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<div id="header">
<h1>
zmq_socket(3) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>zmq_socket -
create 0MQ socket
</p>
</div>
</div>
<div id="content">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>void *zmq_socket (void <em>*context</em>, int <em>type</em>);</strong></p></div>
</div>
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>zmq_socket()</em> function shall create a ØMQ socket within the specified
<em>context</em> and return an opaque handle to the newly created socket. The <em>type</em>
argument specifies the socket type, which determines the semantics of
communication over the socket.</p></div>
<div class="paragraph"><p>The newly created socket is initially unbound, and not associated with any
endpoints. In order to establish a message flow a socket must first be
connected to at least one endpoint with <a href="zmq_connect.html">zmq_connect(3)</a>, or at least one
endpoint must be created for accepting incoming connections with
<a href="zmq_bind.html">zmq_bind(3)</a>.</p></div>
<div class="paragraph"><div class="title">Key differences to conventional sockets</div><p>Generally speaking, conventional sockets present a <em>synchronous</em> interface to
either connection-oriented reliable byte streams (SOCK_STREAM), or
connection-less unreliable datagrams (SOCK_DGRAM). In comparison, ØMQ sockets
present an abstraction of an asynchronous <em>message queue</em>, with the exact
queueing semantics depending on the socket type in use. Where conventional
sockets transfer streams of bytes or discrete datagrams, ØMQ sockets transfer
discrete <em>messages</em>.</p></div>
<div class="paragraph"><p>ØMQ sockets being <em>asynchronous</em> means that the timings of the physical
connection setup and teardown, reconnect and effective delivery are transparent
to the user and organized by ØMQ itself. Further, messages may be <em>queued</em> in
the event that a peer is unavailable to receive them.</p></div>
<div class="paragraph"><p>Conventional sockets allow only strict one-to-one (two peers), many-to-one
(many clients, one server), or in some cases one-to-many (multicast)
relationships. With the exception of <em>ZMQ_PAIR</em>, ØMQ sockets may be connected
<strong>to multiple endpoints</strong> using <em>zmq_connect()</em>, while simultaneously accepting
incoming connections <strong>from multiple endpoints</strong> bound to the socket using
<em>zmq_bind()</em>, thus allowing many-to-many relationships.</p></div>
<div class="paragraph"><div class="title">Socket types</div><p>The following sections present the socket types defined by ØMQ, grouped by the
general <em>messaging pattern</em> which is built from related socket types.</p></div>
<h3 id="_request_reply_pattern">Request-reply pattern</h3><div style="clear:left"></div>
<div class="paragraph"><p>The request-reply pattern is used for sending requests from a <em>client</em> to one
or more instances of a <em>service</em>, and receiving subsequent replies to each
request sent.</p></div>
<h4 id="_zmq_req">ZMQ_REQ</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_REQ</em> is used by a <em>client</em> to send requests to and
receive replies from a <em>service</em>. This socket type allows only an alternating
sequence of <em>zmq_send(request)</em> and subsequent <em>zmq_recv(reply)</em> calls. Each
request sent is load-balanced among all <em>services</em>, and each reply received is
matched with the last issued request.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_REQ</em> socket enters an exceptional state due to having reached the
high water mark for all <em>services</em>, or if there are no <em>services</em> at all, then
any <a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall block until the
exceptional state ends or at least one <em>service</em> becomes available for sending;
messages are not discarded.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_REQ characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_REP</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send, Receive, Send, Receive, …
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Load-balanced
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Last peer
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
<h4 id="_zmq_rep">ZMQ_REP</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_REP</em> is used by a <em>service</em> to receive requests from and
send replies to a <em>client</em>. This socket type allows only an alternating
sequence of <em>zmq_recv(request)</em> and subsequent <em>zmq_send(reply)</em> calls. Each
request received is fair-queued from among all <em>clients</em>, and each reply sent
is routed to the <em>client</em> that issued the last request.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_REP</em> socket enters an exceptional state due to having reached the
high water mark for a <em>client</em>, then any replies sent to the <em>client</em> in
question shall be dropped until the exceptional state ends.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_REP characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_REQ</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive, Send, Receive, Send, …
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing stratagy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Last peer
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
<h3 id="_publish_subscribe_pattern">Publish-subscribe pattern</h3><div style="clear:left"></div>
<div class="paragraph"><p>The publish-subscribe pattern is used for one-to-many distribution of data from
a single <em>publisher</em> to multiple <em>subscribers</em> in a fanout fashion.</p></div>
<h4 id="_zmq_pub">ZMQ_PUB</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PUB</em> is used by a <em>publisher</em> to distribute data.
Messages sent are distributed in a fanout fashion to all connected peers.
