SENDMMSG(2)               (2020-06-09)                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 number of elements of msgvec
          that have been updated.

     RETURN VALUE
          On success, sendmmsg() returns the number of messages sent

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          from msgvec; if this is less than vlen, the caller can retry
          with a further sendmmsg() call to send the remaining mes-
          sages.

          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 datagrams 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 transmis-
          sion, 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.

     EXAMPLES
          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
          main(void)
          {
              int sockfd;
              struct sockaddr_in addr;
              struct mmsghdr msg[2];
              struct iovec msg1[2], msg2;
              int retval;

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              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 5.10 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/.

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