recvmmsg(2) System Calls Manual recvmmsg(2)
NAME
recvmmsg - receive multiple messages on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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 opera-
tion.
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 inter-
val may overrun by a small amount.) If timeout is NULL, then the opera-
tion 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 immedi-
ately.
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.
RETURN VALUE
On success, recvmmsg() returns the number of messages received in ms-
gvec; on error, -1 is returned, and errno is set to indicate the error.
ERRORS
Errors are as for recvmsg(2). In addition, the following error can oc-
cur:
EINVAL timeout is invalid.
See also BUGS.
STANDARDS
Linux.
HISTORY
Linux 2.6.33, glibc 2.12.
BUGS
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 data-
grams are received before the timeout expires, but then no further data-
grams 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.
EXAMPLES
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 <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <time.h>
int
main(void)
{
#define VLEN 10
#define BUFSIZE 200
#define TIMEOUT 1
int sockfd, retval;
char bufs[VLEN][BUFSIZE+1];
struct iovec iovecs[VLEN];
struct mmsghdr msgs[VLEN];
struct timespec timeout;
struct sockaddr_in addr;
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 (size_t 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 (size_t i = 0; i < retval; i++) {
bufs[i][msgs[i].msg_len] = 0;
printf("%zu %s", i+1, bufs[i]);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
clock_gettime(2), recvmsg(2), sendmmsg(2), sendmsg(2), socket(2),
socket(7)
Linux man-pages 6.9.1 2024-06-15 recvmmsg(2)
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