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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
RECVMMSG(2) Linux Programmer's Manual RECVMMSG(2)
recvmmsg - receive multiple messages on a socket
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
int recvmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
unsigned int flags, struct timespec *timeout);
The recvmmsg() system call is an extension of 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 recvmsg(2) is support for a timeout on the
receive operation.
The sockfd argument is the file descriptor of the socket to receive
data from.
The msgvec argument is a pointer to an array of mmsghdr structures.
The size of this array is specified in vlen.
The mmsghdr structure is defined in <sys/socket.h> as:
struct mmsghdr {
struct msghdr msg_hdr; /* Message header */
unsigned int msg_len; /* Number of received bytes for header */
};
The msg_hdr field is a msghdr structure, as described in recvmsg(2).
The 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 recvmsg(2) on the header.
The flags argument contains flags ORed together. The flags are the
same as documented for recvmsg(2), with the following addition:
MSG_WAITFORONE (since Linux 2.6.34)
Turns on MSG_DONTWAIT after the first message has been
received.
The timeout argument points to a struct timespec (see
clock_gettime(2)) defining a timeout (seconds plus nanoseconds) for
the receive operation (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 timeout is NULL, then the operation blocks indefinitely.
A blocking recvmmsg() call blocks until 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 vlen)
and returns immediately.
On return from recvmmsg(), successive elements of msgvec are updated
to contain information about each received message: msg_len contains
the size of the received message; the subfields of msg_hdr are
updated as described in recvmsg(2). The return value of the call
indicates the number of elements of msgvec that have been updated.
On success, recvmmsg() returns the number of messages received in
msgvec; on error, -1 is returned, and errno is set to indicate the
error.
Errors are as for recvmsg(2). In addition, the following error can
occur:
EINVAL timeout is invalid.
The recvmmsg() system call was added in Linux 2.6.33. Support in
glibc was added in version 2.12.
recvmmsg() is Linux-specific.
The timeout argument does not work as intended. The timeout is
checked only after the receipt of each datagram, so that if up to
vlen-1 datagrams are received before the timeout expires, but then no
further datagrams are received, the call will block forever.
The following program uses 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:
$ while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234;
sleep 0.25; done
These datagrams are read by the example application, which can give
the following output:
$ ./a.out
5 messages received
1 11782
2 11345
3 304
4 13514
5 28421
Program source
#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 addr;
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);
}
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
addr.sin_port = htons(1234);
if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -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);
}
clock_gettime(2), recvmsg(2), sendmmsg(2), sendmsg(2), socket(2),
socket(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 RECVMMSG(2)
Pages that refer to this page: recv(2), sendmmsg(2), syscalls(2), signal(7)