.\"	$OpenBSD: send.2,v 1.35 2022/09/09 13:52:59 mbuhl Exp $
.\"	$NetBSD: send.2,v 1.6 1996/01/15 01:17:18 thorpej Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)send.2	8.2 (Berkeley) 2/21/94
.\"
.Dd $Mdocdate: September 9 2022 $
.Dt SEND 2
.Os
.Sh NAME
.Nm send ,
.Nm sendto ,
.Nm sendmsg ,
.Nm sendmmsg
.Nd send a message from a socket
.Sh SYNOPSIS
.In sys/socket.h
.Ft ssize_t
.Fn send "int s" "const void *msg" "size_t len" "int flags"
.Ft ssize_t
.Fn sendto "int s" "const void *msg" "size_t len" "int flags" "const struct sockaddr *to" "socklen_t tolen"
.Ft ssize_t
.Fn sendmsg "int s" "const struct msghdr *msg" "int flags"
.Ft int
.Fn sendmmsg "int s" "const struct mmsghdr *mmsg" "unsigned int vlen" "int flags"
.Sh DESCRIPTION
.Fn send ,
.Fn sendto ,
.Fn sendmsg ,
and
.Fn sendmmsg
are used to transmit a message to another socket.
.Fn send
may be used only when the socket is in a
.Em connected
state, while
.Fn sendto ,
.Fn sendmsg ,
and
.Fn sendmmsg
may be used at any time.
.Pp
The address of the target is given by
.Fa to
with
.Fa tolen
specifying its size.
The length of the message is given by
.Fa len .
If the message is too long to pass atomically through the
underlying protocol, the error
.Er EMSGSIZE
is returned, and
the message is not transmitted.
.Pp
No indication of failure to deliver is implicit in a
.Fn send .
Locally detected errors are indicated by a return value of \-1.
.Pp
If no messages space is available at the socket to hold
the message to be transmitted, then
.Fn send
normally blocks, unless the socket has been placed in
non-blocking I/O mode.
The
.Xr select 2
or
.Xr poll 2
system calls may be used to determine when it is possible to
send more data.
.Pp
The
.Fa flags
parameter may include one or more of the following:
.Pp
.Bl -tag -width "MSG_DONTROUTEXX" -offset indent -compact
.It Dv MSG_DONTROUTE
bypass routing tables, silently ignored
.It Dv MSG_DONTWAIT
don't block
.It Dv MSG_EOR
terminate the record (SOCK_SEQPACKET only)
.It Dv MSG_NOSIGNAL
don't send
.Dv SIGPIPE
.It Dv MSG_OOB
process out-of-band data
.El
.Pp
The flag
.Dv MSG_OOB
is used to send
.Dq out-of-band
data on sockets that support this notion (e.g.,
.Dv SOCK_STREAM ) ;
the underlying protocol must also support
.Dq out-of-band
data.
.Dv MSG_NOSIGNAL
is used to request not to send the
.Dv SIGPIPE
signal if an attempt to send is made on a socket that is shut down for
writing or no longer connected.
.Pp
See
.Xr recv 2
for a description of the
.Fa msghdr
and
.Fa mmsghdr
structures.
.Sh RETURN VALUES
The
.Fn send ,
.Fn sendto ,
and
.Fn sendmsg
calls return the number of characters sent, or \-1
if an error occurred.
The
.Fn sendmmsg
call returns the number of messages sent, or \-1
if an error occurred before the first message has been sent.
.Sh ERRORS
.Fn send ,
.Fn sendto ,
and
.Fn sendmsg
fail if:
.Bl -tag -width Er
.It Bq Er EBADF
An invalid descriptor was specified.
.It Bq Er ENOTSOCK
The argument
.Fa s
is not a socket.
.It Bq Er EFAULT
An invalid user space address was specified for a parameter.
.It Bq Er EMSGSIZE
The socket requires that message be sent atomically,
and the size of the message to be sent made this impossible.
.It Bq Er EAGAIN
The socket is marked non-blocking or the
.Dv MSG_DONTWAIT
flag is set and the requested operation
would block.
.It Bq Er ENOBUFS
The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
.It Bq Er ENOBUFS
The output queue for a network interface was full.
This generally indicates that the interface has stopped sending,
but may be caused by transient congestion.
.It Bq Er EACCES
The connection was blocked by
.Xr pf 4 ,
or
.Dv SO_BROADCAST
is not set on the socket
and a broadcast address was given as the destination.
.It Bq Er EHOSTUNREACH
The destination address specified an unreachable host.
.It Bq Er EINVAL
The
.Fa flags
parameter is invalid.
.It Bq Er EHOSTDOWN
The destination address specified a host that is down.
.It Bq Er ENETDOWN
The destination address specified a network that is down.
.It Bq Er ECONNREFUSED
The destination host rejected the message (or a previous one).
This error can only be returned by connected sockets.
.It Bq Er ENOPROTOOPT
There was a problem sending the message.
This error can only be returned by connected sockets.
.It Bq Er EDESTADDRREQ
The socket is not connected, and no destination address was specified.
.It Bq Er EPIPE
The socket is shut down for writing or not longer connected and the
.Dv MSG_NOSIGNAL
flag is set.
.El
.Pp
In addition,
.Fn send
and
.Fn sendto
may return the following error:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa len
was larger than
.Dv SSIZE_MAX .
.El
.Pp
.Fn sendto
and
.Fn sendmsg
may return the following errors:
.Bl -tag -width Er
.It Bq Er EADDRNOTAVAIL
No suitable address is available on the local machine.
.It Bq Er EAFNOSUPPORT
Addresses in the specified address family cannot be used with this socket.
.It Bq Er EISCONN
The socket is already connected, and a destination address was specified.
.El
.Pp
.Fn sendmsg
may return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa msg_iov
array overflowed an
.Em ssize_t .
.It Bq Er EMSGSIZE
The
.Fa msg_iovlen
member of
.Fa msg
was less than 0 or larger than
.Dv IOV_MAX .
.It Bq Er EMFILE
The message contains control information utilizing
.Xr CMSG_DATA 3
to pass file descriptors, but too many file descriptors
are already in-flight.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr poll 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr write 2 ,
.Xr CMSG_DATA 3
.Sh STANDARDS
The
.Fn send ,
.Fn sendto ,
and
.Fn sendmsg
functions conform to
.St -p1003.1-2008 .
The
.Dv MSG_DONTWAIT
and
.Dv MSG_NOSIGNAL
flags are extensions to that specification.
.Sh HISTORY
The
.Fn send
function call appeared in
.Bx 4.1c .
The
.Fn sendmmsg
syscall first appeared in Linux 3.0 and was added to
.Ox 7.2 .