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.\" Modified Sat Jul 24 01:15:33 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 17:55:49 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Oct 1998 by Andi Kleen
.\"
.TH SEND 2 2002-12-31 "Linux Man Page" "Linux Programmer's Manual"
.SH NAME
send, sendto, sendmsg \- send a message from a socket
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.sp
.BI "ssize_t send(int " s ", const void *" msg ", size_t " len ,
.BI "int " flags );
.br
.BI "ssize_t sendto(int " s ", const void *" msg ", size_t " len ,
.BI "int " flags ", const struct sockaddr *" to ", socklen_t " tolen );
.br
.BI "ssize_t sendmsg(int " s ", const struct msghdr *" msg ,
.BI "int " flags );
.SH DESCRIPTION
.BR Send ,
.BR sendto ,
and
.B sendmsg
are used to transmit a message to another socket.
.B Send
may be used only when the socket is in a 
.I connected
state, while 
.B sendto
and
.B sendmsg
may be used at any time.
.PP
The address of the target is given by
.I to
with 
.I tolen
specifying its size.  The length of the message is given by
.IR len .
If the message is too long to pass atomically through the
underlying protocol, the error
.B EMSGSIZE
is returned, and the message is not transmitted.
.PP
No indication of failure to deliver is implicit in a
.BR send .
Locally detected errors are indicated by a return value of \-1.
.PP
When the message does not fit into the send buffer of the socket,
.B send
normally blocks, unless the socket has been placed in non-blocking I/O
mode.  In non-blocking mode it would return
.B EAGAIN
in this case.
The
.BR select (2)
call may be used to determine when it is possible to send more data.
.PP
The
.I flags
parameter is a flagword and can contain the following flags:
.\" XXX document MSG_PROXY
.TP
.B MSG_OOB
Sends
.I out-of-band
data on sockets that support this notion (e.g.
.BR SOCK_STREAM );
the underlying protocol must also support
.I out-of-band
data.
.TP
.B MSG_DONTROUTE
Dont't use a gateway to send out the packet, only send to hosts on 
directly connected networks. This is usually used only 
by diagnostic or routing programs. This is only defined for protocol
families that route; packet sockets don't.
.TP
.B MSG_DONTWAIT
Enables non-blocking operation; if the operation would block,
.B EAGAIN
is returned (this can also be enabled using the
.B O_NONBLOCK
with the
.B F_SETFL
.BR fcntl (2)).
.TP
.B MSG_NOSIGNAL
Requests not to send 
.B SIGPIPE 
on errors on stream oriented sockets when the other end breaks the
connection. The 
.B EPIPE
error is still returned.
.TP
.BR MSG_CONFIRM " (Linux 2.3+ only)"
Tell the link layer that forward process happened: you got a successful
reply from the other side. If the link layer doesn't get this 
it'll regularly reprobe the neighbour (e.g. via a unicast ARP).
Only valid on 
.B SOCK_DGRAM
and
.B SOCK_RAW
sockets and currently only implemented for IPv4 and IPv6. See
.BR arp (7)
for details.
.PP
The definition of the
.I msghdr
structure follows. See 
.BR recv (2)
and below for an exact description of its fields.
.IP
.RS
.nf
.ta 4n 17n 33n
struct msghdr {
        void    * msg_name;     /* optional address */
        socklen_t       msg_namelen;    /* size of address */
        struct iovec    * msg_iov;      /* scatter/gather array */
        size_t  msg_iovlen;     /* # elements in msg_iov */
        void    * msg_control;  /* ancillary data, see below */
        socklen_t       msg_controllen; /* ancillary data buffer len */
        int     msg_flags;      /* flags on received message */
};
.ta
.fi
.RE
.PP
You may send control information using the 
.I msg_control 
and 
.I msg_controllen 
members. The maximum control buffer length the kernel can process is limited
per socket by the
.B net.core.optmem_max 
sysctl; see
.BR socket (7).
.SH "RETURN VALUE"
The calls return the number of characters sent, or \-1
if an error occurred.
.SH ERRORS
These are some standard errors generated by the socket layer. Additional errors
may be generated and returned from the underlying protocol modules; see their
respective manual pages.
.TP
.B EBADF
An invalid descriptor was specified.
.TP
.B ENOTSOCK
The argument
.I s
is not a socket.
.TP
.B EFAULT
An invalid user space address was specified for a parameter.
.TP
.B EMSGSIZE
The socket requires that message be sent atomically, and the size
of the message to be sent made this impossible.
.TP
.BR EAGAIN " or " EWOULDBLOCK
The socket is marked non-blocking and the requested operation
would block.
.TP
.B 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.
(Normally, this does not occur in Linux. Packets are just silently dropped
when a device queue overflows.)
.TP
.B EINTR
A signal occurred.
.TP
.B ENOMEM
No memory available.
.TP
.B EINVAL
Invalid argument passed. 
.TP
.B EPIPE
The local end has been shut down on a connection oriented socket.
In this case the process
will also receive a 
.B SIGPIPE 
unless 
.B MSG_NOSIGNAL 
is set.
.SH "CONFORMING TO"
4.4BSD, SVr4, POSIX 1003.1g draft (these function calls appeared in 4.2BSD).

.B MSG_CONFIRM 
is a Linux extension.
.SH NOTE
The prototypes given above follow the Single Unix Specification,
as glibc2 also does; the
.I flags
argument was `int' in BSD 4.*, but `unsigned int' in libc4 and libc5;
the
.I len
argument was `int' in BSD 4.* and libc4, but `size_t' in libc5;
the
.I tolen
argument was `int' in BSD 4.* and libc4 and libc5.
See also
.BR accept (2).
.SH "SEE ALSO"
.BR fcntl (2),
.BR recv (2),
.BR select (2),
.BR getsockopt (2),
.BR sendfile (2),
.BR socket (2),
.BR write (2),
.BR socket (7),
.BR ip (7),
.BR tcp (7),
.BR udp (7)