Name

sendmmsg — send multiple messages on a socket

Synopsis

#define _GNU_SOURCE          /* See feature_test_macros(7) */
#include <sys/socket.h>
int sendmmsg( int sockfd,
  struct mmsghdr *msgvec,
  unsigned int vlen,
  unsigned 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 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 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 vlen messages have been sent. A nonblocking call sends as many messages as possible (up to the limit specified by vlen) 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 vlen, 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.

VERSIONS

The sendmmsg() system call was added in Linux 3.0. Support in glibc was added in version 2.14.

CONFORMING TO

sendmmsg() is Linux-specific.

NOTES

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

EXAMPLE

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 <netinet/ip.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>

int
main(void)
{
    int sockfd;
    struct sockaddr_in addr;
    struct mmsghdr msg[2];
    struct iovec msg1[2], msg2;
    int retval;

    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)

COLOPHON

This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright (c) 2012 by Michael Kerrisk <mtk.manpagesgmail.com>
with some material from a draft by
Stephan Mueller <stephan.muelleratsec.com>
in turn based on Andi Kleen's recvmmsg.2 page.

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END