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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 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.
RETURN VALUE
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
Errors are as for recvmsg(2). In addition, the following error can occur:
EINVAL timeout is invalid.
See also BUGS.
VERSIONS
The recvmmsg() system call was added in Linux 2.6.33. Support in glibc was added in glibc
2.12.
STANDARDS
recvmmsg() is Linux-specific.
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 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.
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)