.\" Copyright (c) 2012 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" with some material from a draft by
.\" Stephan Mueller <stephan.mueller@atsec.com>
.\" in turn based on Andi Kleen's recvmmsg.2 page.
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH sendmmsg 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
sendmmsg \- send multiple messages on a socket
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <sys/socket.h>
.PP
.BI "int sendmmsg(int " sockfd ", struct mmsghdr *" msgvec \
", unsigned int " vlen ","
.BI "             int " flags ");"
.fi
.SH DESCRIPTION
The
.BR sendmmsg ()
system call is an extension of
.BR 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.)
.\" See commit 228e548e602061b08ee8e8966f567c12aa079682
.PP
The
.I sockfd
argument is the file descriptor of the socket
on which data is to be transmitted.
.PP
The
.I msgvec
argument is a pointer to an array of
.I mmsghdr
structures.
The size of this array is specified in
.IR vlen .
.PP
The
.I mmsghdr
structure is defined in
.I <sys/socket.h>
as:
.PP
.in +4n
.EX
struct mmsghdr {
    struct msghdr msg_hdr;  /* Message header */
    unsigned int  msg_len;  /* Number of bytes transmitted */
};
.EE
.in
.PP
The
.I msg_hdr
field is a
.I msghdr
structure, as described in
.BR sendmsg (2).
The
.I msg_len
field is used to return the number of bytes sent from the message in
.I msg_hdr
(i.e., the same as the return value from a single
.BR sendmsg (2)
call).
.PP
The
.I flags
argument contains flags ORed together.
The flags are the same as for
.BR sendmsg (2).
.PP
A blocking
.BR sendmmsg ()
call blocks until
.I vlen
messages have been sent.
A nonblocking call sends as many messages as possible
(up to the limit specified by
.IR vlen )
and returns immediately.
.PP
On return from
.BR sendmmsg (),
the
.I msg_len
fields of successive elements of
.I msgvec
are updated to contain the number of bytes transmitted from the corresponding
.IR msg_hdr .
The return value of the call indicates the number of elements of
.I msgvec
that have been updated.
.SH RETURN VALUE
On success,
.BR sendmmsg ()
returns the number of messages sent from
.IR msgvec ;
if this is less than
.IR vlen ,
the caller can retry with a further
.BR sendmmsg ()
call to send the remaining messages.
.PP
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
Errors are as for
.BR sendmsg (2).
An error is returned only if no datagrams could be sent.
See also BUGS.
.\" commit 728ffb86f10873aaf4abd26dde691ee40ae731fe
.\"     ... only return an error if no datagrams could be sent.
.\"     If less than the requested number of messages were sent, the application
.\"     must retry starting at the first failed one and if the problem is
.\"     persistent the error will be returned.
.\"
.\"     This matches the behavior of other syscalls like read/write - it
.\"     is not an error if less than the requested number of elements are sent.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 3.0,
glibc 2.14.
.SH NOTES
The value specified in
.I vlen
is capped to
.B UIO_MAXIOV
(1024).
.\" commit 98382f419f32d2c12d021943b87dea555677144b
.\"     net: Cap number of elements for sendmmsg
.\"
.\"     To limit the amount of time we can spend in sendmmsg, cap the
.\"     number of elements to UIO_MAXIOV (currently 1024).
.\"
.\"     For error handling an application using sendmmsg needs to retry at
.\"     the first unsent message, so capping is simpler and requires less
.\"     application logic than returning EINVAL.
.SH BUGS
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.
.SH EXAMPLES
The example below uses
.BR sendmmsg ()
to send
.I onetwo
and
.I three
in two distinct UDP datagrams using one system call.
The contents of the first datagram originates from a pair of buffers.
.PP
.\" SRC BEGIN (sendmmsg.c)
.EX
#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\en", retval);
\&
    exit(0);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR recvmmsg (2),
.BR sendmsg (2),
.BR socket (2),
.BR socket (7)