Provided by: libzmq-dev_2.1.11-1ubuntu1_i386 bug

NAME

       zmq_socket - create 0MQ socket

SYNOPSIS

       void *zmq_socket (void *context, int type);

DESCRIPTION

       The zmq_socket() function shall create a 0MQ socket within the
       specified context and return an opaque handle to the newly created
       socket. The type argument specifies the socket type, which determines
       the semantics of communication over the socket.

       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 zmq_connect(3), or at least
       one endpoint must be created for accepting incoming connections with
       zmq_bind(3).

       Key differences to conventional sockets. Generally speaking,
       conventional sockets present a synchronous interface to either
       connection-oriented reliable byte streams (SOCK_STREAM), or
       connection-less unreliable datagrams (SOCK_DGRAM). In comparison, 0MQ
       sockets present an abstraction of an asynchronous message queue, with
       the exact queueing semantics depending on the socket type in use. Where
       conventional sockets transfer streams of bytes or discrete datagrams,
       0MQ sockets transfer discrete messages.

       0MQ sockets being asynchronous means that the timings of the physical
       connection setup and tear down, reconnect and effective delivery are
       transparent to the user and organized by 0MQ itself. Further, messages
       may be queued in the event that a peer is unavailable to receive them.

       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 ZMQ_PAIR, 0MQ sockets
       may be connected to multiple endpoints using zmq_connect(), while
       simultaneously accepting incoming connections from multiple endpoints
       bound to the socket using zmq_bind(), thus allowing many-to-many
       relationships.

       Thread safety. 0MQ sockets are not thread safe. Applications MUST NOT
       use a socket from multiple threads except after migrating a socket from
       one thread to another with a "full fence" memory barrier.

       Socket types. The following sections present the socket types defined
       by 0MQ, grouped by the general messaging pattern which is built from
       related socket types.

   Request-reply pattern
       The request-reply pattern is used for sending requests from a client to
       one or more instances of a service, and receiving subsequent replies to
       each request sent.

       ZMQ_REQ
           A socket of type ZMQ_REQ is used by a client to send requests to
           and receive replies from a service. This socket type allows only an
           alternating sequence of zmq_send(request) and subsequent
           zmq_recv(reply) calls. Each request sent is round-robined among all
           services, and each reply received is matched with the last issued
           request.

           When a ZMQ_REQ socket enters an exceptional state due to having
           reached the high water mark for all services, or if there are no
           services at all, then any zmq_send(3) operations on the socket
           shall block until the exceptional state ends or at least one
           service becomes available for sending; messages are not discarded.

           Table 1. Summary of ZMQ_REQ characteristics
           Compatible peer sockets     ZMQ_REP

           Direction                   Bidirectional

           Send/receive pattern        Send, Receive, Send,
                                       Receive, ...

           Outgoing routing strategy   Round-robin

           Incoming routing strategy   Last peer

           ZMQ_HWM option action       Block

       ZMQ_REP
           A socket of type ZMQ_REP is used by a service to receive requests
           from and send replies to a client. This socket type allows only an
           alternating sequence of zmq_recv(request) and subsequent
           zmq_send(reply) calls. Each request received is fair-queued from
           among all clients, and each reply sent is routed to the client that
           issued the last request. If the original requester doesn't exist
           any more the reply is silently discarded.

           When a ZMQ_REP socket enters an exceptional state due to having
           reached the high water mark for a client, then any replies sent to
           the client in question shall be dropped until the exceptional state
           ends.

           Table 2. Summary of ZMQ_REP characteristics
           Compatible peer sockets     ZMQ_REQ

           Direction                   Bidirectional

           Send/receive pattern        Receive, Send, Receive,
                                       Send, ...

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   Last peer

           ZMQ_HWM option action       Drop

       ZMQ_DEALER
           A socket of type ZMQ_DEALER is an advanced pattern used for
           extending request/reply sockets. Each message sent is round-robined
           among all connected peers, and each message received is fair-queued
           from all connected peers.

