1 .\" Copyright (C) 2011 by Andi Kleen <andi@firstfloor.org>
2 .\" and Copyright (c) 2011 by Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
5 .\" Permission is granted to make and distribute verbatim copies of this
6 .\" manual provided the copyright notice and this permission notice are
7 .\" preserved on all copies.
9 .\" Permission is granted to copy and distribute modified versions of this
10 .\" manual under the conditions for verbatim copying, provided that the
11 .\" entire resulting derived work is distributed under the terms of a
12 .\" permission notice identical to this one.
14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
16 .\" responsibility for errors or omissions, or for damages resulting from
17 .\" the use of the information contained herein. The author(s) may not
18 .\" have taken the same level of care in the production of this manual,
19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" the source, must acknowledge the copyright and authors of this work.
26 .\" Syscall added in following commit
27 .\" commit a2e2725541fad72416326798c2d7fa4dafb7d337
28 .\" Author: Arnaldo Carvalho de Melo <acme@redhat.com>
29 .\" Date: Mon Oct 12 23:40:10 2009 -0700
31 .TH RECVMMSG 2 2012-12-24 "Linux" "Linux Programmer's Manual"
33 recvmmsg \- receive multiple messages on a socket
36 .B "#define _GNU_SOURCE"
37 .BI "#include <sys/socket.h>"
39 .BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \
40 ", unsigned int " vlen ","
42 .BI " unsigned int " flags ", struct timespec *" timeout ");"
47 system call is an extension of
49 that allows the caller to receive multiple messages from a socket
50 using a single system call.
51 (This has performance benefits for some applications.)
52 A further extension over
54 is support for a timeout on the receive operation.
58 argument is the file descriptor of the socket to receive data from.
62 argument is a pointer to an array of
65 The size of this array is specified in
70 structure is defined in
77 struct msghdr msg_hdr; /* Message header */
78 unsigned int msg_len; /* Number of received bytes for header */
87 structure, as described in
91 field is the number of bytes returned for the message in the entry.
92 This field has the same value as the return value of a single
98 argument contains flags ORed together.
99 The flags are the same as documented for
101 with the following addition:
103 .BR MSG_WAITFORONE " (since Linux 2.6.34)"
106 after the first message has been received.
113 .BR clock_gettime (2))
114 defining a timeout (seconds plus nanoseconds) for the receive operation.
115 (This interval will be rounded up to the system clock granularity,
116 and kernel scheduling delays mean that the blocking interval
117 may overrun by a small amount.)
122 then the operation blocks indefinitely.
128 messages have been received
129 or until the timeout expires.
130 A nonblocking call reads as many messages as are available
131 (up to the limit specified by
133 and returns immediately.
137 successive elements of
139 are updated to contain information about each received message:
141 contains the size of the received message;
144 are updated as described in
146 The return value of the call indicates the number of elements of
148 that have been updated.
152 returns the number of messages received in
154 on error, \-1 is returned, and
156 is set to indicate the error.
160 In addition, the following error can occur:
168 system call was added in Linux 2.6.33.
169 Support in glibc was added in version 2.12.
175 The following program uses
177 to receive multiple messages on a socket and stores
178 them in multiple buffers.
179 The call returns if all buffers are filled or if the
180 timeout specified has expired.
182 The following snippet periodically generates UDP datagrams
183 containing a random number:
187 .RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "
188 .B " sleep 0.25; done"
192 These datagrams are read by the example application, which
193 can give the following output:
210 #include <netinet/ip.h>
214 #include <sys/socket.h>
222 int sockfd, retval, i;
223 struct sockaddr_in sa;
224 struct mmsghdr msgs[VLEN];
225 struct iovec iovecs[VLEN];
226 char bufs[VLEN][BUFSIZE+1];
227 struct timespec timeout;
229 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
235 sa.sin_family = AF_INET;
236 sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
237 sa.sin_port = htons(1234);
238 if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == \-1) {
243 memset(msgs, 0, sizeof(msgs));
244 for (i = 0; i < VLEN; i++) {
245 iovecs[i].iov_base = bufs[i];
246 iovecs[i].iov_len = BUFSIZE;
247 msgs[i].msg_hdr.msg_iov = &iovecs[i];
248 msgs[i].msg_hdr.msg_iovlen = 1;
251 timeout.tv_sec = TIMEOUT;
254 retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
256 perror("recvmmsg()");
260 printf("%d messages received\\n", retval);
261 for (i = 0; i < retval; i++) {
262 bufs[i][msgs[i].msg_len] = 0;
263 printf("%d %s", i+1, bufs[i]);
269 .BR clock_gettime (2),