1RECVMMSG(2) Linux Programmer's Manual RECVMMSG(2)
2
6 recvmmsg - receive multiple messages on a socket
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <sys/socket.h>
11 int recvmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
13 int flags, struct timespec *timeout);
14
16 The recvmmsg() system call is an extension of recvmsg(2) that allows
17 the caller to receive multiple messages from a socket using a single
18 system call. (This has performance benefits for some applications.) A
19 further extension over recvmsg(2) is support for a timeout on the re‐
20 ceive operation.
21 The sockfd argument is the file descriptor of the socket to receive
23 data from.
24 The msgvec argument is a pointer to an array of mmsghdr structures.
26 The size of this array is specified in vlen.
27 The mmsghdr structure is defined in <sys/socket.h> as:
29 struct mmsghdr {
31 struct msghdr msg_hdr; /* Message header */
32 unsigned int msg_len; /* Number of received bytes for header */
33 };
34 The msg_hdr field is a msghdr structure, as described in recvmsg(2).
36 The msg_len field is the number of bytes returned for the message in
37 the entry. This field has the same value as the return value of a sin‐
38 gle recvmsg(2) on the header.
39 The flags argument contains flags ORed together. The flags are the
41 same as documented for recvmsg(2), with the following addition:
42 MSG_WAITFORONE (since Linux 2.6.34)
44 Turns on MSG_DONTWAIT after the first message has been received.
45 The timeout argument points to a struct timespec (see clock_gettime(2))
47 defining a timeout (seconds plus nanoseconds) for the receive operation
48 (but see BUGS!). (This interval will be rounded up to the system clock
49 granularity, and kernel scheduling delays mean that the blocking inter‐
50 val may overrun by a small amount.) If timeout is NULL, then the oper‐
51 ation blocks indefinitely.
52 A blocking recvmmsg() call blocks until vlen messages have been re‐
54 ceived or until the timeout expires. A nonblocking call reads as many
55 messages as are available (up to the limit specified by vlen) and re‐
56 turns immediately.
57 On return from recvmmsg(), successive elements of msgvec are updated to
59 contain information about each received message: msg_len contains the
60 size of the received message; the subfields of msg_hdr are updated as
61 described in recvmsg(2). The return value of the call indicates the
62 number of elements of msgvec that have been updated.
63
65 On success, recvmmsg() returns the number of messages received in ms‐
66 gvec; on error, -1 is returned, and errno is set to indicate the error.
67
69 Errors are as for recvmsg(2). In addition, the following error can oc‐
70 cur:
71 EINVAL timeout is invalid.
73 See also BUGS.
75
77 The recvmmsg() system call was added in Linux 2.6.33. Support in glibc
78 was added in version 2.12.
79
81 recvmmsg() is Linux-specific.
82
84 The timeout argument does not work as intended. The timeout is checked
85 only after the receipt of each datagram, so that if up to vlen-1 data‐
86 grams are received before the timeout expires, but then no further
87 datagrams are received, the call will block forever.
88 If an error occurs after at least one message has been received, the
90 call succeeds, and returns the number of messages received. The error
91 code is expected to be returned on a subsequent call to recvmmsg(). In
92 the current implementation, however, the error code can be overwritten
93 in the meantime by an unrelated network event on a socket, for example
94 an incoming ICMP packet.
95
97 The following program uses recvmmsg() to receive multiple messages on a
98 socket and stores them in multiple buffers. The call returns if all
99 buffers are filled or if the timeout specified has expired.
100 The following snippet periodically generates UDP datagrams containing a
102 random number:
103 $ while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234;
105 sleep 0.25; done
106 These datagrams are read by the example application, which can give the
108 following output:
109 $ ./a.out
111 5 messages received
112 1 11782
113 2 11345
114 3 304
115 4 13514
116 5 28421
117 Program source
119 #define _GNU_SOURCE
121 #include <netinet/ip.h>
122 #include <stdio.h>
123 #include <stdlib.h>
124 #include <string.h>
125 #include <sys/socket.h>
126 int
128 main(void)
129 {
130 #define VLEN 10
131 #define BUFSIZE 200
132 #define TIMEOUT 1
133 int sockfd, retval;
134 struct sockaddr_in addr;
135 struct mmsghdr msgs[VLEN];
136 struct iovec iovecs[VLEN];
137 char bufs[VLEN][BUFSIZE+1];
138 struct timespec timeout;
139 sockfd = socket(AF_INET, SOCK_DGRAM, 0);
141 if (sockfd == -1) {
142 perror("socket()");
143 exit(EXIT_FAILURE);
144 }
145 addr.sin_family = AF_INET;
147 addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
148 addr.sin_port = htons(1234);
149 if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
150 perror("bind()");
151 exit(EXIT_FAILURE);
152 }
153 memset(msgs, 0, sizeof(msgs));
155 for (int i = 0; i < VLEN; i++) {
156 iovecs[i].iov_base = bufs[i];
157 iovecs[i].iov_len = BUFSIZE;
158 msgs[i].msg_hdr.msg_iov = &iovecs[i];
159 msgs[i].msg_hdr.msg_iovlen = 1;
160 }
161 timeout.tv_sec = TIMEOUT;
163 timeout.tv_nsec = 0;
164 retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
166 if (retval == -1) {
167 perror("recvmmsg()");
168 exit(EXIT_FAILURE);
169 }
170 printf("%d messages received\n", retval);
172 for (int i = 0; i < retval; i++) {
173 bufs[i][msgs[i].msg_len] = 0;
174 printf("%d %s", i+1, bufs[i]);
175 }
176 exit(EXIT_SUCCESS);
177 }
178
180 clock_gettime(2), recvmsg(2), sendmmsg(2), sendmsg(2), socket(2),
181 socket(7)
182
184 This page is part of release 5.13 of the Linux man-pages project. A
185 description of the project, information about reporting bugs, and the
186 latest version of this page, can be found at
187 https://www.kernel.org/doc/man-pages/.
188Linux 2020-11-01 RECVMMSG(2)