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

       sendmmsg - send multiple messages on a socket

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

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

       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 num-
       ber of elements of msgvec that have been updated.

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

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

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

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

       sendmmsg() is Linux-specific.

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

       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 re-
       turned, it will be the same as the one that was lost  on  the  previous

       The  example below uses sendmmsg() to send onetwo and three in two dis-
       tinct 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);


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

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       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2019-03-06                       SENDMMSG(2)
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