Provided by: libzmq3-dev_4.0.4+dfsg-2ubuntu0.1_amd64 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 ZMQ_REQ client to one or
       more ZMQ_REP services, and receiving subsequent replies to each request sent.

       The request-reply pattern is formally defined by http://rfc.zeromq.org/spec:28.

       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.

           If no services are available, then any send operation on the socket shall block until
           at least one service becomes available. The REQ socket shall not discard messages.

           Table 1. Summary of ZMQ_REQ characteristics
           Compatible peer sockets     ZMQ_REP, ZMQ_ROUTER

           Direction                   Bidirectional

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

           Outgoing routing strategy   Round-robin

           Incoming routing strategy   Last peer

           Action in mute state        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 does not exist any more the reply
           is silently discarded.

           Table 2. Summary of ZMQ_REP characteristics
           Compatible peer sockets     ZMQ_REQ, ZMQ_DEALER

           Direction                   Bidirectional

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

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   Last peer

       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.

           When a ZMQ_DEALER socket enters the mute 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 mute 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_REP, ZMQ_DEALER

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Outgoing routing strategy   Round-robin

           Incoming routing strategy   Fair-queued

           Action in mute state        Block

       ZMQ_ROUTER
           A socket of type ZMQ_ROUTER is an advanced socket type 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
           by default, unless ZMQ_ROUTER_MANDATORY socket option is set to 1.

           When a ZMQ_ROUTER socket enters the mute state due to having reached the high water
           mark for all peers, then any messages sent to the socket shall be dropped until the
           mute state ends. Likewise, any messages routed to 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_ROUTER

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Outgoing routing strategy   See text

           Incoming routing strategy   Fair-queued

           Action in mute state        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.

       The publish-subscribe pattern is formally defined by http://rfc.zeromq.org/spec:29.

       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 the mute 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 mute 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, ZMQ_XSUB

           Direction                   Unidirectional

           Send/receive pattern        Send only

           Incoming routing strategy   N/A

           Outgoing routing strategy   Fan out

           Action in mute state        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, ZMQ_XPUB

           Direction                   Unidirectional

           Send/receive pattern        Receive only

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   N/A

       ZMQ_XPUB
           Same as ZMQ_PUB except that you can receive subscriptions from the peers in form of
           incoming messages. Subscription message is a byte 1 (for subscriptions) or byte 0 (for
           unsubscriptions) followed by the subscription body. Messages without a sub/unsub
           prefix are also received, but have no effect on subscription status.

           Table 7. Summary of ZMQ_XPUB characteristics
           Compatible peer sockets     ZMQ_SUB, ZMQ_XSUB

           Direction                   Unidirectional

           Send/receive pattern        Send messages, receive
                                       subscriptions

           Incoming routing strategy   N/A

           Outgoing routing strategy   Fan out

           Action in mute state        Drop

       ZMQ_XSUB
           Same as ZMQ_SUB except that you subscribe by sending subscription messages to the
           socket. Subscription message is a byte 1 (for subscriptions) or byte 0 (for
           unsubscriptions) followed by the subscription body. Messages without a sub/unsub
           prefix may also be sent, but have no effect on subscription status.

           Table 8. Summary of ZMQ_XSUB characteristics
           Compatible peer sockets     ZMQ_PUB, ZMQ_XPUB

           Direction                   Unidirectional

           Send/receive pattern        Receive messages, send
                                       subscriptions

           Incoming routing strategy   Fair-queued

           Outgoing routing strategy   N/A

           Action in mute state        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.

       The pipeline pattern is formally defined by http://rfc.zeromq.org/spec:30.

       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 the mute 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 mute state ends or at least
           one downstream node becomes available for sending; messages are not discarded.

           Table 9. 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

           Action in mute state        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.

           Table 10. 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

           Action in mute state        Block

   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.

       The exclusive pair pattern is formally defined by http://rfc.zeromq.org/spec:31.

       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 the mute 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 11. 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

           Action in mute state        Block

   Native Pattern
       The native pattern is used for communicating with TCP peers and allows asynchronous
       requests and replies in either direction.

       ZMQ_STREAM
           A socket of type ZMQ_STREAM is used to send and receive TCP data from a non-0MQ peer,
           when using the tcp:// transport. A ZMQ_STREAM socket can act as client and/or server,
           sending and/or receiving TCP data asynchronously.

           When receiving TCP data, a ZMQ_STREAM 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 TCP data, a ZMQ_STREAM 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, and
           unroutable messages shall cause an EHOSTUNREACH or EAGAIN error.

           To open a connection to a server, use the zmq_connect call, and then fetch the socket
           identity using the ZMQ_IDENTITY zmq_getsockopt call.

           To close a specific client connection, as a server, send the identity frame followed
           by a zero-length message (see EXAMPLE section).

           The ZMQ_MSGMORE flag is ignored on data frames. You must send one identity frame
           followed by one data frame.

           Also, please note that omitting the ZMQ_MSGMORE flag will prevent sending further data
           (from any client) on the same socket.

           Table 12. Summary of ZMQ_STREAM characteristics
           Compatible peer sockets     none.

           Direction                   Bidirectional

           Send/receive pattern        Unrestricted

           Outgoing routing strategy   See text

           Incoming routing strategy   Fair-queued

           Action in mute state        EAGAIN

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.

EXAMPLE

       Creating a simple HTTP server using ZMQ_STREAM.

           void *ctx = zmq_ctx_new ();
           assert (ctx);
           /* Create ZMQ_STREAM socket */
           void *socket = zmq_socket (ctx, ZMQ_STREAM);
           assert (socket);
           int rc = zmq_bind (socket, "tcp://*:8080");
           assert (rc == 0);
           /* Data structure to hold the ZMQ_STREAM ID */
           uint8_t id [256];
           size_t id_size = 256;
           while (1) {
                   /*  Get HTTP request; ID frame and then request */
                   id_size = zmq_recv (server, id, 256, 0);
                   assert (id_size > 0);
                   /* Prepares the response */
                   char http_response [] =
                           "HTTP/1.0 200 OK\r\n"
                           "Content-Type: text/plain\r\n"
                           "\r\n"
                           "Hello, World!";
                   /* Sends the ID frame followed by the response */
                   zmq_send (socket, id, id_size, ZMQ_SNDMORE);
                   zmq_send (socket, http_response, strlen (http_response), ZMQ_SNDMORE);
                   /* Closes the connection by sending the ID frame followed by a zero response */
                   zmq_send (socket, id, id_size, ZMQ_SNDMORE);
                   zmq_send (socket, 0, 0, ZMQ_SNDMORE);
                   /* NOTE: If we don't use ZMQ_SNDMORE, then we won't be able to send more */
                   /* message to any client */
           }
           zmq_close (socket);
           zmq_ctx_destroy (ctx);

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 page was written by the 0MQ community. To make a change please read the 0MQ
       Contribution Policy at http://www.zeromq.org/docs:contributing.