           Previously this socket was called ZMQ_XREQ and that name remains
           available for backwards compatibility.

           When a ZMQ_DEALER socket enters an exceptional state due to having
           reached the high water mark for all peers, or if there are no peers
           at all, then any zmq_send(3) operations on the socket shall block
           until the exceptional state ends or at least one peer becomes
           available for sending; messages are not discarded.

           When a ZMQ_DEALER socket is connected to a ZMQ_REP socket each
           message sent must consist of an empty message part, the delimiter,
           followed by one or more body parts.

           Table 3. Summary of ZMQ_DEALER characteristics
           Compatible peer sockets     ZMQ_ROUTER, ZMQ_REQ,
                                       ZMQ_REP

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Outgoing routing strategy   Round-robin

           Incoming routing strategy   Fair-queued

           ZMQ_HWM option action       Block

       ZMQ_ROUTER
           A socket of type ZMQ_ROUTER is an advanced pattern used for
           extending request/reply sockets. When receiving messages a
           ZMQ_ROUTER socket shall prepend a message part containing the
           identity of the originating peer to the message before passing it
           to the application. Messages received are fair-queued from among
           all connected peers. When sending messages a ZMQ_ROUTER socket
           shall remove the first part of the message and use it to determine
           the identity of the peer the message shall be routed to. If the
           peer does not exist anymore the message shall be silently
           discarded.

           Previously this socket was called ZMQ_XREP and that name remains
           available for backwards compatibility.

           When a ZMQ_ROUTER socket enters an exceptional state due to having
           reached the high water mark for all peers, or if there are no peers
           at all, then any messages sent to the socket shall be dropped until
           the exceptional state ends. Likewise, any messages routed to a
           non-existent peer or a peer for which the individual high water
           mark has been reached shall also be dropped.

           When a ZMQ_REQ socket is connected to a ZMQ_ROUTER socket, in
           addition to the identity of the originating peer each message
           received shall contain an empty delimiter message part. Hence, the
           entire structure of each received message as seen by the
           application becomes: one or more identity parts, delimiter part,
           one or more body parts. When sending replies to a ZMQ_REQ socket
           the application must include the delimiter part.

           Table 4. Summary of ZMQ_ROUTER characteristics
           Compatible peer sockets     ZMQ_DEALER, ZMQ_REQ,
                                       ZMQ_REP

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Outgoing routing strategy   See text

           Incoming routing strategy   Fair-queued

           ZMQ_HWM option action       Drop

   Publish-subscribe pattern
       The publish-subscribe pattern is used for one-to-many distribution of
       data from a single publisher to multiple subscribers in a fan out
       fashion.

       ZMQ_PUB
           A socket of type ZMQ_PUB is used by a publisher to distribute data.
           Messages sent are distributed in a fan out fashion to all connected
           peers. The zmq_recv(3) function is not implemented for this socket
           type.

           When a ZMQ_PUB socket enters an exceptional state due to having
           reached the high water mark for a subscriber, then any messages
           that would be sent to the subscriber in question shall instead be
           dropped until the exceptional state ends. The zmq_send() function
           shall never block for this socket type.

           Table 5. Summary of ZMQ_PUB characteristics
           Compatible peer sockets     ZMQ_SUB

           Direction                   Unidirectional

           Send/receive pattern        Send only

           Incoming routing strategy   N/A

           Outgoing routing strategy   Fan out

           ZMQ_HWM option action       Drop

       ZMQ_SUB
           A socket of type ZMQ_SUB is used by a subscriber to subscribe to
           data distributed by a publisher. Initially a ZMQ_SUB socket is not
           subscribed to any messages, use the ZMQ_SUBSCRIBE option of
           zmq_setsockopt(3) to specify which messages to subscribe to. The
           zmq_send() function is not implemented for this socket type.

