libzmq/doc/zmq.txt

zmq(7)
======


NAME
----
zmq - 0MQ lightweight messaging kernel


SYNOPSIS
--------
*#include <zmq.h>*

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


DESCRIPTION
-----------
The 0MQ lightweight messaging kernel is a library which extends the standard
socket interfaces with features traditionally provided by specialised
_messaging middleware_ products. 0MQ sockets provide an abstraction of
asynchronous  _message queues_, multiple _messaging patterns_, message
filtering (_subscriptions_), seamless access to multiple _transport protocols_
and more.

This documentation presents an overview of 0MQ concepts, describes how 0MQ
abstracts standard sockets and provides a reference manual for the functions
provided by the 0MQ library.


Context
~~~~~~~
Before using any 0MQ library functions the caller must initialise a 0MQ
'context' using _zmq_init()_. The following functions are provided to handle
initialisation and termination of a 'context':

Initialise 0MQ context::
    linkzmq:zmq_init[3]

Terminate 0MQ context::
    linkzmq:zmq_term[3]


Thread safety
^^^^^^^^^^^^^
A 0MQ 'context' is thread safe and may be shared among as many application
threads as the application has requested using the _app_threads_ parameter to
_zmq_init()_, without any additional locking required on the part of the
caller.  Each 0MQ socket belonging to a particular 'context' may only be used
by *the thread that created it* using _zmq_socket()_.


Multiple contexts
^^^^^^^^^^^^^^^^^
Multiple 'contexts' may coexist within a single application. Thus, an
application can use 0MQ directly and at the same time make use of any number of
additional libraries or components which themselves make use of 0MQ as long as
the above guidelines regarding thread safety are adhered to.


Messages
~~~~~~~~
A 0MQ message is a discrete unit of data passed between applications or
components of the same application. 0MQ messages have no internal structure and
from the point of view of 0MQ itself they are considered to be opaque binary
data.

The following functions are provided to work with messages:

Initialise a message::
    linkzmq:zmq_msg_init[3]
    linkzmq:zmq_msg_init_size[3]
    linkzmq:zmq_msg_init_data[3]

Release a message::
    linkzmq:zmq_msg_close[3]

Access message content::
    linkzmq:zmq_msg_data[3]
    linkzmq:zmq_msg_size[3]

Message manipulation::
    linkzmq:zmq_msg_copy[3]
    linkzmq:zmq_msg_move[3]


Sockets
~~~~~~~
0MQ sockets present an abstraction of a asynchronous _message queue_, with the
exact queueing semantics depending on the socket type in use. See
linkzmq:zmq_socket[3] for the socket types provided.

The following functions are provided to work with sockets:

Creating a socket::
    linkzmq:zmq_socket[3]

Closing a socket::
    linkzmq:zmq_close[3]

Manipulating socket options::
    linkzmq:zmq_getsockopt[3]
    linkzmq:zmq_setsockopt[3]

Establishing a message flow::
    linkzmq:zmq_bind[3]
    linkzmq:zmq_connect[3]

Sending and receiving messages::
    linkzmq:zmq_send[3]
    linkzmq:zmq_recv[3]

.Input/output multiplexing
0MQ provides a mechanism for applications to multiplex input/output events over
a set containing both 0MQ sockets and standard sockets. This mechanism mirrors
the standard _poll()_ system call, and is described in detail in
linkzmq:zmq_poll[3].


Transports
~~~~~~~~~~
A 0MQ 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::
    linkzmq:zmq_tcp[7]

Reliable multicast transport using PGM::
    linkzmq:zmq_pgm[7]

Local inter-process communication transport::
    linkzmq:zmq_ipc[7]

Local in-process (inter-thread) communication transport::
    linkzmq:zmq_inproc[7]


Devices
~~~~~~~
Apart from the 0MQ library the 0MQ distribution includes 'devices' which are
building blocks intended to serve as intermediate nodes in complex messaging
topologies.

The following devices are provided:

Forwarder device for request-response messaging::
    linkzmq:zmq_queue[1]

Forwarder device for publish-subscribe messaging::
    linkzmq:zmq_forwarder[1]

Streamer device for parallelized pipeline messaging::
    linkzmq:zmq_streamer[1]


ERROR HANDLING
--------------
The 0MQ library functions handle errors using the standard conventions found on
POSIX systems. Generally, this means that upon failure a 0MQ 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 _zmq_errno()_ function is provided
to assist in these cases; for details refer to linkzmq:zmq_errno[3].

The _zmq_strerror()_ function is provided to translate 0MQ-specific error codes
into error message strings; for details refer to linkzmq:zmq_strerror[3].


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

Report 0MQ library version::
    linkzmq:zmq_version[3]


LANGUAGE BINDINGS
-----------------
The 0MQ 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 0MQ from other languages
shall refer to this documentation alongside any documentation provided by the
vendor of their language binding.


C++ language binding
~~~~~~~~~~~~~~~~~~~~
The 0MQ distribution includes a $$C++$$ language binding, which is documented
separately in linkzmq:zmq_cpp[7].


Other language bindings
~~~~~~~~~~~~~~~~~~~~~~~
Other language bindings (Python, Ruby, Java and more) are provided by members
of the 0MQ community and pointers can be found on the 0MQ website.