c2e3c52e0d
This change allows creating file descriptors with close-on-exec set in some situations. SOCK_CLOEXEC and SOCK_NONBLOCK can be OR'ed in socket() and socketpair()'s type parameter, and MSG_CMSG_CLOEXEC to recvmsg() makes file descriptors (SCM_RIGHTS) atomically close-on-exec. The numerical values for SOCK_CLOEXEC and SOCK_NONBLOCK are as in NetBSD. MSG_CMSG_CLOEXEC is the first free bit for MSG_*. The SOCK_* flags are not passed to MAC because this may cause incorrect failures and can be done later via fcntl() anyway. On the other hand, audit is expected to cope with the new flags. For MSG_CMSG_CLOEXEC, unp_externalize() is extended to take a flags argument. Reviewed by: kib
350 lines
9.7 KiB
Groff
350 lines
9.7 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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.\" 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 March 19, 2013
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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 *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 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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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 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 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 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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.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 strict
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.Tn ANSI
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or C99 compilation mode.
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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 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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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, 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
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.Fn recvmsg
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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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The close-on-exec flag on received descriptors is set according to the
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.Dv MSG_CMSG_CLOEXEC
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flag passed to
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.Fn recvmsg .
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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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.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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If a sender supplies ancillary data with enough space for the above struct
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tagged as
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.Dv SCM_CREDS
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control message type to the
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.Fn sendmsg
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system call, then kernel will fill in the credential information of the
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sending process and deliver it to the receiver.
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Since receiver usually has no control over a sender, this method of retrieving
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credential information isn't reliable.
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For reliable retrieval of remote side credentials it is advised to use the
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.Dv LOCAL_CREDS
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socket option on the receiving socket.
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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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.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 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 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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