SENDMSG(3P) POSIX Programmer's Manual SENDMSG(3P)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.
NAMEsendmsg — 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 speci‐
fied 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 destina‐
tion address and the buffers for the outgoing message. The
length and format of the address depend on the address fam‐
ily 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 pro‐
tocol).
MSG_OOB Sends out-of-band data on sockets that sup‐
port 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 buf‐
fers 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 deliv‐
ery 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 privi‐
leges 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 connec‐
tion-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 path‐
name component names an existing file that is neither a direc‐
tory 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 resolu‐
tion 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 ALSOgetsockopt(), poll(), pselect(), recv(), recvfrom(), recvmsg(), send(),
sendto(), setsockopt(), shutdown(), socket()
The Base Definitions volume of POSIX.1‐2008, <sys_socket.h>
COPYRIGHT
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 Electri‐
cal 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 http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SENDMSG(3P)
