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When a ZMQ_STREAM socket connection is broken (intentionally, via `shutdown()` or accidentally via client crash or network failure), there is no way for the application to dertermine that it should drop per-connection data (such as buffers). This contribution makes sure the application receives a 0-length message to notify it that the connection has been broken. This is symmetric with the process of closing the connection from within the application (where the application sends a 0-length message to tell ZeroMQ to close the connection). Conflicts: CMakeLists.txt
447 lines
16 KiB
Plaintext
447 lines
16 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 tear down, 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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.Thread safety
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0MQ 'sockets' are _not_ thread safe. Applications MUST NOT use a socket
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from multiple threads except after migrating a socket from one thread to
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another with a "full fence" memory barrier.
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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 ZMQ_REQ _client_
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to one or more ZMQ_REP _services_, and receiving subsequent replies to each
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request sent.
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The request-reply pattern is formally defined by http://rfc.zeromq.org/spec:28.
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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 round-robined among all _services_, and each reply received is
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matched with the last issued request.
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If no services are available, then any send operation on the socket shall
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block until at least one _service_ becomes available. The REQ socket shall
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not discard messages.
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[horizontal]
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.Summary of ZMQ_REQ characteristics
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Compatible peer sockets:: 'ZMQ_REP', 'ZMQ_ROUTER'
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Direction:: Bidirectional
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Send/receive pattern:: Send, Receive, Send, Receive, ...
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Outgoing routing strategy:: Round-robin
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Incoming routing strategy:: Last peer
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Action in mute state:: 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. If the original
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requester does not exist any more the reply is silently discarded.
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[horizontal]
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.Summary of ZMQ_REP characteristics
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Compatible peer sockets:: 'ZMQ_REQ', 'ZMQ_DEALER'
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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 strategy:: Last peer
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ZMQ_DEALER
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^^^^^^^^^^
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A socket of type 'ZMQ_DEALER' is an advanced pattern used for extending
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request/reply sockets. Each message sent is round-robined among all connected
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peers, and each message received is fair-queued from all connected peers.
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When a 'ZMQ_DEALER' socket enters the 'mute' state due to having reached the
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high water mark for all peers, or if there are no peers at all, then any
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linkzmq:zmq_send[3] operations on the socket shall block until the mute
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state ends or at least one peer becomes available for sending; messages are not
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discarded.
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When a 'ZMQ_DEALER' socket is connected to a 'ZMQ_REP' socket each message sent
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must consist of an empty message part, the _delimiter_, followed by one or more
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_body parts_.
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[horizontal]
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.Summary of ZMQ_DEALER characteristics
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Compatible peer sockets:: 'ZMQ_ROUTER', 'ZMQ_REP', 'ZMQ_DEALER'
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Direction:: Bidirectional
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Send/receive pattern:: Unrestricted
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Outgoing routing strategy:: Round-robin
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Incoming routing strategy:: Fair-queued
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Action in mute state:: Block
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ZMQ_ROUTER
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^^^^^^^^^^
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A socket of type 'ZMQ_ROUTER' is an advanced socket type used for extending
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request/reply sockets. When receiving messages a 'ZMQ_ROUTER' socket shall
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prepend a message part containing the _identity_ of the originating peer to the
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message before passing it to the application. Messages received are fair-queued
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from among all connected peers. When sending messages a 'ZMQ_ROUTER' socket shall
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remove the first part of the message and use it to determine the _identity_ of
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the peer the message shall be routed to. If the peer does not exist anymore
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the message shall be silently discarded by default, unless 'ZMQ_ROUTER_MANDATORY'
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socket option is set to '1'.
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When a 'ZMQ_ROUTER' socket enters the 'mute' state due to having reached the
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high water mark for all peers, then any messages sent to the socket shall be dropped
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until the mute state ends. Likewise, any messages routed to a peer for which
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the individual high water mark has been reached shall also be dropped.
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When a 'ZMQ_REQ' socket is connected to a 'ZMQ_ROUTER' socket, in addition to the
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_identity_ of the originating peer each message received shall contain an empty
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_delimiter_ message part. Hence, the entire structure of each received message
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as seen by the application becomes: one or more _identity_ parts, _delimiter_
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part, one or more _body parts_. When sending replies to a 'ZMQ_REQ' socket the
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application must include the _delimiter_ part.
