#include <sys/types.h>
#include <sys/socket.h> ssize_t
send (int s const void *msg size_t len int flags); ssize_t
sendto (int s const void *msg size_t len int flags const struct sockaddr *to socklen_t tolen); ssize_t
sendmsg (int s const struct msghdr *msg int flags);
DESCRIPTION
The
send ();
function,
and
sendto ();
and
sendmsg ();
system calls
are used to transmit a message to another socket.
The
send ();
function
may be used only when the socket is in a
connected
state, while
sendto ();
and
sendmsg ();
may be used at any time.
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.
No indication of failure to deliver is implicit in a
send (.);
Locally detected errors are indicated by a return value of -1.
If no messages space is available at the socket to hold
the message to be transmitted, then
send ();
normally blocks, unless the socket has been placed in
non-blocking I/O mode.
The
select(2)
system call may be used to determine when it is possible to
send more data.
The
Fa flags
argument may include one or more of the following:
#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_EOF 0x00100 /* data completes transaction */
#define MSG_NOSIGNAL 0x20000 /* do not generate SIGPIPE on EOF */
The flag
MSG_OOB
is used to send
``out-of-band''
data on sockets that support this notion (e.g.
SOCK_STREAM )
the underlying protocol must also support
``out-of-band''
data.
MSG_EOR
is used to indicate a record mark for protocols which support the
concept.
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
SOCK_STREAM
sockets in the
PF_INET
protocol family.
MSG_DONTROUTE
is usually used only by diagnostic or routing programs.
MSG_NOSIGNAL
is used to prevent
SIGPIPE
generation when writing a socket that
may be closed.
See
recv(2)
for a description of the
Fa msghdr
structure.
RETURN VALUES
The call returns the number of characters sent, or -1
if an error occurred.
ERRORS
The
send ();
function and
sendto ();
and
sendmsg ();
system calls
fail if:
Bq Er EBADF
An invalid descriptor was specified.
Bq Er EACCES
The destination address is a broadcast address, and
SO_BROADCAST
has not been set on the socket.
Bq Er ENOTSOCK
The argument
Fa s
is not a socket.
Bq Er EFAULT
An invalid user space address was specified for an argument.
Bq Er EMSGSIZE
The socket requires that message be sent atomically,
and the size of the message to be sent made this impossible.
Bq Er EAGAIN
The socket is marked non-blocking and the requested operation
would block.
Bq Er ENOBUFS
The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
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.
Bq Er EHOSTUNREACH
The remote host was unreachable.
Bq Er EISCONN
A destination address was specified and the socket is already connected.
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.
Bq Er EHOSTDOWN
The remote host was down.
Bq Er ENETDOWN
The remote network was down.
Bq Er EPERM
The process using a
SOCK_RAW
socket was jailed and the source
address specified in the IP header did not match the IP
address bound to the prison.
Bq Er EPIPE
The socket is unable to send anymore data
( SBS_CANTSENDMORE
has been set on the socket).
This typically means that the socket
is not connected.
Because
sendmsg ();
does not necessarily block until the data has been transferred, it
is possible to transfer an open file descriptor across an
AF_UNIX
domain socket
(see
recv(2)),
then
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.
