libc/sys/send.2

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.\"     From: @(#)send.2	8.2 (Berkeley) 2/21/94
.\" $FreeBSD: src/lib/libc/sys/send.2,v 1.10.2.6 2001/12/14 18:34:01 ru Exp $
.\"
.Dd October 6, 2010
.Dt SEND 2
.Os
.Sh NAME
.Nm send ,
.Nm sendto ,
.Nm sendmsg
.Nd send a message from a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.Ft ssize_t
.Fn send "int s" "const void *msgbuf" "size_t len" "int flags"
.Ft ssize_t
.Fn sendto "int s" "const void *msgbuf" "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"
.Sh DESCRIPTION
.Fn Send ,
.Fn sendto ,
and
.Fn sendmsg
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
and
.Fn sendmsg
may be used at any time.
.Pp
The socket file descriptor is given by
.Fa s .
.Fa msgbuf
points to a buffer containing the message.
.Fa msg
points to a
.Fa msghdr
structure.
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
call 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:
.Bd -literal
#define	MSG_OOB		0x1   /* process out-of-band data */
#define	MSG_PEEK	0x2   /* peek at incoming message */
#define	MSG_DONTROUTE	0x4   /* bypass routing, use direct interface */
#define MSG_EOR		0x8   /* data completes record */
#define	MSG_EOF		0x100 /* data completes transaction */
#define MSG_NOSIGNAL	0x400 /* No SIGPIPE to unconnected socket stream */
.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.
.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
requests not to send the
.Dv SIGPIPE
signal if an attempt to
.Nm
is made on a stream-oriented socket that is no longer connected.
.Pp
See
.Xr recv 2
for a description of the
.Fa msghdr
structure.
.Sh RETURN VALUES
Upon successful completion the number of characters which were sent is
returned.  Otherwise -1 is returned and the global variable
.Va errno
is set to indicate the error.
.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 EACCES
The destination address is a broadcast address, and
.Dv SO_BROADCAST
has not been set on the socket.
.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 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 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 EPIPE
The socket is unable to send anymore data (SS_CANTSENDMORE has
been set on the socket). This typically means that the socket
is not connected.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn send
function call appeared in
.Bx 4.2 .
.Sh BUGS
Because
.Fn sendmsg
doesn't 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.