mirror of https://github.com/mkerrisk/man-pages
538 lines
15 KiB
Groff
538 lines
15 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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.\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
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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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.\" %%%LICENSE_END
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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 2015-12-28 "Linux" "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 " sockfd ", void *" buf ", size_t " len ", int " flags );
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.sp
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.BI "ssize_t recvfrom(int " sockfd ", void *" buf ", size_t " len ", int " flags ,
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.BI " struct sockaddr *" src_addr ", socklen_t *" addrlen );
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.sp
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.BI "ssize_t recvmsg(int " sockfd ", 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 recv (),
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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.
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They may be used
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to receive data on both connectionless and connection-oriented sockets.
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This page first describes common features of all three system calls,
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and then describes the differences between the calls.
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.PP
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All three calls 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, 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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is set to
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.BR EAGAIN " or " EWOULDBLOCK .
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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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An application can use
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.BR select (2),
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.BR poll (2),
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or
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.BR epoll (7)
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to determine when more data arrives on a socket.
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.SS The flags argument
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The
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.I flags
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argument is formed by ORing one or more of the following values:
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.TP
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.BR MSG_CMSG_CLOEXEC " (" recvmsg "() only; since Linux 2.6.23)"
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Set the close-on-exec flag for the file descriptor received
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via a UNIX domain file descriptor using the
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.B SCM_RIGHTS
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operation (described in
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.BR unix (7)).
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This flag is useful for the same reasons as the
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.B O_CLOEXEC
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flag of
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.BR open (2).
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.TP
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.BR MSG_DONTWAIT " (since Linux 2.2)"
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Enables nonblocking operation; if the operation would block,
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the call fails with the error
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.BR EAGAIN " or " EWOULDBLOCK .
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This provides similar behavior to setting the
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.B O_NONBLOCK
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flag (via the
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.BR fcntl (2)
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.B F_SETFL
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operation), but differs in that
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.B MSG_DONTWAIT
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is a per-call option, whereas
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.B O_NONBLOCK
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is a setting on the open file description (see
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.BR open (2)),
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which will affect all threads in the calling process
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and as well as other processes that hold file descriptors
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referring to the same open file description.
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.TP
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.BR MSG_ERRQUEUE " (since Linux 2.2)"
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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.
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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 +4n
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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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uint32_t ee_errno; /* error number */
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uint8_t ee_origin; /* where the error originated */
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uint8_t ee_type; /* type */
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uint8_t ee_code; /* code */
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uint8_t ee_pad; /* padding */
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uint32_t ee_info; /* additional information */
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uint32_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
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.IP
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.I ee_errno
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contains the
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.I errno
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number of the queued error.
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.I 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.
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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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.I 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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.I sockaddr
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are undefined.
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The payload of the packet 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.
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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
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return data from the beginning of the
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receive queue without removing that data from the queue.
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Thus, a
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subsequent receive call will return the same data.
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.TP
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.BR MSG_TRUNC " (since Linux 2.2)"
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For raw
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.RB ( AF_PACKET ),
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Internet datagram (since Linux 2.4.27/2.6.8),
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netlink (since Linux 2.6.22), and UNIX datagram (since Linux 3.4) sockets:
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return the real length of the packet or datagram,
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even when it was longer than the passed buffer.
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For use with Internet stream sockets, see
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.BR tcp (7).
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.TP
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.BR MSG_WAITALL " (since Linux 2.2)"
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This flag requests that the operation block until the full request is
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satisfied.
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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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.\"
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.SS recvfrom()
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.BR recvfrom ()
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places the received message into the buffer
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.IR buf .
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The caller must specify the size of the buffer in
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.IR len .
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If
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.I src_addr
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is not NULL,
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and the underlying protocol provides the source address of the message,
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that source address is placed in the buffer pointed to by
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.IR src_addr .
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.\" (Note: for datagram sockets in both the UNIX and Internet domains,
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.\" .I src_addr
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.\" is filled in.
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.\" .I src_addr
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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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In this case,
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.I addrlen
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is a value-result argument.
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Before the call,
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it should be initialized to the size of the buffer associated with
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.IR src_addr .
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Upon return,
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.I addrlen
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is updated to contain the actual size of the source address.
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The returned address is truncated if the buffer provided is too small;
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in this case,
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.I addrlen
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will return a value greater than was supplied to the call.
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If the caller is not interested in the source address,
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.I src_addr
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and
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.I addrlen
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should be specified as NULL.
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.\"
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.SS recv()
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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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It is equivalent to the call:
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recvfrom(fd, buf, len, flags, NULL, 0));
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.\"
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.SS recvmsg()
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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 arguments.
