SENDMMSG
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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,
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 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 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).
EXAMPLE
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);
}
SEE ALSO
recvmmsg(2),
sendmsg(2),
socket(2),
socket(7)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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