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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
     returned, 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); }