           Table 6. Summary of ZMQ_SUB characteristics
           Compatible peer sockets     ZMQ_PUB

           Direction                   Unidirectional

           Send/receive pattern        Receive only

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   N/A

           ZMQ_HWM option action       Drop

   Pipeline pattern
       The pipeline pattern is used for distributing data to nodes arranged in
       a pipeline. Data always flows down the pipeline, and each stage of the
       pipeline is connected to at least one node. When a pipeline stage is
       connected to multiple nodes data is round-robined among all connected
       nodes.

       ZMQ_PUSH
           A socket of type ZMQ_PUSH is used by a pipeline node to send
           messages to downstream pipeline nodes. Messages are round-robined
           to all connected downstream nodes. The zmq_recv() function is not
           implemented for this socket type.

           When a ZMQ_PUSH socket enters an exceptional state due to having
           reached the high water mark for all downstream nodes, or if there
           are no downstream nodes at all, then any zmq_send(3) operations on
           the socket shall block until the exceptional state ends or at least
           one downstream node becomes available for sending; messages are not
           discarded.

           Deprecated alias: ZMQ_DOWNSTREAM.

           Table 7. Summary of ZMQ_PUSH characteristics
           Compatible peer sockets     ZMQ_PULL

           Direction                   Unidirectional

           Send/receive pattern        Send only

           Incoming routing strategy   N/A

           Outgoing routing strategy   Round-robin

           ZMQ_HWM option action       Block

       ZMQ_PULL
           A socket of type ZMQ_PULL is used by a pipeline node to receive
           messages from upstream pipeline nodes. Messages are fair-queued
           from among all connected upstream nodes. The zmq_send() function is
           not implemented for this socket type.

           Deprecated alias: ZMQ_UPSTREAM.

           Table 8. Summary of ZMQ_PULL characteristics
           Compatible peer sockets     ZMQ_PUSH

           Direction                   Unidirectional

           Send/receive pattern        Receive only

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   N/A

           ZMQ_HWM option action       N/A

   Exclusive pair pattern
       The exclusive pair pattern is used to connect a peer to precisely one
       other peer. This pattern is used for inter-thread communication across
       the inproc transport.

       ZMQ_PAIR
           A socket of type ZMQ_PAIR can only be connected to a single peer at
           any one time. No message routing or filtering is performed on
           messages sent over a ZMQ_PAIR socket.

           When a ZMQ_PAIR 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 zmq_send(3) operations on the socket shall
           block until the peer becomes available for sending; messages are
           not discarded.

               Note
               ZMQ_PAIR sockets are designed for inter-thread communication
               across the zmq_inproc(7) transport and do not implement
               functionality such as auto-reconnection. ZMQ_PAIR sockets are
               considered experimental and may have other missing or broken
               aspects.

           Table 9. Summary of ZMQ_PAIR characteristics
           Compatible peer sockets     ZMQ_PAIR

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Incoming routing strategy   N/A

           Outgoing routing strategy   N/A

           ZMQ_HWM option action       Block

RETURN VALUE

       The zmq_socket() function shall return an opaque handle to the newly
       created socket if successful. Otherwise, it shall return NULL and set
       errno to one of the values defined below.

ERRORS

       EINVAL
           The requested socket type is invalid.

       EFAULT
           The provided context is invalid.

       EMFILE
           The limit on the total number of open 0MQ sockets has been reached.

       ETERM
           The context specified was terminated.

SEE ALSO

       zmq_init(3) zmq_setsockopt(3) zmq_bind(3) zmq_connect(3) zmq_send(3)
       zmq_recv(3) zmq_inproc(7) zmq(7)

AUTHORS

       This 0MQ manual page was written by Martin Sustrik
       <sustrik@250bpm.com[1]> and Martin Lucina <mato@kotelna.sk[2]>.

NOTES

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

        2. mato@kotelna.sk
           mailto:mato@kotelna.sk