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[horizontal]
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.Summary of ZMQ_ROUTER characteristics
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Compatible peer sockets:: 'ZMQ_DEALER', 'ZMQ_REQ', 'ZMQ_ROUTER'
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Direction:: Bidirectional
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Send/receive pattern:: Unrestricted
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Outgoing routing strategy:: See text
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Incoming routing strategy:: Fair-queued
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Action in mute state:: 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 fan out fashion.
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The publish-subscribe pattern is formally defined by http://rfc.zeromq.org/spec:29.
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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 fan out 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 the 'mute' 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 mute state
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ends. The _zmq_send()_ function shall never block for this socket type.
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[horizontal]
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.Summary of ZMQ_PUB characteristics
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Compatible peer sockets:: 'ZMQ_SUB', 'ZMQ_XSUB'
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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:: Fan out
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Action in mute state:: 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', 'ZMQ_XPUB'
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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_XPUB
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^^^^^^^^
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Same as ZMQ_PUB except that you can receive subscriptions from the peers
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in form of incoming messages. Subscription message is a byte 1 (for
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subscriptions) or byte 0 (for unsubscriptions) followed by the subscription
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body. Messages without a sub/unsub prefix are also received, but have no
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effect on subscription status.
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[horizontal]
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.Summary of ZMQ_XPUB characteristics
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Compatible peer sockets:: 'ZMQ_SUB', 'ZMQ_XSUB'
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Direction:: Unidirectional
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Send/receive pattern:: Send messages, receive subscriptions
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Incoming routing strategy:: N/A
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Outgoing routing strategy:: Fan out
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Action in mute state:: Drop
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ZMQ_XSUB
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^^^^^^^^
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Same as ZMQ_SUB except that you subscribe by sending subscription messages to
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the socket. Subscription message is a byte 1 (for subscriptions) or byte 0
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(for unsubscriptions) followed by the subscription body. Messages without a
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sub/unsub prefix may also be sent, but have no effect on subscription status.
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[horizontal]
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.Summary of ZMQ_XSUB characteristics
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Compatible peer sockets:: 'ZMQ_PUB', 'ZMQ_XPUB'
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Direction:: Unidirectional
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Send/receive pattern:: Receive messages, send subscriptions
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Incoming routing strategy:: Fair-queued
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Outgoing routing strategy:: N/A
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Action in mute state:: Drop
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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 round-robined among all connected _nodes_.
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The pipeline pattern is formally defined by http://rfc.zeromq.org/spec:30.
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ZMQ_PUSH
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^^^^^^^^
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A socket of type 'ZMQ_PUSH' is used by a pipeline _node_ to send messages
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to downstream pipeline _nodes_. Messages are round-robined 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_PUSH' socket enters the 'mute' state due to having reached the
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high water mark for all downstream _nodes_, or if there are no downstream
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_nodes_ at all, then any linkzmq:zmq_send[3] operations on the socket shall
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block until the mute state ends or at least one downstream _node_
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becomes available for sending; messages are not discarded.
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[horizontal]
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.Summary of ZMQ_PUSH characteristics
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Compatible peer sockets:: 'ZMQ_PULL'
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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:: Round-robin
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Action in mute state:: Block
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ZMQ_PULL
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^^^^^^^^
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A socket of type 'ZMQ_PULL' is used by a pipeline _node_ to receive messages
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from upstream pipeline _nodes_. Messages are fair-queued from among all
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connected upstream _nodes_. The _zmq_send()_ function is not implemented for
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this socket type.
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[horizontal]
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.Summary of ZMQ_PULL characteristics
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Compatible peer sockets:: 'ZMQ_PUSH'
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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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Action in mute state:: Block
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Exclusive pair pattern
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~~~~~~~~~~~~~~~~~~~~~~
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The exclusive pair pattern is used to connect a peer to precisely one other
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peer. This pattern is used for inter-thread communication across the inproc
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transport.
