Manpages - sendmmsg.2

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",unsignedint vlen ,"
   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. See also BUGS.

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).

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.

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;

      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);
  }

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

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