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280 lines
8.0 KiB
Groff
280 lines
8.0 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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.\"
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.TH RECV 2 "16 January 1997" GNO "System Calls"
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.SH NAME
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.BR recv ,
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.BR recvfrom ,
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.BR recvmsg
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\- receive a message from a socket
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.SH SYNOPSIS
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.br
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#include <sys/types.h>
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.br
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#include <sys/socket.h>
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.sp 1
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int
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\fBrecv\fR
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(int \fIs\fR, void *\fIbuf\fR, size_t \fIlen\fR, unsigned int \fIflags\fR);
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.sp 1
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int
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\fBrecvfrom\fR
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(int \fIs\fR, void *\fIbuf\fR, size_t \fIlen\fR, unsigned int \fIflags\fR,
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struct sockaddr *\fIfrom\fR, int *\fIfromlen\fR);
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.sp 1
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int
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\fBrecvmsg\fR
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(int \fIs\fR, struct msghdr *\fImsg\fR, unsigned int \fIflags\fR);
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.SH DESCRIPTION
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.BR Recvfrom
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and
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.BR recvmsg
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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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.LP
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If
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.I from
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is non-NULL, and the socket is not connection-oriented,
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the source address of the message is filled in.
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.I Fromlen
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is a value-result parameter, initialized to the size of
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the buffer associated with
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.IR 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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.LP
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The
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.BR recv
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call is normally used only on a
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.IR connected
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socket (see
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.BR connect (2))
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and is identical to
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.BR recvfrom
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with a NULL
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.I from
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parameter.
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As it is redundant, it may not be supported in future releases.
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.LP
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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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.BR socket (2)).
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.LP
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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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.BR fcntl (2))
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in which case the value -1 is returned and the external variable
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.IR errno
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set to 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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.BR SO_RCVLOWAT
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and
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.BR SO_RCVTIMEO
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described in
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.BR getsockopt (2).
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.LP
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The
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.BR select (2)
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call may be used to determine when more data arrive.
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.LP
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The
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.I flags
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argument to a recv call is formed by
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.IR or 'ing
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one or more of the values:
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.RS
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.TP
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MSG_OOB
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process out-of-band data
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.TP
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MSG_PEEK
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peek at incoming message
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.TP
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MSG_WAITALL
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wait for full request or error
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.RE
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.LP
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The
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.BR 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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.LP
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The
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.BR recvmsg
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call uses a
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.I msghdr
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structure to minimize the number of directly supplied parameters.
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This structure has the following form, as defined in <sys/socket.h>:
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.nf
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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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.fi
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Here
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.I msg_name
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and
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.I msg_namelen
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specify the destination address if the socket is unconnected;
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.I msg_name
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may be given as a null pointer if no names are desired or required.
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.I Msg_iov
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and
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.I msg_iovlen
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describe scatter gather locations, as discussed in
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.BR read (2).
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.IR Msg_control ,
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which has length
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.IR 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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.nf
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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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.fi
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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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.BR accept
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call.
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.LP
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Open file descriptors are now passed as ancillary data for
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.BR AF_UNIX
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domain sockets, with
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.I cmsg_level
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set to
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.BR SOL_SOCKET
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and
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.I cmsg_type
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set to
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.BR SCM_RIGHTS .
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.LP
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The
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.I msg_flags
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field is set on return according to the message received.
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.BR 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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.BR SOCK_SEQPACKET ).
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.BR 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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.BR 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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.BR MSG_OOB
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is returned to indicate that expedited or out-of-band data were received.
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.LP
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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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.RS
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.IP \fBEBADF\fR
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The argument
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.I s
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is an invalid descriptor.
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.IP \fBENOTCONN\fR
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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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.BR connect (2)
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and
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.BR accept (2)).
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.IP \fBENOTSOCK\fR
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The argument
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.I s
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does not refer to a socket.
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.IP \fBEAGAIN\fR
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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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.IP \fBEINTR\fR
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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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.IP \fBEFAULT\fR
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The receive buffer pointer(s) point outside the process's
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address space.
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.RE
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.SH CONVORMANCE
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The GNO prototypes of these routines differ slightly from that of
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4.4BSD.
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.SH SEE ALSO
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.BR fcntl (2),
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.BR read (2),
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.BR select (2),
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.BR getsockopt (2),
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.BR socket (2)
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.SH HISTORY
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The
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.BR recvmsg
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function call appeared in 4.2BSD.
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The first appearance in GNO was in v2.0.5.
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