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The exclusive pair pattern is formally defined by http://rfc.zeromq.org/spec:31.
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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 the 'mute' 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 designed for inter-thread communication across
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the linkzmq:zmq_inproc[7] transport and do not implement functionality such
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as auto-reconnection. 'ZMQ_PAIR' sockets are considered experimental and may
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have other missing or broken aspects.
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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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Action in mute state:: Block
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Native Pattern
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~~~~~~~~~~~~~~
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The native pattern is used for communicating with TCP peers and allows
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asynchronous requests and replies in either direction.
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ZMQ_STREAM
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^^^^^^^^^^
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A socket of type 'ZMQ_STREAM' is used to send and receive TCP data from a
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non-0MQ peer, when using the tcp:// transport. A 'ZMQ_STREAM' socket can
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act as client and/or server, sending and/or receiving TCP data asynchronously.
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When receiving TCP data, a 'ZMQ_STREAM' socket shall prepend a message part
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containing the _identity_ of the originating peer to the message before passing
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it to the application. Messages received are fair-queued from among all
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connected peers.
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When sending TCP data, a 'ZMQ_STREAM' socket shall remove the first part of the
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message and use it to determine the _identity_ of the peer the message shall be
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routed to, and unroutable messages shall cause an EHOSTUNREACH or EAGAIN error.
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To open a connection to a server, use the zmq_connect call, and then fetch the
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socket identity using the ZMQ_IDENTITY zmq_getsockopt call.
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To close a specific client connection, as a server, send the identity frame
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followed by a zero-length message (see EXAMPLE section). Similarly, when the
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peer disconnects (or the connection is lost), a zero-length message will be
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received by the application.
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The ZMQ_SNDMORE flag is ignored on data frames. You must send one identity frame
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followed by one data frame.
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Also, please note that omitting the ZMQ_SNDMORE flag will prevent sending further
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data (from any client) on the same socket.
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[horizontal]
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.Summary of ZMQ_STREAM characteristics
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Compatible peer sockets:: none.
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Direction:: Bidirectional
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Send/receive pattern:: Unrestricted
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Outgoing routing strategy:: See text
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Incoming routing strategy:: Fair-queued
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Action in mute state:: EAGAIN
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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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*EFAULT*::
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The provided 'context' is invalid.
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*EMFILE*::
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The limit on the total number of open 0MQ sockets has been reached.
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*ETERM*::
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The context specified was terminated.
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EXAMPLE
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-------
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.Creating a simple HTTP server using ZMQ_STREAM
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----
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void *ctx = zmq_ctx_new ();
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assert (ctx);
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/* Create ZMQ_STREAM socket */
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void *socket = zmq_socket (ctx, ZMQ_STREAM);
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assert (socket);
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int rc = zmq_bind (socket, "tcp://*:8080");
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assert (rc == 0);
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/* Data structure to hold the ZMQ_STREAM ID */
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uint8_t id [256];
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size_t id_size = 256;
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while (1) {
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/* Get HTTP request; ID frame and then request */
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id_size = zmq_recv (server, id, 256, 0);
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assert (id_size > 0);
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/* Prepares the response */
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char http_response [] =
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"HTTP/1.0 200 OK\r\n"
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"Content-Type: text/plain\r\n"
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"\r\n"
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"Hello, World!";
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/* Sends the ID frame followed by the response */
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zmq_send (socket, id, id_size, ZMQ_SNDMORE);
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zmq_send (socket, http_response, strlen (http_response), ZMQ_SNDMORE);
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/* Closes the connection by sending the ID frame followed by a zero response */
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zmq_send (socket, id, id_size, ZMQ_SNDMORE);
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zmq_send (socket, 0, 0, ZMQ_SNDMORE);
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/* NOTE: If we don't use ZMQ_SNDMORE, then we won't be able to send more */
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/* message to any client */
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}
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zmq_close (socket);
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zmq_ctx_destroy (ctx);
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----
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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_inproc[7]
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linkzmq:zmq[7]
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AUTHORS
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-------
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This page was written by the 0MQ community. To make a change please
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read the 0MQ Contribution Policy at <http://www.zeromq.org/docs:contributing>.
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