2011-09-29 04:12:37 +00:00
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.\" Copyright (C) 2011 by Andi Kleen <andi@firstfloor.org>
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2011-09-09 20:56:04 +00:00
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.\" and Copyright (c) 2011 by Michael Kerrisk <mtk.manpages@gmail.com>
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2011-09-29 04:12:37 +00:00
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.\"
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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.\" manual page may be incorrect or out-of-date.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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.\"
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2011-10-05 04:45:34 +00:00
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.\" FIXME: This page could be improved with an example program.
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.\"
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2011-10-04 05:58:49 +00:00
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.TH RECVMMSG 2 2011-10-04 "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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.B "#define _GNU_SOURCE"
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.BI "#include <sys/socket.h>"
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.BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \
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", unsigned int " vlen ","
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.br
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.BI " unsigned 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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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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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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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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.in +4n
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.nf
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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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.fi
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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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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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.B MSG_WAITFORONE
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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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If
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.I timeout
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is
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.I NULL
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then the operation blocks indefinitely.
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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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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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.SH VERSIONS
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The
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.BR recvmmsg ()
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system call was added in Linux 2.6.32.
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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 SEE ALSO
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.BR clock_gettime (2),
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.BR recvmsg (2),
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2011-09-09 21:17:48 +00:00
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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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