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[.n], unsigned int n,
             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 n.

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 n 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 n) 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.

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 n-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)