bea2668321
sendto(2), sendmsg(2) and sendmmsg(2) return ENOTCONN if a destination
address is specified and the socket is not connected and the socket
protocol does not automatically connect ("implied connect"). Document
that. Also document the fact that the destination address is ignored
for connection-mode sockets if the socket is already connected.
PR: 245817
Submitted by: Erik Inge Bolsø <knan-bfo@modirum.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24530
296 lines
7.8 KiB
Groff
296 lines
7.8 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" From: @(#)send.2 8.2 (Berkeley) 2/21/94
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.\" $FreeBSD$
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.\"
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.Dd April 27, 2020
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.Dt SEND 2
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.Os
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.Sh NAME
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.Nm send ,
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.Nm sendto ,
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.Nm sendmsg ,
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.Nm sendmmsg
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.Nd send message(s) from a socket
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/socket.h
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.Ft ssize_t
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.Fn send "int s" "const void *msg" "size_t len" "int flags"
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.Ft ssize_t
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.Fn sendto "int s" "const void *msg" "size_t len" "int flags" "const struct sockaddr *to" "socklen_t tolen"
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.Ft ssize_t
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.Fn sendmsg "int s" "const struct msghdr *msg" "int flags"
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.Ft ssize_t
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.Fn sendmmsg "int s" "struct mmsghdr * restrict msgvec" "size_t vlen" "int flags"
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.Sh DESCRIPTION
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The
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.Fn send
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and
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.Fn sendmmsg
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functions,
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and
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.Fn sendto
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and
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.Fn sendmsg
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system calls
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are used to transmit one or more messages (with the
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.Fn sendmmsg
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call) to
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another socket.
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The
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.Fn send
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function
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may be used only when the socket is in a
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.Em connected
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state.
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The functions
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.Fn sendto ,
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.Fn sendmsg
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and
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.Fn sendmmsg
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may be used at any time if the socket is connectionless-mode.
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If the socket is connection-mode, the protocol
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must support implied connect (currently
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.Xr tcp 4
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is the only protocol with support) or the socket must be in a
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connected state before use.
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.Pp
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The address of the target is given by
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.Fa to
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with
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.Fa tolen
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specifying its size, or the equivalent
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.Fa msg_name
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and
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.Fa msg_namelen
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in
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.Fa struct msghdr .
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If the socket is in a connected state, the target address passed to
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.Fn sendto ,
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.Fn sendmsg
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or
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.Fn sendmmsg
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is ignored.
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The length of the message is given by
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.Fa len .
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If the message is too long to pass atomically through the
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underlying protocol, the error
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.Er EMSGSIZE
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is returned, and
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the message is not transmitted.
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.Pp
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The
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.Fn sendmmsg
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function sends multiple messages at a call.
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They are given by the
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.Fa msgvec
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vector along with
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.Fa vlen
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specifying the vector size.
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The number of octets sent per each message is placed in the
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.Fa msg_len
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field of each processed element of the vector after transmission.
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.Pp
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No indication of failure to deliver is implicit in a
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.Fn send .
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Locally detected errors are indicated by a return value of -1.
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.Pp
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If no messages space is available at the socket to hold
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the message to be transmitted, then
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.Fn send
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normally blocks, unless the socket has been placed in
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non-blocking I/O mode.
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The
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.Xr select 2
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system call may be used to determine when it is possible to
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send more data.
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.Pp
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The
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.Fa flags
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argument may include one or more of the following:
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.Bd -literal
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#define MSG_OOB 0x00001 /* process out-of-band data */
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#define MSG_DONTROUTE 0x00004 /* bypass routing, use direct interface */
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#define MSG_EOR 0x00008 /* data completes record */
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#define MSG_DONTWAIT 0x00080 /* do not block */
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#define MSG_EOF 0x00100 /* data completes transaction */
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#define MSG_NOSIGNAL 0x20000 /* do not generate SIGPIPE on EOF */
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.Ed
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.Pp
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The flag
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.Dv MSG_OOB
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is used to send
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.Dq out-of-band
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data on sockets that support this notion (e.g.\&
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.Dv SOCK_STREAM ) ;
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the underlying protocol must also support
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.Dq out-of-band
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data.
