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,
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.
See also BUGS.
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.
If an error occurs after at least one message has been received, the call
succeeds, and returns the number of messages received. The error code is
expected to be returned on a subsequent call to
recvmmsg(). In the
current implementation, however, the error code can be overwritten in the
meantime by an unrelated network event on a socket, for example an incoming
ICMP packet.
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
#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)