libc/sys/send.2

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.\"     From: @(#)send.2	8.2 (Berkeley) 2/21/94
.\"
.Dd April 27, 2020
.Dt SEND 2
.Os
.Sh NAME
.Nm send ,
.Nm sendto ,
.Nm sendmsg ,
.Nm sendmmsg
.Nd send message(s) from a socket
.Sh LIBRARY
.Lb libc
.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 ssize_t
.Fn sendmmsg "int s" "struct mmsghdr * restrict msgvec" "size_t vlen" "int flags"
.Sh DESCRIPTION
The
.Fn send
and
.Fn sendmmsg
functions,
and
.Fn sendto
and
.Fn sendmsg
system calls
are used to transmit one or more messages (with the
.Fn sendmmsg
call) to
another socket.
The
.Fn send
function
may be used only when the socket is in a
.Em connected
state.
The functions
.Fn sendto ,
.Fn sendmsg
and
.Fn sendmmsg
may be used at any time if the socket is connectionless-mode.
If the socket is connection-mode, the protocol
must support implied connect (currently
.Xr tcp 4
is the only protocol with support) or the socket must be in a
connected state before use.
.Pp
The address of the target is given by
.Fa to
with
.Fa tolen
specifying its size, or the equivalent
.Fa msg_name
and
.Fa msg_namelen
in
.Fa struct msghdr .
If the socket is in a connected state, the target address passed to
.Fn sendto ,
.Fn sendmsg
or
.Fn sendmmsg
is ignored.
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
The
.Fn sendmmsg
function sends multiple messages at a call.
They are given by the
.Fa msgvec
vector along with
.Fa vlen
specifying the vector size.
The number of octets sent per each message is placed in the
.Fa msg_len
field of each processed element of the vector after transmission.
.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
system call may be used to determine when it is possible to
send more data.
.Pp
The
.Fa flags
argument may include one or more of the following:
.Bd -literal
#define	MSG_OOB		0x00001 /* process out-of-band data */
#define	MSG_DONTROUTE	0x00004 /* bypass routing, use direct interface */
#define	MSG_EOR		0x00008 /* data completes record */
#define	MSG_DONTWAIT	0x00080 /* do not block */
#define	MSG_EOF		0x00100 /* data completes transaction */
#define	MSG_NOSIGNAL	0x20000 /* do not generate SIGPIPE on EOF */
.Ed
.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_EOR
is used to indicate a record mark for protocols which support the
concept.
The
.Dv MSG_DONTWAIT
flag request the call to return when it would block otherwise.
.Dv MSG_EOF
requests that the sender side of a socket be shut down, and that an
appropriate indication be sent at the end of the specified data;
this flag is only implemented for
.Dv SOCK_STREAM
sockets in the
.Dv PF_INET
protocol family.
.Dv MSG_DONTROUTE
is usually used only by diagnostic or routing programs.
.Dv MSG_NOSIGNAL
is used to prevent
.Dv SIGPIPE
generation when writing a socket that
may be closed.
.Pp
See
.Xr recv 2
for a description of the
.Fa msghdr
structure and the
.Fa mmsghdr
structure.
.Sh RETURN VALUES
The
.Fn send ,
.Fn sendto
and
.Fn sendmsg
calls
return the number of octets sent.
The
.Fn sendmmsg
call returns the number of messages sent.
If an error occurred a value of -1 is returned.
.Sh ERRORS
The
.Fn send
and
.Fn sendmmsg
functions and
.Fn sendto
and
.Fn sendmsg
system calls
fail if:
.Bl -tag -width Er
.It Bq Er EBADF
An invalid descriptor was specified.
.It Bq Er EACCES
The destination address is a broadcast address, and
.Dv SO_BROADCAST
has not been set on the socket.
.It Bq Er ENOTCONN
The socket is connection-mode but is not connected.
.It Bq Er ENOTSOCK
The argument
.Fa s
is not a socket.
.It Bq Er EFAULT
An invalid user space address was specified for an argument.
.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
.Dv MSG_DONTWAIT
is specified, 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 EHOSTUNREACH
The remote host was unreachable.
.It Bq Er EISCONN
A destination address was specified and the socket is already connected.
.It Bq Er ECONNREFUSED
The socket received an ICMP destination unreachable message
from the last message sent.
This typically means that the
receiver is not listening on the remote port.
.It Bq Er EHOSTDOWN
The remote host was down.
.It Bq Er ENETDOWN
The remote network was down.
.It Bq Er EADDRNOTAVAIL
The process using a
.Dv SOCK_RAW
socket was jailed and the source
address specified in the IP header did not match the IP
address bound to the prison.
.It Bq Er EPIPE
The socket is unable to send anymore data
.Dv ( SBS_CANTSENDMORE
has been set on the socket).
This typically means that the socket
is not connected.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr write 2 ,
.Xr CMSG_DATA 3
.Sh HISTORY
The
.Fn send
function appeared in
.Bx 4.2 .
The
.Fn sendmmsg
function appeared in
.Fx 11.0 .
.Sh BUGS
Because
.Fn sendmsg
does not necessarily block until the data has been transferred, it
is possible to transfer an open file descriptor across an
.Dv AF_UNIX
domain socket
(see
.Xr recv 2 ) ,
then
.Fn close
it before it has actually been sent, the result being that the receiver
gets a closed file descriptor.
It is left to the application to
implement an acknowledgment mechanism to prevent this from happening.