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.
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);
}
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.
This page is part of release 5.10 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/.