Section: POSIX Programmer's Manual (3P)
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.
--- send a message on a socket using a message structure
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
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
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
(overriding the pre-specified peer address), or the function shall
return -1 and set
If the socket is connection-mode, the destination address in
shall be ignored.
function takes the following arguments:
Specifies the socket file descriptor.
Points to a
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
member is ignored.
Specifies the type of message transmission. The application may
specify 0 or the following flag:
Terminates a record (if supported by the protocol).
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.
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
error shall still be returned.
specify zero or more buffers containing the data to be sent.
points to an array of
shall be set to the dimension of this array. In each
field specifies a storage area and the
field gives its size in bytes. Some of these sizes can be zero. The
data from each storage area indicated by
is sent in turn.
Successful completion of a call to
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
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
function shall fail.
If the socket protocol supports broadcast and the specified address is
a broadcast address for the socket protocol,
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
Upon successful completion,
shall return the number of bytes sent. Otherwise, -1 shall
be returned and
set to indicate the error.
function shall fail if:
- EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
Addresses in the specified address family cannot be used with this
argument is not a valid file descriptor.
A connection was forcibly closed by a peer.
A signal interrupted
before any data was transmitted.
The sum of the
values overflows an
The message is too large to be sent all at once (as the socket
requires), or the
member of the
structure pointed to by
is less than or equal to 0 or is greater than
The socket is connection-mode but is not connected.
argument does not refer to a socket.
argument is associated with a socket that does not support one or more
of the values set in
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
shall fail if:
An I/O error occurred while reading from or writing to the file
A loop exists in symbolic links encountered during resolution of the
pathname in the socket address.
The length of a component of a pathname is longer than
A component of the pathname does not name an existing file or the path
name is an empty string.
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
character and ends with one or more trailing
characters and the last pathname component names an existing file that
is neither a directory nor a symbolic link to a directory.
function may fail if:
Search permission is denied for a component of the path prefix; or
write access to the named socket is denied.
The socket is not connection-mode and does not have its peer address
set, and no destination address was specified.
The destination host cannot be reached (probably because the host is
down or a remote router cannot reach it).
An I/O error occurred while reading from or writing to the file system.
A destination address was specified and the socket is already
The local network interface used to reach the destination is down.
No route to the network is present.
Insufficient resources were available in the system to perform the
Insufficient memory was available to fulfill the request.
If the address family of the socket is AF_UNIX, then
may fail if:
symbolic links were encountered during resolution of the pathname in
the socket address.
The length of a pathname exceeds
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
The following sections are informative.
functions can be used to determine when it is possible to send more
The Base Definitions volume of POSIX.1-2008,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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
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