libc/sys/send.2

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.\"     @(#)send.2	8.2 (Berkeley) 2/21/94
.\"
.Dd June 22, 2012
.Dt SEND 2
.Os
.Sh NAME
.Nm send ,
.Nm sendto ,
.Nm sendmsg ,
.Nm sendmmsg
.Nd send a message 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 int
.Fn sendmmsg "int s" "struct mmsghdr *mmsg" "unsigned int vlen" "unsigned 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
.Fn sendmmsg
call be used to send multiple messages in the same call using an array of
.Fa mmsghdr
elements with the following form, as defined in
.Ao Pa sys/socket.h Ac :
.Pp
.Bd -literal
struct mmsghdr {
	struct msghdr	msg_hdr;	/* the message to be sent */
	unsigned int	msg_len;	/* number of bytes transmitted */
};
.Ed
.Pp
The
.Fa msg_len
member contains the number of bytes sent for each
.Fa msg_hdr
member.
The array has
.Fa vlen
elements, which is limited to
.Dv 1024 .
If there is an error, a number fewer than
.Fa vlen
may be returned, and the error may be retrieved using
.Xr getsockopt 2
with
.Dv SO_ERROR .
.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
call may be used to determine when it is possible to
send more data.
Unfortunately this does not work when the interface queue which is used to
send the message is full, and the call returns
.Er ENOBUFS .
.Pp
The
.Fa flags
parameter may include one or more of the following:
.Bd -literal
#define	MSG_OOB		0x0001 /* process out-of-band data */
#define	MSG_PEEK	0x0002 /* peek at incoming message */
#define	MSG_DONTROUTE	0x0004 /* bypass routing, use direct interface */
#define	MSG_EOR		0x0008 /* data completes record */
#define	MSG_NOSIGNAL	0x0400 /* 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.
.\" .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.
.Dv MSG_NOSIGNAL
is used to prevent
.Dv SIGPIPE
generation when writing a socket that
may be closed.
.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 occured.
.Sh ERRORS
.Fn send ,
.Fn sendto ,
.Fn sendmsg ,
and
.Fn sendmmsg
fail if:
.Bl -tag -width Er
.It Bq Er EACCES
The SO_BROADCAST option is not set on the socket, and a broadcast address
was given as the destination.
.It Bq Er EAFNOSUPPORT
Addresses in the specified address family cannot be used with this socket.
.It Bq Er EAGAIN|EWOULDBLOCK
The socket is marked non-blocking and the requested operation
would block.
.It Bq Er EBADF
An invalid descriptor was specified.
.It Bq Er EDSTADDRREQ
In a non-connected socket a destination address has not been specified.
.It Bq Er EFAULT
An invalid user space address was specified for a parameter.
.It Bq Er EHOSTDOWN
The destination is a host on the local subnet and does not respond to
.Xr arp 4 .
.It Bq Er EHOSTUNREACH
The destination for the message is unreachable.
.It Bq Er EINVAL
The total length of the I/O is more than can be expressed by the ssize_t
return value.
.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 ENOBUFS
The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
.Pp
An alternative reason: 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 ENOTSOCK
The argument
.Fa s
is not a socket.
.It Bq Er EPIPE
In a connected socket the connection has been broken.
.El
.Pp
.Fn sendto
will also fail if:
.Bl -tag -width Er
.It Bq Er EISCONN
A destination address was specified and the socket is already connected.
.El
.Pp
.Fn sendmsg
and
.Fn sendmmsg
will also fail if:
.Bl -tag -width Er
.It Bq Er EMSGSIZE
The
.Fa msg_iovlen
member of the
.Fa msg
structure is less than or equal to 0
or is greater than
.Brq Dv IOV_MAX .
.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 .
The
.Fn sendmmsg
function call appeared in
.Tn Linux 3.0
and
.Nx 7.0 .