freebsd-skq/lib/libc/sys/send.2

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.\"     From: @(#)send.2	8.2 (Berkeley) 2/21/94
.\" $FreeBSD$
.\"
.Dd February 15, 1995
.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 *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"
.Sh DESCRIPTION
The
.Fn send
function,
and
.Fn sendto
and
.Fn sendmsg
system calls
are used to transmit a message to another socket.
The
.Fn send
function
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 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
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		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 */
.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, and is used to implement Transaction
.Tn TCP
(see
.Xr ttcp 4 ) .
.Dv MSG_DONTROUTE
is usually used only by diagnostic or routing programs.
.Pp
See
.Xr recv 2
for a description of the
.Fa msghdr
structure.
.Sh RETURN VALUES
The call returns the number of characters sent, or -1
if an error occurred.
.Sh ERRORS
The
.Fn send
function 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 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 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 ENETDOWN
The remote network 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 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 acknowlegment mechanism to prevent this from happening.
.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 appeared in
.Bx 4.2 .