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.

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;
    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 (int 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 (int 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)