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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 2016-10-08 "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.
175 argument does not work as intended.
176 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371
177 .\" http://thread.gmane.org/gmane.linux.man/5677
178 The timeout is checked only after the receipt of each datagram,
181 datagrams are received before the timeout expires,
182 but then no further datagrams are received, the call will block forever.
185 The following program uses
187 to receive multiple messages on a socket and stores
188 them in multiple buffers.
189 The call returns if all buffers are filled or if the
190 timeout specified has expired.
192 The following snippet periodically generates UDP datagrams
193 containing a random number:
197 .RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "
198 .B " sleep 0.25; done"
202 These datagrams are read by the example application, which
203 can give the following output:
220 #include <netinet/ip.h>
224 #include <sys/socket.h>
232 int sockfd, retval, i;
233 struct sockaddr_in addr;
234 struct mmsghdr msgs[VLEN];
235 struct iovec iovecs[VLEN];
236 char bufs[VLEN][BUFSIZE+1];
237 struct timespec timeout;
239 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
245 addr.sin_family = AF_INET;
246 addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
247 addr.sin_port = htons(1234);
248 if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == \-1) {
253 memset(msgs, 0, sizeof(msgs));
254 for (i = 0; i < VLEN; i++) {
255 iovecs[i].iov_base = bufs[i];
256 iovecs[i].iov_len = BUFSIZE;
257 msgs[i].msg_hdr.msg_iov = &iovecs[i];
258 msgs[i].msg_hdr.msg_iovlen = 1;
261 timeout.tv_sec = TIMEOUT;
264 retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
266 perror("recvmmsg()");
270 printf("%d messages received\\n", retval);
271 for (i = 0; i < retval; i++) {
272 bufs[i][msgs[i].msg_len] = 0;
273 printf("%d %s", i+1, bufs[i]);
279 .BR clock_gettime (2),