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,
                unsigned 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
   number 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
   sendmmsg() 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
   datagrams could be sent.


   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).


   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);



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


   This page is part of release 4.09 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


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