mirror of https://github.com/mkerrisk/man-pages
293 lines
7.4 KiB
Groff
293 lines
7.4 KiB
Groff
.\" Copyright (C) 2011 by Andi Kleen <andi@firstfloor.org>
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.\" and Copyright (c) 2011 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Syscall added in following commit
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.\" commit a2e2725541fad72416326798c2d7fa4dafb7d337
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.\" Author: Arnaldo Carvalho de Melo <acme@redhat.com>
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.\" Date: Mon Oct 12 23:40:10 2009 -0700
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.\"
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.TH RECVMMSG 2 2020-06-09 "Linux" "Linux Programmer's Manual"
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.SH NAME
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recvmmsg \- receive multiple messages on a socket
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.SH SYNOPSIS
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.nf
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.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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.BI "#include <sys/socket.h>"
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.PP
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.BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \
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", unsigned int " vlen ","
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.BI " int " flags ", struct timespec *" timeout ");"
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.fi
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.SH DESCRIPTION
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The
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.BR recvmmsg ()
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system call is an extension of
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.BR recvmsg (2)
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that allows the caller to receive multiple messages from a socket
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using a single system call.
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(This has performance benefits for some applications.)
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A further extension over
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.BR recvmsg (2)
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is support for a timeout on the receive operation.
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.PP
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The
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.I sockfd
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argument is the file descriptor of the socket to receive data from.
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.PP
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The
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.I msgvec
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argument is a pointer to an array of
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.I mmsghdr
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structures.
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The size of this array is specified in
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.IR vlen .
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.PP
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The
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.I mmsghdr
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structure is defined in
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.I <sys/socket.h>
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as:
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.PP
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.in +4n
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.EX
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struct mmsghdr {
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struct msghdr msg_hdr; /* Message header */
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unsigned int msg_len; /* Number of received bytes for header */
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};
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.EE
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.in
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.PP
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The
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.I msg_hdr
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field is a
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.I msghdr
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structure, as described in
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.BR recvmsg (2).
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The
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.I msg_len
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field is the number of bytes returned for the message in the entry.
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This field has the same value as the return value of a single
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.BR recvmsg (2)
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on the header.
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.PP
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The
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.I flags
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argument contains flags ORed together.
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The flags are the same as documented for
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.BR recvmsg (2),
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with the following addition:
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.TP
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.BR MSG_WAITFORONE " (since Linux 2.6.34)"
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Turns on
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.B MSG_DONTWAIT
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after the first message has been received.
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.PP
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The
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.I timeout
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argument points to a
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.I struct timespec
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(see
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.BR clock_gettime (2))
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defining a timeout (seconds plus nanoseconds) for the receive operation
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.RI ( "but see BUGS!" ).
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(This interval will be rounded up to the system clock granularity,
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and kernel scheduling delays mean that the blocking interval
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may overrun by a small amount.)
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If
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.I timeout
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is NULL, then the operation blocks indefinitely.
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.PP
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A blocking
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.BR recvmmsg ()
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call blocks until
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.I vlen
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messages have been received
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or until the timeout expires.
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A nonblocking call reads as many messages as are available
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(up to the limit specified by
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.IR vlen )
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and returns immediately.
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.PP
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On return from
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.BR recvmmsg (),
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successive elements of
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.IR msgvec
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are updated to contain information about each received message:
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.I msg_len
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contains the size of the received message;
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the subfields of
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.I msg_hdr
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are updated as described in
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.BR recvmsg (2).
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The return value of the call indicates the number of elements of
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.I msgvec
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that have been updated.
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.SH RETURN VALUE
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On success,
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.BR recvmmsg ()
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returns the number of messages received in
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.IR msgvec ;
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on error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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Errors are as for
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.BR recvmsg (2).
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In addition, the following error can occur:
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.TP
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.B EINVAL
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.I timeout
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is invalid.
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.PP
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See also BUGS.
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.SH VERSIONS
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The
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.BR recvmmsg ()
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system call was added in Linux 2.6.33.
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Support in glibc was added in version 2.12.
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.SH CONFORMING TO
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.BR recvmmsg ()
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is Linux-specific.
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.SH BUGS
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The
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.I timeout
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argument does not work as intended.
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.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371
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.\" http://thread.gmane.org/gmane.linux.man/5677
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The timeout is checked only after the receipt of each datagram,
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so that if up to
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.I vlen\-1
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datagrams are received before the timeout expires,
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but then no further datagrams are received, the call will block forever.
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.PP
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If an error occurs after at least one message has been received,
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the call succeeds, and returns the number of messages received.
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The error code is expected to be returned on a subsequent call to
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.BR recvmmsg ().
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In the current implementation, however, the error code can be overwritten
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in the meantime by an unrelated network event on a socket,
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for example an incoming ICMP packet.
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.SH EXAMPLES
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The following program uses
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.BR recvmmsg ()
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to receive multiple messages on a socket and stores
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them in multiple buffers.
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The call returns if all buffers are filled or if the
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timeout specified has expired.
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.PP
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The following snippet periodically generates UDP datagrams
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containing a random number:
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.PP
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.in +4n
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.EX
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.RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234;"
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.B " sleep 0.25; done"
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.EE
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.in
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.PP
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These datagrams are read by the example application, which
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can give the following output:
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.PP
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.in +4n
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.EX
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.RB "$" " ./a.out"
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5 messages received
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1 11782
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2 11345
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3 304
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4 13514
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5 28421
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.EE
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.in
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.SS Program source
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\&
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.EX
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#define _GNU_SOURCE
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#include <netinet/ip.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/socket.h>
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int
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main(void)
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{
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#define VLEN 10
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#define BUFSIZE 200
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#define TIMEOUT 1
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int sockfd, retval, i;
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struct sockaddr_in addr;
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struct mmsghdr msgs[VLEN];
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struct iovec iovecs[VLEN];
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char bufs[VLEN][BUFSIZE+1];
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struct timespec timeout;
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sockfd = socket(AF_INET, SOCK_DGRAM, 0);
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if (sockfd == \-1) {
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perror("socket()");
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exit(EXIT_FAILURE);
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}
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addr.sin_family = AF_INET;
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addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
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addr.sin_port = htons(1234);
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if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == \-1) {
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perror("bind()");
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exit(EXIT_FAILURE);
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}
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memset(msgs, 0, sizeof(msgs));
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for (i = 0; i < VLEN; i++) {
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iovecs[i].iov_base = bufs[i];
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iovecs[i].iov_len = BUFSIZE;
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msgs[i].msg_hdr.msg_iov = &iovecs[i];
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msgs[i].msg_hdr.msg_iovlen = 1;
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}
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timeout.tv_sec = TIMEOUT;
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timeout.tv_nsec = 0;
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retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
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if (retval == \-1) {
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perror("recvmmsg()");
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exit(EXIT_FAILURE);
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}
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printf("%d messages received\en", retval);
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for (i = 0; i < retval; i++) {
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bufs[i][msgs[i].msg_len] = 0;
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printf("%d %s", i+1, bufs[i]);
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}
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exit(EXIT_SUCCESS);
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}
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.EE
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.SH SEE ALSO
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.BR clock_gettime (2),
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.BR recvmsg (2),
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.BR sendmmsg (2),
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.BR sendmsg (2),
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.BR socket (2),
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.BR socket (7)
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