mirror of https://github.com/mkerrisk/man-pages
439 lines
13 KiB
Groff
439 lines
13 KiB
Groff
.\" Copyright (c) 1983, 1990, 1991 The Regents of the University of California.
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.\" 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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.\" $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $
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.\"
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.\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>
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.\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 1998,1999 by Andi Kleen
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.\" 2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin
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.\"
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.TH RECV 2 2002-12-31 "Linux Man Page" "Linux Programmer's Manual"
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.SH NAME
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recv, recvfrom, recvmsg \- receive a message from a socket
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.SH SYNOPSIS
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.\" .B #include <sys/uio.h>
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.\" .br
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.nf
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.B #include <sys/types.h>
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.br
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.B #include <sys/socket.h>
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.sp
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.BI "ssize_t recv(int " s ", void *" buf ", size_t " len ", int " flags );
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.sp
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.BI "ssize_t recvfrom(int " s ", void *" buf ", size_t " len ", int " flags ,
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.BI " struct sockaddr *" from ", socklen_t *" fromlen );
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.sp
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.BI "ssize_t recvmsg(int " s ", struct msghdr *" msg ", int " flags );
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.fi
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.SH DESCRIPTION
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The
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.BR recvfrom ()
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and
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.BR recvmsg ()
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calls are used to receive messages from a socket, and may be used
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to receive data on a socket whether or not it is connection-oriented.
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.PP
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If
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.I from
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is not NULL, and the underlying protocol provides the source address,
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this source address is filled in.
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.\" (Note: for datagram sockets in both the Unix and Internet domains,
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.\" .I from
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.\" is filled in.
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.\" .I from
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.\" is also filled in for stream sockets in the Unix domain, but is not
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.\" filled in for stream sockets in the Internet domain.)
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.\" [The above notes on AF_UNIX and AF_INET sockets apply as at
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.\" Kernel 2.4.18. (MTK, 22 Jul 02)]
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The argument
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.I fromlen
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is a value-result parameter, initialized to the size of the buffer
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associated with
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.IR from ,
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and modified on return to indicate the actual size of the address stored
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there.
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.PP
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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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.I 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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.PP
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All three routines return the length of the message on successful
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completion. If a message is too long to fit in the supplied buffer, excess
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bytes may be discarded depending on the type of socket the message is
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received from.
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.PP
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If no messages are available at the socket, the receive calls wait for a
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message to arrive, unless 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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.I errno
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set to
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.BR EAGAIN .
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The receive calls normally return any data available, up to the requested
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amount, rather than waiting for receipt of the full amount requested.
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.PP
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The
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.BR select (2)
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or
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.BR poll (2)
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call may be used to determine when more data arrives.
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.PP
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The
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.I flags
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argument to a
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.BR recv ()
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call is formed by
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.IR OR 'ing
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one or more of the following values:
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.TP
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.B MSG_DONTWAIT
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Enables non-blocking operation; if the operation would block,
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.B EAGAIN
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is returned (this can also be enabled using the
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.B O_NONBLOCK
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with the
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.B F_SETFL
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.BR fcntl (2)).
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.TP
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.B MSG_ERRQUEUE
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This flag
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specifies that queued errors should be received from the socket error queue.
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The error is passed in
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an ancillary message with a type dependent on the protocol (for IPv4
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.BR IP_RECVERR ).
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The user should supply a buffer of sufficient size. See
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.BR cmsg (3)
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and
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.BR ip (7)
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for more information.
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The payload of the original packet that caused the error
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is passed as normal data via
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.IR msg_iovec .
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The original destination address of the datagram that caused the error
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is supplied via
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.IR msg_name .
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.IP
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For local errors, no address is passed (this can be checked with the
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.I cmsg_len
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member of the
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.IR cmsghdr ).
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For error receives, the
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.B MSG_ERRQUEUE
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is set in the
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.IR msghdr .
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After an error has been passed, the pending socket error
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is regenerated based on the next queued error and will be passed
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on the next socket operation.
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The error is supplied in a
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.I sock_extended_err
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structure:
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.in +0.25i
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.nf
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#define SO_EE_ORIGIN_NONE 0
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#define SO_EE_ORIGIN_LOCAL 1
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#define SO_EE_ORIGIN_ICMP 2
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#define SO_EE_ORIGIN_ICMP6 3
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struct sock_extended_err
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{
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u_int32_t ee_errno; /* error number */
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u_int8_t ee_origin; /* where the error originated */
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u_int8_t ee_type; /* type */
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u_int8_t ee_code; /* code */
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u_int8_t ee_pad;
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u_int32_t ee_info; /* additional information */
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u_int32_t ee_data; /* other data */
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/* More data may follow */
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};
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struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
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.fi
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.in -0.25i
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.IP
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.B ee_errno
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contains the errno number of the queued error.
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.B ee_origin
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is the origin code of where the error originated.
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The other fields are protocol specific. The macro
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.B SOCK_EE_OFFENDER
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returns a pointer to the address of the network object
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where the error originated from given a pointer to the ancillary message.
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If this address is not known, the
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.I sa_family
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member of the
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.B sockaddr
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contains
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.B AF_UNSPEC
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and the other fields of the
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.B sockaddr
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are undefined. The payload of the packet
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that caused the error is passed as normal data.
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.IP
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For local errors, no address is passed (this
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can be checked with the
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.I cmsg_len
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member of the
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.IR cmsghdr ).
