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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.1‐2008, <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  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 .