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SENDMSG(3P) POSIX Programmer's Manual SENDMSG(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.
sendmsg — send a message on a socket using a message structure
#include <sys/socket.h>
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
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.
Upon successful completion, sendmsg() shall return the number of
bytes sent. Otherwise, −1 shall be returned and errno set to indicate
the error.
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.
Done.
The select() and poll() functions can be used to determine when it is
possible to send more data.
None.
None.
getsockopt(3p), poll(3p), pselect(3p), recv(3p), recvfrom(3p),
recvmsg(3p), send(3p), sendto(3p), setsockopt(3p), shutdown(3p),
socket(3p)
The Base Definitions volume of POSIX.1‐2008, sys_socket.h(0p)
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 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SENDMSG(3P)
Pages that refer to this page: netinet_in.h(0p), sys_socket.h(0p), pselect(3p), recv(3p), recvfrom(3p), recvmsg(3p), send(3p), sendto(3p), socket(3p)
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