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For error receives,
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the
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.B MSG_ERRQUEUE
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is set in the
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.IR msghdr .
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After an error has been passed, the pending socket error
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is regenerated based on the next queued error and will be passed
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on the next socket operation.
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.TP
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.B MSG_OOB
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This flag requests receipt of out-of-band data that would not be received
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in the normal data stream. Some protocols place expedited data
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at the head of the normal data queue, and thus this flag cannot
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be used with such protocols.
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.TP
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.B MSG_PEEK
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This flag causes the receive operation to return data from the beginning of the
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receive queue without removing that data from the queue. Thus, a
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subsequent receive call will return the same data.
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.TP
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.B MSG_TRUNC
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Return the real length of the packet, even when it was longer than
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the passed buffer. Only valid for packet sockets.
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.TP
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.B MSG_WAITALL
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This flag requests that the operation block until the full request is
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satisfied. However, the call may still return less data than requested if
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a signal is caught, an error or disconnect occurs, or the next data to be
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received is of a different type than that returned.
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.PP
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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. This
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structure has the following form, as defined in
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.IR <sys/socket.h> :
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.in +0.25i
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.nf
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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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size_t 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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.fi
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.in -0.25i
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.PP
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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 source 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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The fields
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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 readv (2).
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The field
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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 or
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miscellaneous ancillary data. When
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.BR recvmsg ()
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is called,
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.I msg_controllen
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should contain the length of the available buffer in
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.IR msg_control ;
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upon return from a successful call it will contain the length
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of the control message sequence.
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.PP
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The messages are of the form:
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.in +0.25i
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.nf
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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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.fi
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.in -0.25i
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.PP
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Ancillary data should only be accessed by the macros defined in
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.BR cmsg (3).
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.PP
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As an example, Linux uses this auxiliary data mechanism to pass extended
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errors, IP options or file descriptors over Unix sockets.
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.PP
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The
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.I msg_flags
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field in the msghdr is set on return of
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.BR recvmsg ().
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It can contain several flags:
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.TP
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.B MSG_EOR
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indicates end-of-record; the data returned completed a record (generally
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used with sockets of type
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.BR SOCK_SEQPACKET ).
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.TP
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.B MSG_TRUNC
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indicates that the trailing portion of a datagram was discarded because the
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datagram was larger than the buffer supplied.
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.TP
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.B MSG_CTRUNC
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indicates that some control data were discarded due to lack of space in the
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buffer for ancillary data.
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.TP
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.B MSG_OOB
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is returned to indicate that expedited or out-of-band data were received.
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.TP
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.B MSG_ERRQUEUE
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indicates that no data was received but an extended error from the socket
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error queue.
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.SH "RETURN VALUE"
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These calls return the number of bytes received, or \-1
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if an error occurred. The return value will be 0 when the
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peer has performed an orderly shutdown.
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.SH ERRORS
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These are some standard errors generated by the socket layer. Additional errors
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may be generated and returned from the underlying protocol modules; see their
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manual pages.
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.TP
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.B EAGAIN
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The socket is marked non-blocking and the receive operation
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would block, or a receive timeout had been set and the timeout expired
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before data was received.
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.TP
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.B EBADF
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The argument
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.I s
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is an invalid descriptor.
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.TP
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.B ECONNREFUSED
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A remote host refused to allow the network connection (typically
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because it is not running the requested service).
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.TP
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.B 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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.TP
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.B 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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.TP
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.B EINVAL
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Invalid argument passed.
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.\" e.g., msg_namelen < 0 for recvmsg() or fromlen < 0 for recvfrom()
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.TP
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.B ENOMEM
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Could not allocate memory for
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.BR recvmsg ().
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.TP
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.B 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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.BR connect (2)
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and
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.BR accept (2)).
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.TP
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.B ENOTSOCK
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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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.SH "CONFORMING TO"
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4.4BSD (these function calls first appeared in 4.2BSD),
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POSIX 1003.1-2001.
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.LP
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POSIX only describes the
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.BR MSG_OOB ,
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.BR MSG_PEEK ,
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and
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.B MSG_WAITALL
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flags.
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.SH NOTES
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The prototypes given above follow glibc2.
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The Single Unix Specification agrees, except that it has return values
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of type `ssize_t' (while 4.x BSD and libc4 and libc5 all have `int').
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The
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.I flags
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argument is `int' in 4.x BSD, but `unsigned int' in libc4 and libc5.
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The
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.I len
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argument is `int' in 4.x BSD, but `size_t' in libc4 and libc5.
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The
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.I fromlen
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argument is `int *' in 4.x BSD, libc4 and libc5.
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The present `socklen_t *' was invented by POSIX.
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See also
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.BR accept (2).
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According to POSIX.1-2001, the
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.I msg_controllen
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field of the
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.I msghdr
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structure should be typed as
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.IR socklen_t ,
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but glibc currently (2.4) types it as
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.IR size_t .
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.\" glibc bug raised 12 Mar 2006
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
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.\" The problem is an underlying kernel issue: the size of the
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.\" __kernel_size_t type used to type this field varies
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.\" across architectures, but socklen_t is always 32 bits.
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.SH "SEE ALSO"
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.BR fcntl (2),
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.BR getsockopt (2),
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.BR read (2),
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.BR select (2),
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.BR shutdown (2),
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.BR socket (2),
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.BR cmsg (3),
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.BR sockatmark (3)
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