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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,
		    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 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.

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  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 sa;
	   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);
	   }

	   sa.sin_family = AF_INET;
	   sa.sin_addr.s_addr =	htonl(INADDR_LOOPBACK);
	   sa.sin_port = htons(1234);
	   if (connect(sockfd, (struct sockaddr	*) &sa,	sizeof(sa)) == -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 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-07-08			   SENDMMSG(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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