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https://github.com/freebsd/freebsd-src.git
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Kyle Evans
9b16399225
With exception to sockopt functionality, implement the so_options flag in unix(4) itself. The general argument for the flag is that SCM_RIGHTS can be used maliciously for, e.g., a DoS that the receiving side can't avoid if it is expecting other control messages. This option gives the receiver a way to disable SCM_RIGHTS on the sender-side, surfacing an EPERM to them instead. This seems to match the semantics that Linux offers. If an SCM_RIGHTS was already sent before we disabled SO_PASSRIGHTS, then a subsequent recvmsg(2) will silently discard any in-flight files. This has the downside of punting a file with the potential to hang over to the deferred-close task, but perhaps usage of the option would discourage folks from attempting to take advantage of that possibility anyways. Various manpages updated to describe the new behavior. The ru_msgsnd accounting here might need to be re-evaluated: some error paths will increment it while others will not, and it isn't really clearly labelled. At some point in the operation enough work has been done that one could consider it enough resources to count, but it's not obvious that it should work this way. This change doesn't attempt to address that. Reviewed by: glebius Differential Revision: https://reviews.freebsd.org/D57423
303 lines
7.9 KiB
Plaintext
303 lines
7.9 KiB
Plaintext
.\" 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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.Dd June 3, 2026
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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 EPERM
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The
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.Fa msg
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contained an
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.Dv SCM_RIGHTS
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control message, and the receiving
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.Xr unix 4
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socket is configured to reject new
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.Dv SCM_RIGHTS .
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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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.Xr unix 4
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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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