+++ /dev/null
-.\" Copyright (C) 2011 by Andi Kleen <andi@firstfloor.org>
-.\" and Copyright (c) 2011 by Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
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-.\" preserved on all copies.
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-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
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-.\" manual page may be incorrect or out-of-date. The author(s) assume no
-.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein. The author(s) may not
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-.\"
-.\" Syscall added in following commit
-.\" commit a2e2725541fad72416326798c2d7fa4dafb7d337
-.\" Author: Arnaldo Carvalho de Melo <acme@redhat.com>
-.\" Date: Mon Oct 12 23:40:10 2009 -0700
-.\"
-.TH RECVMMSG 2 2014-06-13 "Linux" "Linux Programmer's Manual"
-.SH NAME
-recvmmsg \- receive multiple messages on a socket
-.SH SYNOPSIS
-.nf
-.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
-.BI "#include <sys/socket.h>"
-
-.BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \
-", unsigned int " vlen ","
-.br
-.BI " unsigned int " flags ", struct timespec *" timeout ");"
-.fi
-.SH DESCRIPTION
-The
-.BR recvmmsg ()
-system call is an extension of
-.BR recvmsg (2)
-that allows the caller to receive multiple messages from a socket
-using a single system call.
-(This has performance benefits for some applications.)
-A further extension over
-.BR recvmsg (2)
-is support for a timeout on the receive operation.
-
-The
-.I sockfd
-argument is the file descriptor of the socket to receive data from.
-
-The
-.I msgvec
-argument is a pointer to an array of
-.I mmsghdr
-structures.
-The size of this array is specified in
-.IR vlen .
-
-The
-.I mmsghdr
-structure is defined in
-.I <sys/socket.h>
-as:
-
-.in +4n
-.nf
-struct mmsghdr {
- struct msghdr msg_hdr; /* Message header */
- unsigned int msg_len; /* Number of received bytes for header */
-};
-.fi
-.in
-.PP
-The
-.I msg_hdr
-field is a
-.I msghdr
-structure, as described in
-.BR recvmsg (2).
-The
-.I msg_len
-field is the number of bytes returned for the message in the entry.
-This field has the same value as the return value of a single
-.BR recvmsg (2)
-on the header.
-
-The
-.I flags
-argument contains flags ORed together.
-The flags are the same as documented for
-.BR recvmsg (2),
-with the following addition:
-.TP
-.BR MSG_WAITFORONE " (since Linux 2.6.34)"
-Turns on
-.B MSG_DONTWAIT
-after the first message has been received.
-.PP
-The
-.I timeout
-argument points to a
-.I struct timespec
-(see
-.BR clock_gettime (2))
-defining a timeout (seconds plus nanoseconds) for the receive operation
-.RI ( "but see BUGS!" ).
-(This interval will be rounded up to the system clock granularity,
-and kernel scheduling delays mean that the blocking interval
-may overrun by a small amount.)
-If
-.I timeout
-is NULL, then the operation blocks indefinitely.
-
-A blocking
-.BR recvmmsg ()
-call blocks until
-.I vlen
-messages have been received
-or until the timeout expires.
-A nonblocking call reads as many messages as are available
-(up to the limit specified by
-.IR vlen )
-and returns immediately.
-
-On return from
-.BR recvmmsg (),
-successive elements of
-.IR msgvec
-are updated to contain information about each received message:
-.I msg_len
-contains the size of the received message;
-the subfields of
-.I msg_hdr
-are updated as described in
-.BR recvmsg (2).
-The return value of the call indicates the number of elements of
-.I msgvec
-that have been updated.
-.SH RETURN VALUE
-On success,
-.BR recvmmsg ()
-returns the number of messages received in
-.IR msgvec ;
-on error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-Errors are as for
-.BR recvmsg (2).
-In addition, the following error can occur:
-.TP
-.B EINVAL
-.I timeout
-is invalid.
-.SH VERSIONS
-The
-.BR recvmmsg ()
-system call was added in Linux 2.6.33.
-Support in glibc was added in version 2.12.
-.SH CONFORMING TO
-.BR recvmmsg ()
-is Linux-specific.
-.SH EXAMPLE
-.PP
-The following program uses
-.BR recvmmsg ()
-to receive multiple messages on a socket and stores
-them in multiple buffers.
-The call returns if all buffers are filled or if the
-timeout specified has expired.
-
-The following snippet periodically generates UDP datagrams
-containing a random number:
-.in +4n
-.nf
-
-.RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "
-.B " sleep 0.25; done"
-.fi
-.in
-
-These datagrams are read by the example application, which
-can give the following output:
-.in +4n
-.nf
-
-.RB "$" " ./a.out"
-5 messages received
-1 11782
-2 11345
-3 304
-4 13514
-5 28421
-.fi
-.in
-.SS Program source
-\&
-.nf
-#define _GNU_SOURCE
-#include <netinet/ip.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <sys/socket.h>
-
-int
-main(void)
-{
-#define VLEN 10
-#define BUFSIZE 200
-#define TIMEOUT 1
- int sockfd, retval, i;
- struct sockaddr_in sa;
- struct mmsghdr msgs[VLEN];
- struct iovec iovecs[VLEN];
- char bufs[VLEN][BUFSIZE+1];
- struct timespec timeout;
-
- sockfd = socket(AF_INET, SOCK_DGRAM, 0);
- if (sockfd == \-1) {
- perror("socket()");
- exit(EXIT_FAILURE);
- }
-
- sa.sin_family = AF_INET;
- sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
- sa.sin_port = htons(1234);
- if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == \-1) {
- perror("bind()");
- exit(EXIT_FAILURE);
- }
-
- memset(msgs, 0, sizeof(msgs));
- for (i = 0; i < VLEN; i++) {
- iovecs[i].iov_base = bufs[i];
- iovecs[i].iov_len = BUFSIZE;
- msgs[i].msg_hdr.msg_iov = &iovecs[i];
- msgs[i].msg_hdr.msg_iovlen = 1;
- }
-
- timeout.tv_sec = TIMEOUT;
- timeout.tv_nsec = 0;
-
- retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
- if (retval == \-1) {
- perror("recvmmsg()");
- exit(EXIT_FAILURE);
- }
-
- printf("%d messages received\\n", retval);
- for (i = 0; i < retval; i++) {
- bufs[i][msgs[i].msg_len] = 0;
- printf("%d %s", i+1, bufs[i]);
- }
- exit(EXIT_SUCCESS);
-}
-.fi
-.SH BUGS
-The
-.I timeout
-argument does not work as intended.
-.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371
-.\" http://thread.gmane.org/gmane.linux.man/5677
-The timeout is checked only after the receipt of each datagram,
-so that if up to
-.I vlen\-1
-datagrams are received before the timeout expires,
-but then no further datagrams are received, the call will block forever.
-.SH SEE ALSO
-.BR clock_gettime (2),
-.BR recvmsg (2),
-.BR sendmmsg (2),
-.BR sendmsg (2),
-.BR socket (2),
-.BR socket (7)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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