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.Dv MSG_EOR
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is used to indicate a record mark for protocols which support the
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concept.
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The
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.Dv MSG_DONTWAIT
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flag request the call to return when it would block otherwise.
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.Dv MSG_EOF
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requests that the sender side of a socket be shut down, and that an
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appropriate indication be sent at the end of the specified data;
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this flag is only implemented for
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.Dv SOCK_STREAM
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sockets in the
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.Dv PF_INET
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protocol family.
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.Dv MSG_DONTROUTE
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is usually used only by diagnostic or routing programs.
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.Dv MSG_NOSIGNAL
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is used to prevent
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.Dv SIGPIPE
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generation when writing a socket that
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may be closed.
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.Pp
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See
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.Xr recv 2
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for a description of the
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.Fa msghdr
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structure and the
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.Fa mmsghdr
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structure.
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.Sh RETURN VALUES
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The
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.Fn send ,
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.Fn sendto
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and
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.Fn sendmsg
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calls
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return the number of octets sent.
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The
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.Fn sendmmsg
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call returns the number of messages sent.
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If an error occurred a value of -1 is returned.
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.Sh ERRORS
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The
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.Fn send
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and
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.Fn sendmmsg
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functions and
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.Fn sendto
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and
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.Fn sendmsg
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system calls
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fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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An invalid descriptor was specified.
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.It Bq Er EACCES
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The destination address is a broadcast address, and
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.Dv SO_BROADCAST
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has not been set on the socket.
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.It Bq Er ENOTCONN
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The socket is connection-mode but is not connected.
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.It Bq Er ENOTSOCK
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The argument
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.Fa s
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is not a socket.
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.It Bq Er EFAULT
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An invalid user space address was specified for an argument.
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.It Bq Er EMSGSIZE
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The socket requires that message be sent atomically,
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and the size of the message to be sent made this impossible.
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.It Bq Er EAGAIN
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The socket is marked non-blocking, or
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.Dv MSG_DONTWAIT
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is specified, and the requested operation would block.
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.It Bq Er ENOBUFS
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The system was unable to allocate an internal buffer.
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The operation may succeed when buffers become available.
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.It Bq Er ENOBUFS
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The output queue for a network interface was full.
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This generally indicates that the interface has stopped sending,
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but may be caused by transient congestion.
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.It Bq Er EHOSTUNREACH
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The remote host was unreachable.
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.It Bq Er EISCONN
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A destination address was specified and the socket is already connected.
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.It Bq Er ECONNREFUSED
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The socket received an ICMP destination unreachable message
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from the last message sent.
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This typically means that the
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receiver is not listening on the remote port.
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.It Bq Er EHOSTDOWN
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The remote host was down.
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.It Bq Er ENETDOWN
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The remote network was down.
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.It Bq Er EADDRNOTAVAIL
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The process using a
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.Dv SOCK_RAW
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socket was jailed and the source
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address specified in the IP header did not match the IP
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address bound to the prison.
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.It Bq Er EPIPE
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The socket is unable to send anymore data
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.Dv ( SBS_CANTSENDMORE
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has been set on the socket).
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This typically means that the socket
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is not connected.
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.El
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.Sh SEE ALSO
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.Xr connect 2 ,
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.Xr fcntl 2 ,
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.Xr getsockopt 2 ,
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.Xr recv 2 ,
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.Xr select 2 ,
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.Xr socket 2 ,
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.Xr write 2 ,
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.Xr CMSG_DATA 3
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.Sh HISTORY
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The
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.Fn send
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function appeared in
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.Bx 4.2 .
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The
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.Fn sendmmsg
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function appeared in
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.Fx 11.0 .
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.Sh BUGS
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Because
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.Fn sendmsg
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does not necessarily block until the data has been transferred, it
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is possible to transfer an open file descriptor across an
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.Dv AF_UNIX
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domain socket
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(see
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.Xr recv 2 ) ,
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then
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.Fn close
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it before it has actually been sent, the result being that the receiver
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gets a closed file descriptor.
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It is left to the application to
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implement an acknowledgment mechanism to prevent this from happening.
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