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

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,
                    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 num-
       ber 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 send-
       mmsg() 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  data-
       grams could be sent.  See also BUGS.

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).

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

EXAMPLE
       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
       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 5.05 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/.

Linux                             2019-03-06                       SENDMMSG(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON