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7c9b09bc51
Mostly Flow control and additions to zmq_socket(3) Removed/changed lots of text regarding message queues More fixes for 2.0.7 changes
261 lines
8.9 KiB
Plaintext
261 lines
8.9 KiB
Plaintext
zmq_socket(3)
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=============
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NAME
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----
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zmq_socket - create 0MQ socket
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SYNOPSIS
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--------
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*void *zmq_socket (void '*context', int 'type');*
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DESCRIPTION
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-----------
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The 'zmq_socket()' function shall create a 0MQ socket within the specified
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'context' and return an opaque handle to the newly created socket. The 'type'
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argument specifies the socket type, which determines the semantics of
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communication over the socket.
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The newly created socket is initially unbound, and not associated with any
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endpoints. In order to establish a message flow a socket must first be
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connected to at least one endpoint with linkzmq:zmq_connect[3], or at least one
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endpoint must be created for accepting incoming connections with
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linkzmq:zmq_bind[3].
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.Key differences to conventional sockets
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Generally speaking, conventional sockets present a _synchronous_ interface to
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either connection-oriented reliable byte streams (SOCK_STREAM), or
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connection-less unreliable datagrams (SOCK_DGRAM). In comparison, 0MQ sockets
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present an abstraction of an asynchronous _message queue_, with the exact
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queueing semantics depending on the socket type in use. Where conventional
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sockets transfer streams of bytes or discrete datagrams, 0MQ sockets transfer
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discrete _messages_.
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0MQ sockets being _asynchronous_ means that the timings of the physical
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connection setup and teardown, reconnect and effective delivery are transparent
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to the user and organized by 0MQ itself. Further, messages may be _queued_ in
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the event that a peer is unavailable to receive them.
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Conventional sockets allow only strict one-to-one (two peers), many-to-one
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(many clients, one server), or in some cases one-to-many (multicast)
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relationships. With the exception of 'ZMQ_PAIR', 0MQ sockets may be connected
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*to multiple endpoints* using _zmq_connect()_, while simultaneously accepting
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incoming connections *from multiple endpoints* bound to the socket using
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_zmq_bind()_, thus allowing many-to-many relationships.
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.Socket types
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The following sections present the socket types defined by 0MQ, grouped by the
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general _messaging pattern_ which is built from related socket types.
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Request-reply pattern
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~~~~~~~~~~~~~~~~~~~~~
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The request-reply pattern is used for sending requests from a _client_ to one
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or more instances of a _service_, and receiving subsequent replies to each
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request sent.
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ZMQ_REQ
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^^^^^^^
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A socket of type 'ZMQ_REQ' is used by a _client_ to send requests to and
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receive replies from a _service_. This socket type allows only an alternating
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sequence of _zmq_send(request)_ and subsequent _zmq_recv(reply)_ calls. Each
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request sent is load-balanced among all _services_, and each reply received is
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matched with the last issued request.
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When a 'ZMQ_REQ' socket enters an exceptional state due to having reached the
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high water mark for all _services_, or if there are no _services_ at all, then
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any linkzmq:zmq_send[3] operations on the socket shall block until the
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exceptional state ends or at least one _service_ becomes available for sending;
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messages are not discarded.
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[horizontal]
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.Summary of ZMQ_REQ characteristics
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Compatible peer sockets:: 'ZMQ_REP'
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Direction:: Bidirectional
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Send/receive pattern:: Send, Receive, Send, Receive, ...
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Outgoing routing strategy:: Load-balanced
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Incoming routing strategy:: Last peer
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ZMQ_HWM option action:: Block
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ZMQ_REP
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^^^^^^^
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A socket of type 'ZMQ_REP' is used by a _service_ to receive requests from and
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send replies to a _client_. This socket type allows only an alternating
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sequence of _zmq_recv(request)_ and subsequent _zmq_send(reply)_ calls. Each
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request received is fair-queued from among all _clients_, and each reply sent
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is routed to the _client_ that issued the last request.
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When a 'ZMQ_REP' socket enters an exceptional state due to having reached the
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high water mark for a _client_, then any replies sent to the _client_ in
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question shall be dropped until the exceptional state ends.
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[horizontal]
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.Summary of ZMQ_REP characteristics
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Compatible peer sockets:: 'ZMQ_REQ'
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Direction:: Bidirectional
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Send/receive pattern:: Receive, Send, Receive, Send, ...
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Incoming routing strategy:: Fair-queued
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Outgoing routing stratagy:: Last peer
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ZMQ_HWM option action:: Drop
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Publish-subscribe pattern
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The publish-subscribe pattern is used for one-to-many distribution of data from
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a single _publisher_ to multiple _subscribers_ in a fanout fashion.
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ZMQ_PUB
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^^^^^^^
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A socket of type 'ZMQ_PUB' is used by a _publisher_ to distribute data.
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Messages sent are distributed in a fanout fashion to all connected peers.
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The linkzmq:zmq_recv[3] function is not implemented for this socket type.
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When a 'ZMQ_PUB' socket enters an exceptional state due to having reached the
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high water mark for a _subscriber_, then any messages that would be sent to the
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_subscriber_ in question shall instead be dropped until the exceptional state
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ends.
