Provided by: libzmq3-dev_4.1.4-7ubuntu0.1_amd64 
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 connection, send the identity frame followed by a zero-length
message (see EXAMPLE section).
When a connection is made, a zero-length message will be received by the application.
Similarly, when the peer disconnects (or the connection is lost), a zero-length
message will be received by the application.
The ZMQ_SNDMORE flag is ignored on data frames. You must send one identity frame
followed by one data frame.
Also, please note that omitting the ZMQ_SNDMORE 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;
/* Data structure to hold the ZMQ_STREAM received data */
uint8_t raw [256];
size_t raw_size = 256;
while (1) {
/* Get HTTP request; ID frame and then request */
id_size = zmq_recv (socket, id, 256, 0);
assert (id_size > 0);
do {
raw_size = zmq_recv (socket, raw, 256, 0);
assert (raw_size >= 0);
} while (raw_size == 256);
/* 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
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Contribution Policy at http://www.zeromq.org/docs:contributing.