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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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sendmmsg(2) System Calls Manual sendmmsg(2)
sendmmsg - send multiple messages on a socket
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
int sendmmsg(int sockfd, struct mmsghdr msgvec[.n], unsigned int n,
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 n.
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 n messages have been sent.
A nonblocking call sends as many messages as possible (up to the
limit specified by n) 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 n, 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.
Linux.
Linux 3.0, glibc 2.14.
The value specified in n 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 <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
int
main(void)
{
int retval;
int sockfd;
struct iovec msg1[2], msg2;
struct mmsghdr msg[2];
struct sockaddr_in addr;
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sendmmsg(2)
Pages that refer to this page: recvmmsg(2), send(2), syscalls(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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