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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 2014-06-13 "Linux" "Linux Programmer's Manual"
33 recvmmsg \- receive multiple messages on a socket
36 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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 .RI ( "but see BUGS!" ).
116 (This interval will be rounded up to the system clock granularity,
117 and kernel scheduling delays mean that the blocking interval
118 may overrun by a small amount.)
121 is NULL, then the operation blocks indefinitely.
127 messages have been received
128 or until the timeout expires.
129 A nonblocking call reads as many messages as are available
130 (up to the limit specified by
132 and returns immediately.
136 successive elements of
138 are updated to contain information about each received message:
140 contains the size of the received message;
143 are updated as described in
145 The return value of the call indicates the number of elements of
147 that have been updated.
151 returns the number of messages received in
153 on error, \-1 is returned, and
155 is set to indicate the error.
159 In addition, the following error can occur:
167 system call was added in Linux 2.6.33.
168 Support in glibc was added in version 2.12.
174 The following program uses
176 to receive multiple messages on a socket and stores
177 them in multiple buffers.
178 The call returns if all buffers are filled or if the
179 timeout specified has expired.
181 The following snippet periodically generates UDP datagrams
182 containing a random number:
186 .RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "
187 .B " sleep 0.25; done"
191 These datagrams are read by the example application, which
192 can give the following output:
209 #include <netinet/ip.h>
213 #include <sys/socket.h>
221 int sockfd, retval, i;
222 struct sockaddr_in sa;
223 struct mmsghdr msgs[VLEN];
224 struct iovec iovecs[VLEN];
225 char bufs[VLEN][BUFSIZE+1];
226 struct timespec timeout;
228 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
234 sa.sin_family = AF_INET;
235 sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
236 sa.sin_port = htons(1234);
237 if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == \-1) {
242 memset(msgs, 0, sizeof(msgs));
243 for (i = 0; i < VLEN; i++) {
244 iovecs[i].iov_base = bufs[i];
245 iovecs[i].iov_len = BUFSIZE;
246 msgs[i].msg_hdr.msg_iov = &iovecs[i];
247 msgs[i].msg_hdr.msg_iovlen = 1;
250 timeout.tv_sec = TIMEOUT;
253 retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
255 perror("recvmmsg()");
259 printf("%d messages received\\n", retval);
260 for (i = 0; i < retval; i++) {
261 bufs[i][msgs[i].msg_len] = 0;
262 printf("%d %s", i+1, bufs[i]);
270 argument does not work as intended.
271 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371
272 .\" http://thread.gmane.org/gmane.linux.man/5677
273 The timeout is checked only after the receipt of each datagram,
276 datagrams are received before the timeout expires,
277 but then no further datagrams are received, the call will block forever.
279 .BR clock_gettime (2),
286 This page is part of release 3.79 of the Linux
289 A description of the project,
290 information about reporting bugs,
291 and the latest version of this page,
293 \%http://www.kernel.org/doc/man\-pages/.