sendmmsg(2)

NAME
sendmmsg - send 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 sendmmsg(int sockfd, struct mmsghdr msgvec[.n], unsigned int n,
             int flags);

DESCRIPTION
The sendmmsg() system call is an extension of sendmsg(2) that allows the caller to transmit multiple messages on a socket using a single system call. (This has performance benefits for some applications.)

The sockfd argument is the file descriptor of the socket on which data is to be transmitted.

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 bytes transmitted */
    };

The msg_hdr field is a msghdr structure, as described in sendmsg(2). The msg_len field is used to return the number of bytes sent from the message in msg_hdr (i.e., the same as the return value from a single sendmsg(2) call).

The flags argument contains flags ORed together. The flags are the same as for sendmsg(2).

A blocking sendmmsg() call blocks until n messages have been sent. A nonblocking call sends as many messages as possible (up to the limit specified by n) and returns immediately.

On return from sendmmsg(), the msg_len fields of successive elements of msgvec are updated to contain the number of bytes transmitted from the corresponding msg_hdr. The return value of the call indicates the number of elements of msgvec that have been updated.

RETURN VALUE
On success, sendmmsg() returns the number of messages sent from msgvec; if this is less than n, the caller can retry with a further sendmmsg() call to send the remaining messages.

On error, -1 is returned, and errno is set to indicate the error.

ERRORS
Errors are as for sendmsg(2). An error is returned only if no datagrams could be sent. See also BUGS.

STANDARDS
Linux.

HISTORY
Linux 3.0, glibc 2.14.

NOTES
The value specified in n is capped to UIO_MAXIOV (1024).

BUGS
If an error occurs after at least one message has been sent, the call succeeds, and returns the number of messages sent. The error code is lost. The caller can retry the transmission, starting at the first failed message, but there is no guarantee that, if an error is returned, it will be the same as the one that was lost on the previous call.

EXAMPLES
The example below uses sendmmsg() to send onetwo and three in two distinct UDP datagrams using one system call. The contents of the first datagram originates from a pair of buffers.

    #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 <sys/types.h>

    int
    main(void)
    {
        int                 retval;
        int                 sockfd;
        struct iovec        msg1[2], msg2;
        struct mmsghdr      msg[2];
        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 (connect(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
            perror("connect()");
            exit(EXIT_FAILURE);
        }

        memset(msg1, 0, sizeof(msg1));
        msg1[0].iov_base = "one";
        msg1[0].iov_len = 3;
        msg1[1].iov_base = "two";
        msg1[1].iov_len = 3;

        memset(&msg2, 0, sizeof(msg2));
        msg2.iov_base = "three";
        msg2.iov_len = 5;

        memset(msg, 0, sizeof(msg));
        msg[0].msg_hdr.msg_iov = msg1;
        msg[0].msg_hdr.msg_iovlen = 2;

        msg[1].msg_hdr.msg_iov = &msg2;
        msg[1].msg_hdr.msg_iovlen = 1;

        retval = sendmmsg(sockfd, msg, 2, 0);
        if (retval == -1)
            perror("sendmmsg()");
        else
            printf("%d messages sent\n", retval);

        exit(0);
    }

SEE ALSO
recvmmsg(2), sendmsg(2), socket(2), socket(7)