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.

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 .

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 .