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[horizontal]
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.Summary of ZMQ_PUB characteristics
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Compatible peer sockets:: 'ZMQ_SUB'
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Direction:: Unidirectional
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Send/receive pattern:: Send only
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Incoming routing strategy:: N/A
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Outgoing routing strategy:: Fanout
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ZMQ_HWM option action:: Drop
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ZMQ_SUB
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^^^^^^^
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A socket of type 'ZMQ_SUB' is used by a _subscriber_ to subscribe to data
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distributed by a _publisher_. Initially a 'ZMQ_SUB' socket is not subscribed to
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any messages, use the 'ZMQ_SUBSCRIBE' option of linkzmq:zmq_setsockopt[3] to
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specify which messages to subscribe to. The _zmq_send()_ function is not
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implemented for this socket type.
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[horizontal]
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.Summary of ZMQ_SUB characteristics
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Compatible peer sockets:: 'ZMQ_PUB'
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Direction:: Unidirectional
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Send/receive pattern:: Receive only
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Incoming routing strategy:: Fair-queued
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Outgoing routing strategy:: N/A
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ZMQ_HWM option action:: N/A
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Pipeline pattern
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~~~~~~~~~~~~~~~~
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The pipeline pattern is used for distributing data to _nodes_ arranged in
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a pipeline. Data always flows down the pipeline, and each stage of the pipeline
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is connected to at least one _node_. When a pipeline stage is connected to
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multiple _nodes_ data is load-balanced among all connected _nodes_.
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ZMQ_DOWNSTREAM
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^^^^^^^^^^^^^^
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A socket of type 'ZMQ_DOWNSTREAM' is used by a pipeline _node_ to send messages
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to downstream pipeline _nodes_. Messages are load-balanced to all connected
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downstream _nodes_. The _zmq_recv()_ function is not implemented for this
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socket type.
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When a 'ZMQ_DOWNSTREAM' socket enters an exceptional state due to having
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reached the high water mark for all downstream _nodes_, or if there are no
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downstream _nodes_ at all, then any linkzmq:zmq_send[3] operations on the
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socket shall block until the exceptional state ends or at least one downstream
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_node_ becomes available for sending; messages are not discarded.
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[horizontal]
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.Summary of ZMQ_DOWNSTREAM characteristics
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Compatible peer sockets:: 'ZMQ_UPSTREAM'
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Direction:: Unidirectional
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Send/receive pattern:: Send only
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Incoming routing strategy:: N/A
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Outgoing routing strategy:: Load-balanced
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ZMQ_HWM option action:: Block
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ZMQ_UPSTREAM
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^^^^^^^^^^^^
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A socket of type 'ZMQ_UPSTREAM' is used by a pipeline _node_ to receive
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messages from upstream pipeline _nodes_. Messages are fair-queued from among
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all connected upstream _nodes_. The _zmq_send()_ function is not implemented
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for this socket type.
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[horizontal]
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.Summary of ZMQ_UPSTREAM characteristics
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Compatible peer sockets:: 'ZMQ_DOWNSTREAM'
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Direction:: Unidirectional
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Send/receive pattern:: Receive only
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Incoming routing strategy:: Fair-queued
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Outgoing routing strategy:: N/A
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ZMQ_HWM option action:: N/A
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Exclusive pair pattern
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~~~~~~~~~~~~~~~~~~~~~~
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The exclusive pair is an advanced pattern used for communicating exclusively
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between two peers.
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ZMQ_PAIR
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^^^^^^^^
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A socket of type 'ZMQ_PAIR' can only be connected to a single peer at any one
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time. No message routing or filtering is performed on messages sent over a
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'ZMQ_PAIR' socket.
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When a 'ZMQ_PAIR' socket enters an exceptional state due to having reached the
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high water mark for the connected peer, or if no peer is connected, then
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any linkzmq:zmq_send[3] operations on the socket shall block until the peer
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becomes available for sending; messages are not discarded.
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NOTE: 'ZMQ_PAIR' sockets are experimental, and are currently missing several
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features such as auto-reconnection.
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[horizontal]
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.Summary of ZMQ_PAIR characteristics
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Compatible peer sockets:: 'ZMQ_PAIR'
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Direction:: Bidirectional
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Send/receive pattern:: Unrestricted
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Incoming routing strategy:: N/A
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Outgoing routing strategy:: N/A
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ZMQ_HWM option action:: Block
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RETURN VALUE
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------------
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The _zmq_socket()_ function shall return an opaque handle to the newly created
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socket if successful. Otherwise, it shall return NULL and set 'errno' to one of
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the values defined below.
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ERRORS
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------
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*EINVAL*::
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The requested socket 'type' is invalid.
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*EMTHREAD*::
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The maximum number of sockets within this 'context' has been exceeded.
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SEE ALSO
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--------
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linkzmq:zmq_init[3]
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linkzmq:zmq_setsockopt[3]
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linkzmq:zmq_bind[3]
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linkzmq:zmq_connect[3]
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linkzmq:zmq_send[3]
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linkzmq:zmq_recv[3]
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linkzmq:zmq[7]
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AUTHORS
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-------
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The 0MQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and
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Martin Lucina <mato@kotelna.sk>.
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