendmsg()
does not guarantee delivery of the message. A return value of -1
indicates only locally-detected errors.
If space is not available at the sending socket to hold the message to
be transmitted and the socket file descriptor does not have O_NONBLOCK
set, the
sendmsg()
function shall block until space is available. If space is not
available at the sending socket to hold the message to be transmitted
and the socket file descriptor does have O_NONBLOCK set, the
sendmsg()
function shall fail.
If the socket protocol supports broadcast and the specified address is
a broadcast address for the socket protocol,
sendmsg()
shall fail if the SO_BROADCAST option is not set for the socket.
The socket in use may require the process to have appropriate
privileges to use the
sendmsg()
function.
RETURN VALUE
Upon successful completion,
sendmsg()
shall return the number of bytes sent. Otherwise, -1 shall
be returned and
errno
set to indicate the error.
ERRORS
The
sendmsg()
function shall fail if:
- EAGAIN or EWOULDBLOCK
-
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
- EAFNOSUPPORT
-
Addresses in the specified address family cannot be used with this
socket.
- EBADF
-
The
socket
argument is not a valid file descriptor.
- ECONNRESET
-
A connection was forcibly closed by a peer.
- EINTR
-
A signal interrupted
sendmsg()
before any data was transmitted.
- EINVAL
-
The sum of the
iov_len
values overflows an
ssize_t.
- EMSGSIZE
-
The message is too large to be sent all at once (as the socket
requires), or the
msg_iovlen
member of the
msghdr
structure pointed to by
message
is less than or equal to 0 or is greater than
{IOV_MAX}.
- ENOTCONN
-
The socket is connection-mode but is not connected.
- ENOTSOCK
-
The
socket
argument does not refer to a socket.
- EOPNOTSUPP
-
The
socket
argument is associated with a socket that does not support one or more
of the values set in
flags.
- EPIPE
-
The socket is shut down for writing, or the socket is connection-mode
and is no longer connected. In the latter case, and if the socket is of
type SOCK_STREAM or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set,
the SIGPIPE signal is generated to the calling thread.
If the address family of the socket is AF_UNIX, then
sendmsg()
shall fail if:
- EIO
-
An I/O error occurred while reading from or writing to the file
system.
- ELOOP
-
A loop exists in symbolic links encountered during resolution of the
pathname in the socket address.
- ENAMETOOLONG
-
The length of a component of a pathname is longer than
{NAME_MAX}.
- ENOENT
-
A component of the pathname does not name an existing file or the path
name is an empty string.
- ENOTDIR
-
A component of the path prefix of the pathname in the socket address
names an existing file that is neither a directory nor a symbolic link
to a directory, or the pathname in the socket address contains at least
one non-<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file that
is neither a directory nor a symbolic link to a directory.
The
sendmsg()
function may fail if:
- EACCES
-
Search permission is denied for a component of the path prefix; or
write access to the named socket is denied.
- EDESTADDRREQ
-
The socket is not connection-mode and does not have its peer address
set, and no destination address was specified.
- EHOSTUNREACH
-
The destination host cannot be reached (probably because the host is
down or a remote router cannot reach it).
- EIO
-
An I/O error occurred while reading from or writing to the file system.
- EISCONN
-
A destination address was specified and the socket is already
connected.
- ENETDOWN
-
The local network interface used to reach the destination is down.
- ENETUNREACH
-
No route to the network is present.
- ENOBUFS
-
Insufficient resources were available in the system to perform the
operation.
- ENOMEM
-
Insufficient memory was available to fulfill the request.
If the address family of the socket is AF_UNIX, then
sendmsg()
may fail if:
- ELOOP
-
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the pathname in
the socket address.
- ENAMETOOLONG
-
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
The following sections are informative.
EXAMPLES
Done.
APPLICATION USAGE
The
select()
and
poll()
functions can be used to determine when it is possible to send more
data.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
getsockopt(),
poll(),
pselect(),
recv(),
recvfrom(),
recvmsg(),
send(),
sendto(),
setsockopt(),
shutdown(),
socket()
The Base Definitions volume of POSIX.1-2017,
<sys_socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
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