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

           xs_init(3)

       Terminate Crossroads context

           xs_term(3)

       Set Crossroads context options

           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

           xs_msg_init(3) xs_msg_init_size(3) xs_msg_init_data(3)

       Release a message

           xs_msg_close(3)

       Access message content

           xs_msg_data(3) xs_msg_size(3)

       Message manipulation

           xs_msg_copy(3) xs_msg_move(3)

       Retrieve message option

           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 xs_socket(3) for the socket
       types provided.

       The following functions are provided to work with sockets:

       Creating a socket

           xs_socket(3)

       Closing a socket

           xs_close(3)

       Manipulating socket options

           xs_getsockopt(3) xs_setsockopt(3)

       Creating and modifiying topologies

           xs_bind(3) xs_connect(3) xs_shutdown(3)

       Sending and receiving messages

           xs_send(3) xs_recv(3)

       Sending and receiving messages (zero-copy)

           xs_sendmsg(3) 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
       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

           xs_tcp(7)

       Reliable multicast transport using PGM

           xs_pgm(7)

       Local inter-process communication transport

           xs_ipc(7)

       Local in-process (inter-thread) communication transport

           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 xs_errno(3).

       The xs_strerror() function is provided to translate Crossroads-specific error codes into
       error message strings; for details refer to xs_strerror(3).

MISCELLANEOUS

       The following miscellaneous functions are provided:

       Report Crossroads library version

           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 xs_zmq(7) for documentation on this option.

AUTHORS

       The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and
       Martin Lucina <martin@lucina.net[2]>.

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.

NOTES

        1. sustrik@250bpm.com
           mailto:sustrik@250bpm.com

        2. martin@lucina.net
           mailto:martin@lucina.net