xs(7)
=====


NAME
----
xs - Crossroads I/O, a lightweight messaging layer


SYNOPSIS
--------
*#include <xs/xs.h>*

*cc* ['flags'] 'files' *-lxs* ['libraries']


DESCRIPTION
-----------
Crossroads I/O is a library for building scalable and high performance
distributed applications. It fits between classic BSD sockets,
JMS/AMQP-style message queues, and enterprise message-oriented middleware.

Crossroads I/O extends the standard socket interfaces with features
traditionally provided by specialised _messaging middleware_ products.
Crossroads sockets provide an abstraction of asynchronous  _message
queues_, multiple _messaging patterns_, message filtering
(_subscriptions_), seamless access to multiple _transport protocols_ and
more.

Crossroads I/O provides a native C API for applications. Support for many
more languages is provided by the community through language bindings which
can be found at the Crossroads website.

This documentation presents an overview of Crossroads concepts, describes how
Crossroads abstract standard sockets and provides a reference manual for the
functions provided by the Crossroads library.


Context
~~~~~~~
Before using any Crossroads library functions the caller must initialise a
'context' using _xs_init()_. The following functions are provided to handle
initialisation and termination of a 'context':

Initialise Crossroads context::
    linkxs:xs_init[3]

Terminate Crossroads context::
    linkxs:xs_term[3]

Set Crossroads context options::
    linkxs:xs_setctxopt[3]


Thread safety
^^^^^^^^^^^^^
A 'context' is thread safe and may be shared among as many application
threads as necessary, without any additional locking required on the part
of the caller.

The individual 'sockets' within a 'context' are _not_ thread safe --
applications may _not_ use a single 'socket' concurrently from multiple
threads.

A 'socket' _may_ be _migrated_ from one thread to another, by issuing a
full memory barrier between individual calls on the 'socket'.  For example,
this means applications can create a socket in one thread with
_xs_socket()_ and then pass it to a _newly created_ thread as part of
thread initialization via a structure passed as an argument to
_pthread_create()_.


Multiple contexts
^^^^^^^^^^^^^^^^^
Multiple 'contexts' may coexist within a single application. Thus, an
application can use Crossroads directly and at the same time make use of any
number of additional libraries or components which themselves make use of
Crossroads.


Messages
~~~~~~~~
A Crossroads message is a discrete unit of data passed between applications
or components of the same application. Crossroads messages have no internal
structure and from the point of view of Crossroads themselves they are
considered to be opaque binary data.

Applications using the Crossroads library send and receive messages
directly from/to buffers provided by the application, using the Crossroads
functions _xs_send()_ and _xs_recv()_.

Alternatively, applications desiring zero-copy messaging and/or reference
counted allocation of messages can use the message handling functions
described in this section, and send and receive messages using
_xs_sendmsg()_ and _xs_recvmsg()_ respectively.  These two approaches are
interchangeable.

The following functions are provided to work with messages using zero-copy
and/or reference-counted allocation of messages:

Initialise a message::
    linkxs:xs_msg_init[3]
    linkxs:xs_msg_init_size[3]
    linkxs:xs_msg_init_data[3]

Release a message::
    linkxs:xs_msg_close[3]

Access message content::
    linkxs:xs_msg_data[3]
    linkxs:xs_msg_size[3]

Message manipulation::
    linkxs:xs_msg_copy[3]
    linkxs:xs_msg_move[3]

Retrieve message option::
    linkxs:xs_getmsgopt[3]


Sockets
~~~~~~~
Crossroads sockets present an abstraction of a asynchronous _message queue_,
with the exact queueing semantics depending on the socket type in use. See
linkxs:xs_socket[3] for the socket types provided.

The following functions are provided to work with sockets:

Creating a socket::
    linkxs:xs_socket[3]

Closing a socket::
    linkxs:xs_close[3]

Manipulating socket options::
    linkxs:xs_getsockopt[3]
    linkxs:xs_setsockopt[3]

Creating and modifiying topologies::
    linkxs:xs_bind[3]
    linkxs:xs_connect[3]
    linkxs:xs_shutdown[3]

Sending and receiving messages::
    linkxs:xs_send[3]
    linkxs:xs_recv[3]

Sending and receiving messages (zero-copy)::
    linkxs:xs_sendmsg[3]
    linkxs:xs_recvmsg[3]

.Input/output multiplexing
Crossroads provides a mechanism for applications to multiplex input/output events
over a set containing both Crossroads sockets and standard sockets. This
mechanism mirrors the standard _poll()_ system call, and is described in detail
in linkxs:xs_poll[3].


Transports
~~~~~~~~~~
A Crossroads socket can use multiple different underlying transport mechanisms.
Each transport mechanism is suited to a particular purpose and has its own
advantages and drawbacks.

The following transport mechanisms are provided:

Unicast transport using TCP::
    linkxs:xs_tcp[7]

Reliable multicast transport using PGM::
    linkxs:xs_pgm[7]

Local inter-process communication transport::
    linkxs:xs_ipc[7]

Local in-process (inter-thread) communication transport::
    linkxs:xs_inproc[7]


ERROR HANDLING
--------------
The Crossroads library functions handle errors using the standard conventions
found on POSIX systems. Generally, this means that upon failure a Crossroads
library function shall return either a NULL value (if returning a pointer) or
a negative value (if returning an integer), and the actual error code shall be
stored in the 'errno' variable.

On non-POSIX systems some users may experience issues with retrieving the
correct value of the 'errno' variable. The _xs_errno()_ function is provided
to assist in these cases; for details refer to linkxs:xs_errno[3].

The _xs_strerror()_ function is provided to translate Crossroads-specific error
codes into error message strings; for details refer to linkxs:xs_strerror[3].


MISCELLANEOUS
-------------
The following miscellaneous functions are provided:

Report Crossroads library version::
    linkxs:xs_version[3]


LANGUAGE BINDINGS
-----------------
The Crossroads library provides interfaces suitable for calling from programs in
any language; this documentation documents those interfaces as they would be
used by C programmers. The intent is that programmers using Crossroads from
other languages shall refer to this documentation alongside any documentation
provided by the vendor of their language binding.


ZEROMQ COMPATIBILITY
--------------------
The Crossroads library provides an optional drop-in 'libzmq' compatibility
library for ZeroMQ applications.  See linkxs:xs_zmq[7] for documentation on
this option.


AUTHORS
-------
The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com>
and Martin Lucina <martin@lucina.net>.


COPYING
-------
Free use of the Crossroads library software is granted under the terms of
the GNU Lesser General Public License (LGPL). For details see the files
`COPYING` and `COPYING.LESSER` included with the libxs distribution.

As a special exception, the copyright holders of libxs grant you the right
to link the library statically with your software. Refer to the end of
the `COPYING.LESSER` file included with the libxs distribution for details.