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This structure is defined as follows in
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.IR <sys/socket.h> :
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.in +4n
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.nf
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struct iovec { /* Scatter/gather array items */
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void *iov_base; /* Starting address */
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size_t iov_len; /* Number of bytes to transfer */
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};
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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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size_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
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.PP
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The
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.I msg_name
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field points to a caller-allocated buffer that is used to
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return the source address if the socket is unconnected.
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The caller should set
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.I msg_namelen
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to the size of this buffer before this call;
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upon return from a successful call,
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.I msg_namelen
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will contain the length of the returned address.
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If the application does not need to know the source address,
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.I msg_name
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can be specified as NULL.
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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.
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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 +4n
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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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unsigned char cmsg_data[]; */
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};
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.fi
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.in
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.PP
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Ancillary data should be accessed only 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 ancillary data mechanism to pass extended
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errors, IP options, or file descriptors over UNIX domain 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
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.I msghdr
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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.
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In the event of an error,
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.I errno
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is set to indicate the error.
|
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When a stream socket peer has performed an orderly shutdown,
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the return value will be 0 (the traditional "end-of-file" return).
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Datagram sockets in various domains (e.g., the UNIX and Internet domains)
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permit zero-length datagrams.
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When such a datagram is received, the return value is 0.
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|
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The value 0 may also be returned if the requested number of bytes
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to receive from a stream socket was 0.
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.SH ERRORS
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|
These are some standard errors generated by the socket layer.
|
|
Additional errors
|
|
may be generated and returned from the underlying protocol modules;
|
|
see their manual pages.
|
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.TP
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.BR EAGAIN " or " EWOULDBLOCK
|
|
.\" Actually EAGAIN on Linux
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The socket is marked nonblocking and the receive operation
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|
would block, or a receive timeout had been set and the timeout expired
|
|
before data was received.
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|
POSIX.1 allows either error to be returned for this case,
|
|
and does not require these constants to have the same value,
|
|
so a portable application should check for both possibilities.
|
|
.TP
|
|
.B EBADF
|
|
The argument
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.I sockfd
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|
is an invalid file descriptor.
|
|
.TP
|
|
.B ECONNREFUSED
|
|
A remote host refused to allow the network connection (typically
|
|
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
|
|
any data were available; see
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.BR signal (7).
|
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.TP
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.B EINVAL
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|
Invalid argument passed.
|
|
.\" e.g., msg_namelen < 0 for recvmsg() or addrlen < 0 for recvfrom()
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.TP
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.B ENOMEM
|
|
Could not allocate memory for
|
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.BR recvmsg ().
|
|
.TP
|
|
.B ENOTCONN
|
|
The socket is associated with a connection-oriented protocol
|
|
and has not been connected (see
|
|
.BR connect (2)
|
|
and
|
|
.BR accept (2)).
|
|
.TP
|
|
.B ENOTSOCK
|
|
The file descriptor
|
|
.I sockfd
|
|
does not refer to a socket.
|
|
.SH CONFORMING TO
|
|
POSIX.1-2001, POSIX.1-2008,
|
|
4.4BSD (these interfaces first appeared in 4.2BSD).
|
|
.LP
|
|
POSIX.1 describes only the
|
|
.BR MSG_OOB ,
|
|
.BR MSG_PEEK ,
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|
and
|
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.B MSG_WAITALL
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|
flags.
|
|
.SH NOTES
|
|
The
|
|
.I socklen_t
|
|
type was invented by POSIX.
|
|
See also
|
|
.BR accept (2).
|
|
|
|
According to POSIX.1,
|
|
.\" POSIX.1-2001, POSIX.1-2008
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|
the
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.I msg_controllen
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|
field of the
|
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.I msghdr
|
|
structure should be typed as
|
|
.IR socklen_t ,
|
|
but glibc currently types it as
|
|
.IR size_t .
|
|
.\" glibc bug raised 12 Mar 2006
|
|
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
|
|
.\" 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.
|
|
|
|
See
|
|
.BR recvmmsg (2)
|
|
for information about a Linux-specific system call
|
|
that can be used to receive multiple datagrams in a single call.
|
|
.SH EXAMPLE
|
|
An example of the use of
|
|
.BR recvfrom ()
|
|
is shown in
|
|
.BR getaddrinfo (3).
|
|
.SH SEE ALSO
|
|
.BR fcntl (2),
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.BR getsockopt (2),
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|
.BR read (2),
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|
.BR recvmmsg (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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|
.BR socket (7)
|