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, unsigned int 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)