xs_sendmsg(3)
=============


NAME
----
xs_sendmsg - send a message part on a socket (zero-copy)


SYNOPSIS
--------
*int xs_sendmsg (void '*socket', xs_msg_t '*msg', int 'flags');*


DESCRIPTION
-----------
The _xs_sendmsg()_ function shall queue the message referenced by the 'msg'
argument to be sent to the socket referenced by the 'socket' argument.  The
'flags' argument is a combination of the flags defined below:

*XS_DONTWAIT*::
Specifies that the operation should be performed in non-blocking mode. If the
message cannot be queued on the 'socket', the _xs_sendmsg()_ function shall
fail with 'errno' set to EAGAIN.

*XS_SNDMORE*::
Specifies that the message being sent is a multi-part message, and that further
message parts are to follow. Refer to the section regarding multi-part messages
below for a detailed description.

The _xs_msg_t_ structure passed to _xs_sendmsg()_ is nullified during the
call. If you want to send the same message to multiple sockets you have to copy
it using (e.g. using _xs_msg_copy()_).

NOTE: A successful invocation of _xs_sendmsg()_ does not indicate that the
message has been transmitted to the network, only that it has been queued on
the 'socket' and Crossroads have assumed responsibility for the message.


Multi-part messages
~~~~~~~~~~~~~~~~~~~
A Crossroads message is composed of 1 or more message parts. Each message
part is an independent 'xs_msg_t' in its own right. Crossroads ensure atomic
delivery of messages; peers shall receive either all _message parts_ of a
message or none at all. The total number of message parts is unlimited except
by available memory.

An application that sends multipart messages must use the _XS_SNDMORE_ flag
when sending each data part except the final one.


RETURN VALUE
------------
The _xs_sendmsg()_ function shall return number of bytes in the message
if successful. Otherwise it shall return `-1` and set 'errno' to one of the
values defined below.


ERRORS
------
*EAGAIN*::
Non-blocking mode was requested and the message cannot be sent at the moment.
*ENOTSUP*::
The _xs_sendmsg()_ operation is not supported by this socket type.
*EFSM*::
The _xs_sendmsg()_ operation cannot be performed on this socket at the moment
due to the socket not being in the appropriate state.  This error may occur with
socket types that switch between several states, such as XS_REP.  See the
_messaging patterns_ section of linkxs:xs_socket[3] for more information.
*ETERM*::
The 'context' associated with the specified 'socket' was terminated.
*ENOTSOCK*::
The provided 'socket' was invalid.
*EINTR*::
The operation was interrupted by delivery of a signal before the message was
sent.
*EFAULT*::
Invalid message.


EXAMPLE
-------
.Filling in a message and sending it to a socket
----
/* Create a new message, allocating 6 bytes for message content */
xs_msg_t msg;
int rc = xs_msg_init_size (&msg, 6);
assert (rc == 0);
/* Fill in message content with 'AAAAAA' */
memset (xs_msg_data (&msg), 'A', 6);
/* Send the message to the socket */
rc = xs_sendmsg (socket, &msg, 0);
assert (rc == 6);
----

.Sending a multi-part message
----
/* Send a multi-part message consisting of three parts to socket */
rc = xs_sendmsg (socket, &part1, XS_SNDMORE);
rc = xs_sendmsg (socket, &part2, XS_SNDMORE);
/* Final part; no more parts to follow */
rc = xs_sendmsg (socket, &part3, 0);
----


SEE ALSO
--------
Applications that do not require zero-copy messaging can use the simpler
linkxs:xs_send[3] instead of _xs_sendmsg()_.

linkxs:xs_recvmsg[3]
linkxs:xs_socket[7]
linkxs:xs[7]


AUTHORS
-------
This man page was written by Martin Sustrik <sustrik@250bpm.com>, Martin
Lucina <martin@lucina.net> and Pieter Hintjens <ph@imatix.com>.
