sendmmsg(2) - Linux manual page (original) (raw)

sendmmsg(2) System Calls Manual sendmmsg(2)

NAME top

   sendmmsg - send multiple messages on a socket

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#define _GNU_SOURCE** /* See feature_test_macros(7) */
   **#include <sys/socket.h>**

   **int sendmmsg(int** _sockfd_**, struct mmsghdr** _msgvec_**[.**_n_**], unsigned int** _n_**,**
                **int** _flags_**);**

DESCRIPTION top

   The **sendmmsg**() system call is an extension of [sendmsg(2)](../man2/sendmsg.2.html) 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 _n_.

   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 _msghdr_ field is a _msghdr_ structure, as described in
   [sendmsg(2)](../man2/sendmsg.2.html).  The _msglen_ field is used to return the number of
   bytes sent from the message in _msghdr_ (i.e., the same as the
   return value from a single [sendmsg(2)](../man2/sendmsg.2.html) call).

   The _flags_ argument contains flags ORed together.  The flags are
   the same as for [sendmsg(2)](../man2/sendmsg.2.html).

   A blocking **sendmmsg**() call blocks until _n_ messages have been sent.
   A nonblocking call sends as many messages as possible (up to the
   limit specified by _n_) and returns immediately.

   On return from **sendmmsg**(), the _msglen_ fields of successive
   elements of _msgvec_ are updated to contain the number of bytes
   transmitted from the corresponding _msghdr_.  The return value of
   the call indicates the number of elements of _msgvec_ that have been
   updated.

RETURN VALUE top

   On success, **sendmmsg**() returns the number of messages sent from
   _msgvec_; if this is less than _n_, the caller can retry with a
   further **sendmmsg**() call to send the remaining messages.

   On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   Errors are as for [sendmsg(2)](../man2/sendmsg.2.html).  An error is returned only if no
   datagrams could be sent.  See also BUGS.

STANDARDS top

   Linux.

HISTORY top

   Linux 3.0, glibc 2.14.

NOTES top

   The value specified in _n_ is capped to **UIO_MAXIOV** (1024).

BUGS top

   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.

EXAMPLES top

   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 <arpa/inet.h>
   #include <netinet/in.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <sys/socket.h>
   #include <sys/types.h>

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

       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 top

   [recvmmsg(2)](../man2/recvmmsg.2.html), [sendmsg(2)](../man2/sendmsg.2.html), [socket(2)](../man2/socket.2.html), [socket(7)](../man7/socket.7.html)

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Linux man-pages 6.10 2024-11-17 sendmmsg(2)

Pages that refer to this page:recvmmsg(2), send(2), syscalls(2)