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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, 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.
Linux.
Linux 3.0, glibc 2.14.
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 <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)
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version of the page, or you believe there is a better or more up-
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Linux man-pages 6.9.1 2024-06-15 sendmmsg(2)
Pages that refer to this page: recvmmsg(2), send(2), syscalls(2)
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HTML rendering created 2024-06-26 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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