The <a href="zmq_recv.html">zmq_recv(3)</a> function is not implemented for this socket type.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_PUB</em> socket enters an exceptional state due to having reached the
high water mark for a <em>subscriber</em>, then any messages that would be sent to the
<em>subscriber</em> in question shall instead be dropped until the exceptional state
ends.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_PUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_SUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fanout
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
<h4 id="_zmq_sub">ZMQ_SUB</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_SUB</em> is used by a <em>subscriber</em> to subscribe to data
distributed by a <em>publisher</em>. Initially a <em>ZMQ_SUB</em> socket is not subscribed to
any messages, use the <em>ZMQ_SUBSCRIBE</em> option of <a href="zmq_setsockopt.html">zmq_setsockopt(3)</a> to
specify which messages to subscribe to. The <em>zmq_send()</em> function is not
implemented for this socket type.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_SUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
</table></div>
<h3 id="_pipeline_pattern">Pipeline pattern</h3><div style="clear:left"></div>
<div class="paragraph"><p>The pipeline pattern is used for distributing data to <em>nodes</em> arranged in
a pipeline. Data always flows down the pipeline, and each stage of the pipeline
is connected to at least one <em>node</em>. When a pipeline stage is connected to
multiple <em>nodes</em> data is load-balanced among all connected <em>nodes</em>.</p></div>
<h4 id="_zmq_downstream">ZMQ_DOWNSTREAM</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_DOWNSTREAM</em> is used by a pipeline <em>node</em> to send messages
to downstream pipeline <em>nodes</em>. Messages are load-balanced to all connected
downstream <em>nodes</em>. The <em>zmq_recv()</em> function is not implemented for this
socket type.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_DOWNSTREAM</em> socket enters an exceptional state due to having
reached the high water mark for all downstream <em>nodes</em>, or if there are no
downstream <em>nodes</em> at all, then any <a href="zmq_send.html">zmq_send(3)</a> operations on the
socket shall block until the exceptional state ends or at least one downstream
<em>node</em> becomes available for sending; messages are not discarded.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_DOWNSTREAM characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_UPSTREAM</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Load-balanced
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
<h4 id="_zmq_upstream">ZMQ_UPSTREAM</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_UPSTREAM</em> is used by a pipeline <em>node</em> to receive
messages from upstream pipeline <em>nodes</em>. Messages are fair-queued from among
all connected upstream <em>nodes</em>. The <em>zmq_send()</em> function is not implemented
for this socket type.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_UPSTREAM characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_DOWNSTREAM</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
</table></div>
<h3 id="_exclusive_pair_pattern">Exclusive pair pattern</h3><div style="clear:left"></div>
<div class="paragraph"><p>The exclusive pair is an advanced pattern used for communicating exclusively
between two peers.</p></div>
<h4 id="_zmq_pair">ZMQ_PAIR</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PAIR</em> can only be connected to a single peer at any one
time. No message routing or filtering is performed on messages sent over a
<em>ZMQ_PAIR</em> socket.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_PAIR</em> socket enters an exceptional state due to having reached the
high water mark for the connected peer, or if no peer is connected, then
any <a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall block until the peer
becomes available for sending; messages are not discarded.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content"><em>ZMQ_PAIR</em> sockets are experimental, and are currently missing several
features such as auto-reconnection.</td>
</tr></table>
</div>
<div class="hdlist"><div class="title">Summary of ZMQ_PAIR characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PAIR</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unrestricted
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
ZMQ_HWM option action
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
<h2 id="_return_value">RETURN VALUE</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>zmq_socket()</em> function shall return an opaque handle to the newly created
socket if successful. Otherwise, it shall return NULL and set <em>errno</em> to one of
the values defined below.</p></div>
</div>
<h2 id="_errors">ERRORS</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>EINVAL</strong>
</dt>
<dd>
<p>
The requested socket <em>type</em> is invalid.
</p>
</dd>
<dt class="hdlist1">
<strong>EMTHREAD</strong>
</dt>
<dd>
<p>
The maximum number of sockets within this <em>context</em> has been exceeded.
</p>
</dd>
</dl></div>
</div>
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="zmq_init.html">zmq_init(3)</a>
<a href="zmq_setsockopt.html">zmq_setsockopt(3)</a>
<a href="zmq_bind.html">zmq_bind(3)</a>
<a href="zmq_connect.html">zmq_connect(3)</a>
<a href="zmq_send.html">zmq_send(3)</a>
<a href="zmq_recv.html">zmq_recv(3)</a>
<a href="zmq.html">zmq(7)</a></p></div>
</div>
<h2 id="_authors">AUTHORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>The ØMQ documentation was written by Martin Sustrik <<a href="mailto:sustrik@250bpm.com">sustrik@250bpm.com</a>> and
Martin Lucina <<a href="mailto:mato@kotelna.sk">mato@kotelna.sk</a>>.</p></div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2010-06-02 18:30:09 CEST
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