375 lines
9.9 KiB
Groff
375 lines
9.9 KiB
Groff
.\" Copyright (c) 1983, 1990, 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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.\" @(#)recv.2 8.3 (Berkeley) 2/21/94
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.\" $FreeBSD$
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.\"
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.Dd August 19, 2018
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.Dt RECV 2
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.Os
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.Sh NAME
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.Nm recv ,
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.Nm recvfrom ,
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.Nm recvmsg ,
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.Nm recvmmsg
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.Nd receive 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 recv "int s" "void *buf" "size_t len" "int flags"
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.Ft ssize_t
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.Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen"
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.Ft ssize_t
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.Fn recvmsg "int s" "struct msghdr *msg" "int flags"
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.Ft ssize_t
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.Fn recvmmsg "int s" "struct mmsghdr * restrict msgvec" "size_t vlen" "int flags" "const struct timespec * restrict timeout"
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.Sh DESCRIPTION
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The
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.Fn recvfrom ,
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.Fn recvmsg ,
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and
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.Fn recvmmsg
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system calls
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are used to receive messages from a socket,
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and may be used to receive data on a socket whether or not
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it is connection-oriented.
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.Pp
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If
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.Fa from
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is not a null pointer
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and the socket is not connection-oriented,
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the source address of the message is filled in.
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The
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.Fa fromlen
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argument
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is a value-result argument, initialized to the size of
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the buffer associated with
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.Fa from ,
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and modified on return to indicate the actual size of the
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address stored there.
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.Pp
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The
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.Fn recv
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function is normally used only on a
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.Em connected
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socket (see
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.Xr connect 2 )
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and is identical to
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.Fn recvfrom
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with a
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null pointer passed as its
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.Fa from
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argument.
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.Pp
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The
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.Fn recvmmsg
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function is used to receive multiple
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messages at a call.
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Their number is supplied by
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.Fa vlen .
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The messages are placed in the buffers described by
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.Fa msgvec
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vector, after reception.
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The size of each received message is placed in the
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.Fa msg_len
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field of each element of the vector.
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If
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.Fa timeout
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is NULL the call blocks until the data is available for each
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supplied message buffer.
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Otherwise it waits for data for the specified amount of time.
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If the timeout expired and there is no data received,
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a value 0 is returned.
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The
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.Xr ppoll 2
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system call is used to implement the timeout mechanism,
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before first receive is performed.
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.Pp
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The
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.Fn recv ,
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.Fn recvfrom
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and
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.Fn recvmsg
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return the length of the message on successful
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completion, whereas
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.Fn recvmmsg
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returns the number of received messages.
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If a message is too long to fit in the supplied buffer,
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excess bytes may be discarded depending on the type of socket
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the message is received from (see
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.Xr socket 2 ) .
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.Pp
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If no messages are available at the socket, the
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receive call waits for a message to arrive, unless
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the socket is non-blocking (see
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.Xr fcntl 2 )
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in which case the value
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\-1 is returned and the global variable
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.Va errno
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is set to
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.Er EAGAIN .
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The receive calls except
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.Fn recvmmsg
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normally return any data available,
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up to the requested amount,
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rather than waiting for receipt of the full amount requested;
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this behavior is affected by the socket-level options
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.Dv SO_RCVLOWAT
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and
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.Dv SO_RCVTIMEO
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described in
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.Xr getsockopt 2 .
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The
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.Fn recvmmsg
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function implements this behaviour for each message in the vector.
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.Pp
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The
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.Xr select 2
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system call may be used to determine when more data arrives.
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.Pp
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The
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.Fa flags
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argument to a
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.Fn recv
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function is formed by
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.Em or Ap ing
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one or more of the values:
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.Bl -column ".Dv MSG_CMSG_CLOEXEC" -offset indent
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.It Dv MSG_OOB Ta process out-of-band data
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.It Dv MSG_PEEK Ta peek at incoming message
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.It Dv MSG_WAITALL Ta wait for full request or error
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.It Dv MSG_DONTWAIT Ta do not block
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.It Dv MSG_CMSG_CLOEXEC Ta set received fds close-on-exec
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.It Dv MSG_WAITFORONE Ta do not block after receiving the first message
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(only for
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.Fn recvmmsg
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)
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.El
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.Pp
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The
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.Dv MSG_OOB
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flag requests receipt of out-of-band data
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that would not be received in the normal data stream.
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Some protocols place expedited data at the head of the normal
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data queue, and thus this flag cannot be used with such protocols.
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The
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.Dv MSG_PEEK
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flag causes the receive operation to return data
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from the beginning of the receive queue without removing that
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data from the queue.
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Thus, a subsequent receive call will return the same data.
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The
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.Dv MSG_WAITALL
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flag requests that the operation block until
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the full request is satisfied.
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However, the call may still return less data than requested
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if a signal is caught, an error or disconnect occurs,
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or the next data to be received is of a different type than that returned.
