sendmmsg(2) — Linux manual page


SENDMMSG(2)               Linux Programmer's Manual              SENDMMSG(2)

NAME         top

       sendmmsg - send multiple messages on a socket

SYNOPSIS         top

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int sendmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
                    int flags);

DESCRIPTION         top

       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         top

       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         top

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

VERSIONS         top

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

CONFORMING TO         top

       sendmmsg() is Linux-specific.

NOTES         top

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

BUGS         top

       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         top

       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 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) {

           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) {

           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)
               printf("%d messages sent\n", retval);


SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                      SENDMMSG(2)

Pages that refer to this page: recvmmsg(2)send(2)sendmsg(2)sendto(2)syscalls(2)