306 lines
8.7 KiB
Groff
306 lines
8.7 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. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. 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 February 21, 1994
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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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.Nd receive a message 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/types.h
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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 * restrict 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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.Sh DESCRIPTION
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The
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.Fn recvfrom
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and
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.Fn recvmsg
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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 non-nil, 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 nil
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.Fa from
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argument.
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As it is redundant, it may not be supported in future releases.
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.Pp
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All three routines return the length of the message on successful
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completion.
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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 nonblocking (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 external variable
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.Va errno
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set to
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.Er EAGAIN .
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The receive calls 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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.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 arrive.
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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 MSG_WAITALL -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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.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 MSG_PEEK 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 MSG_WAITALL 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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.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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.Pp
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.Bd -literal
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struct msghdr {
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caddr_t msg_name; /* optional address */
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u_int msg_namelen; /* size of address */
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struct iovec *msg_iov; /* scatter/gather array */
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u_int msg_iovlen; /* # elements in msg_iov */
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caddr_t msg_control; /* ancillary data, see below */
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u_int 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 destination 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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u_int 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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.Pp
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.Ed
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As an example, one could use this to learn of changes in the data-stream
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in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting
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a recvmsg with no data buffer provided immediately after an
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.Fn accept
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system call.
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.Pp
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Open file descriptors are now passed as ancillary data for
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.Dv AF_UNIX
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domain sockets, with
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.Fa cmsg_level
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set to
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.Dv SOL_SOCKET
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and
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.Fa cmsg_type
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set to
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.Dv SCM_RIGHTS .
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.Pp
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Process credentials can also be passed as ancillary data for
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.Dv AF_UNIX
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domain sockets using a
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.Fa cmsg_type
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of
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.Dv SCM_CREDS .
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In this case,
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.Fa cmsg_data
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should be a structure of type
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.Fa cmsgcred ,
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which is defined in
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.In sys/socket.h
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as follows:
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.Pp
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.Bd -literal
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struct cmsgcred {
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pid_t cmcred_pid; /* PID of sending process */
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uid_t cmcred_uid; /* real UID of sending process */
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uid_t cmcred_euid; /* effective UID of sending process */
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gid_t cmcred_gid; /* real GID of sending process */
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short cmcred_ngroups; /* number or groups */
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gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
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};
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.Ed
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.Pp
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The kernel will fill in the credential information of the sending process
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and deliver it to the receiver.
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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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.Sh RETURN VALUES
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These calls return the number of bytes received, or -1
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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 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 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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.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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