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The
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.Dv MSG_DONTWAIT
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flag requests the call to return when it would block otherwise.
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If no data is available,
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.Va errno
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is set to
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.Er EAGAIN .
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This flag is not available in
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.St -ansiC
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or
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.St -isoC-99
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compilation mode.
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The
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.Dv MSG_WAITFORONE
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flag sets MSG_DONTWAIT after the first message has been received.
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This flag is only relevant for
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.Fn recvmmsg .
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.Pp
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The
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.Fn recvmsg
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system call uses a
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.Fa msghdr
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structure to minimize the number of directly supplied arguments.
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This structure has the following form, as defined in
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.In sys/socket.h :
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.Bd -literal
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struct msghdr {
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void *msg_name; /* optional address */
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socklen_t msg_namelen; /* size of address */
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struct iovec *msg_iov; /* scatter/gather array */
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int msg_iovlen; /* # elements in msg_iov */
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void *msg_control; /* ancillary data, see below */
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socklen_t msg_controllen;/* ancillary data buffer len */
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int msg_flags; /* flags on received message */
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};
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.Ed
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.Pp
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Here
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.Fa msg_name
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and
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.Fa msg_namelen
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specify the source address if the socket is unconnected;
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.Fa msg_name
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may be given as a null pointer if no names are desired or required.
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The
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.Fa msg_iov
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and
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.Fa msg_iovlen
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arguments
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describe scatter gather locations, as discussed in
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.Xr read 2 .
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The
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.Fa msg_control
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argument,
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which has length
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.Fa msg_controllen ,
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points to a buffer for other protocol control related messages
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or other miscellaneous ancillary data.
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The messages are of the form:
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.Bd -literal
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struct cmsghdr {
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socklen_t cmsg_len; /* data byte count, including hdr */
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int cmsg_level; /* originating protocol */
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int cmsg_type; /* protocol-specific type */
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/* followed by
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u_char cmsg_data[]; */
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};
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.Ed
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.Pp
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As an example, the SO_TIMESTAMP socket option returns a reception
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timestamp for UDP packets.
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.Pp
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With
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.Dv AF_UNIX
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domain sockets, ancillary data can be used to pass file descriptors and
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process credentials.
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See
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.Xr unix 4
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for details.
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.Pp
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The
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.Fa msg_flags
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field is set on return according to the message received.
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.Dv MSG_EOR
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indicates end-of-record;
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the data returned completed a record (generally used with sockets of type
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.Dv SOCK_SEQPACKET ) .
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.Dv MSG_TRUNC
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indicates that
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the trailing portion of a datagram was discarded because the datagram
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was larger than the buffer supplied.
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.Dv MSG_CTRUNC
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indicates that some
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control data were discarded due to lack of space in the buffer
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for ancillary data.
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.Dv MSG_OOB
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is returned to indicate that expedited or out-of-band data were received.
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.Pp
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The
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.Fn recvmmsg
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system call uses the
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.Fa mmsghdr
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structure, defined as follows in the
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.In sys/socket.h
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header:
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.Bd -literal
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struct mmsghdr {
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struct msghdr msg_hdr; /* message header */
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ssize_t msg_len; /* message length */
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};
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.Ed
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.Pp
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On data reception the
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.Fa msg_len
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field is updated to the length of the received message.
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.Sh RETURN VALUES
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These calls except
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.Fn recvmmsg
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return the number of bytes received.
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.Fn recvmmsg
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returns the number of messages received.
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A value of -1 is returned if an error occurred.
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.Sh ERRORS
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The calls fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The argument
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.Fa s
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is an invalid descriptor.
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.It Bq Er ECONNRESET
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The remote socket end is forcibly closed.
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.It Bq Er ENOTCONN
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The socket is associated with a connection-oriented protocol
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and has not been connected (see
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.Xr connect 2
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and
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.Xr accept 2 ) .
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.It Bq Er ENOTSOCK
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The argument
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.Fa s
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does not refer to a socket.
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.It Bq Er EMSGSIZE
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The
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.Fn recvmsg
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system call
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was used to receive rights (file descriptors) that were in flight on the
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connection.
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However, the receiving program did not have enough free file
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descriptor slots to accept them.
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In this case the descriptors are
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closed, any pending data can be returned by another call to
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.Fn recvmsg .
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.It Bq Er EAGAIN
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The socket is marked non-blocking and the receive operation
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would block, or
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a receive timeout had been set
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and the timeout expired before data were received.
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.It Bq Er EINTR
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The receive was interrupted by delivery of a signal before
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any data were available.
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.It Bq Er EFAULT
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The receive buffer pointer(s) point outside the process's
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address space.
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.El
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.Sh SEE ALSO
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.Xr fcntl 2 ,
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.Xr getsockopt 2 ,
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.Xr read 2 ,
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.Xr select 2 ,
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.Xr socket 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 recv
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function appeared in
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.Bx 4.2 .
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The
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.Fn recvmmsg
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function appeared in
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.Fx 11.0 .
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