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SENDMMSG(2)                Linux Programmer's Manual               SENDMMSG(2)



NAME
       sendmmsg - send multiple messages on a socket

SYNOPSIS
       #define _GNU_SOURCE
       #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 mes-
       sages 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  num-
       ber  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 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
       Errors are as for sendmsg(2).  An error is returned only if no datagrams 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).

SEE ALSO
       recvmmsg(2), sendmsg(2), socket(2), socket(7)



Linux                             2011-09-09                       SENDMMSG(2)