sendmmsg(2)

1SENDMMSG(2)                Linux Programmer's Manual               SENDMMSG(2)
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NAME

6       sendmmsg - send multiple messages on a socket
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SYNOPSIS

9       #define _GNU_SOURCE
10       #include <sys/socket.h>
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12       int sendmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
13                    unsigned int flags);
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DESCRIPTION

16       The  sendmmsg()  system  call is an extension of sendmsg(2) that allows
17       the caller to transmit multiple messages on a  socket  using  a  single
18       system call.  (This has performance benefits for some applications.)
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20       The  sockfd argument is the file descriptor of the socket on which data
21       is to be transmitted.
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23       The msgvec argument is a pointer to an  array  of  mmsghdr  structures.
24       The size of this array is specified in vlen.
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26       The mmsghdr structure is defined in <sys/socket.h> as:
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28           struct mmsghdr {
29               struct msghdr msg_hdr;  /* Message header */
30               unsigned int  msg_len;  /* Number of bytes transmitted */
31           };
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33       The  msg_hdr  field  is a msghdr structure, as described in sendmsg(2).
34       The msg_len field is used to return the number of bytes sent  from  the
35       message  in  msg_hdr  (i.e., the same as the return value from a single
36       sendmsg(2) call).
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38       The flags argument contains flags ORed together.   The  flags  are  the
39       same as for sendmsg(2).
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41       A  blocking  sendmmsg() call blocks until vlen messages have been sent.
42       A nonblocking call sends as many messages as possible (up to the  limit
43       specified by vlen) and returns immediately.
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45       On return from sendmmsg(), the msg_len fields of successive elements of
46       msgvec are updated to contain the number of bytes transmitted from  the
47       corresponding msg_hdr.  The return value of the call indicates the num‐
48       ber of elements of msgvec that have been updated.
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RETURN VALUE

51       On success, sendmmsg() returns the number of messages sent from msgvec;
52       if  this  is  less than vlen, the caller can retry with a further send‐
53       mmsg() call to send the remaining messages.
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55       On error, -1 is returned, and errno is set to indicate the error.
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ERRORS

58       Errors are as for sendmsg(2).  An error is returned only  if  no  data‐
59       grams could be sent.
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VERSIONS

62       The  sendmmsg()  system  call was added in Linux 3.0.  Support in glibc
63       was added in version 2.14.
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CONFORMING TO

66       sendmmsg() is Linux-specific.
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NOTES

69       The value specified in vlen is capped to UIO_MAXIOV (1024).
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