Provided by: libzmq3-dev_4.3.2-2ubuntu1_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 has both thread safe socket type and not thread safe socket types.
Applications MUST NOT use a not thread safe socket from multiple threads except after
migrating a socket from one thread to another with a "full fence" memory barrier.
Following are the thread safe sockets: * ZMQ_CLIENT * ZMQ_SERVER * ZMQ_DISH * ZMQ_RADIO *
ZMQ_SCATTER * ZMQ_GATHER
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.
Client-server pattern
The client-server pattern is used to allow a single ZMQ_SERVER server talk to one or more
ZMQ_CLIENT clients. The client always starts the conversation, after which either peer can
send messages asynchronously, to the other.
The client-server pattern is formally defined by http://rfc.zeromq.org/spec:41.
Note
Server-client is still in draft phase.
ZMQ_CLIENT
A ZMQ_CLIENT socket talks to a ZMQ_SERVER socket. Either peer can connect, though the
usual and recommended model is to bind the ZMQ_SERVER and connect the ZMQ_CLIENT.
If the ZMQ_CLIENT socket has established a connection, zmq_send(3) will accept
messages, queue them, and send them as rapidly as the network allows. The outgoing
buffer limit is defined by the high water mark for the socket. If the outgoing buffer
is full, or if there is no connected peer, zmq_send(3) will block, by default. The
ZMQ_CLIENT socket will not drop messages.
When a ZMQ_CLIENT socket is connected to multiple ZMQ_SERVER sockets, outgoing
messages are distributed between connected peers on a round-robin basis. Likewise, the
ZMQ_CLIENT socket receives messages fairly from each connected peer. This usage is
sensible only for stateless protocols.
ZMQ_CLIENT sockets are threadsafe and can be used from multiple threads at the same
time. Note that replies from a ZMQ_SERVER socket will go to the first client thread
that calls zmq_msg_recv(3). If you need to get replies back to the originating thread,
use one ZMQ_CLIENT socket per thread.
Note
ZMQ_CLIENT sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on
sends not ZMQ_RCVMORE on receives. This limits them to single part data. The
intention is to extend the API to allow scatter/gather of multi-part data.
Table 1. Summary of ZMQ_CLIENT characteristics
Compatible peer sockets ZMQ_SERVER
Direction Bidirectional
Send/receive pattern Unrestricted
Outgoing routing strategy Round-robin
Incoming routing strategy Fair-queued
Action in mute state Block
ZMQ_SERVER
A ZMQ_SERVER socket talks to a set of ZMQ_CLIENT sockets. A ZMQ_SERVER socket can only
reply to an incoming message: the ZMQ_CLIENT peer must always initiate a conversation.
Each received message has a routing_id that is a 32-bit unsigned integer. The
application can fetch this with zmq_msg_routing_id(3). To send a message to a given
ZMQ_CLIENT peer the application must set the peer’s routing_id on the message, using
zmq_msg_set_routing_id(3).
If the routing_id is not specified, or does not refer to a connected client peer, the
send call will fail with EHOSTUNREACH. If the outgoing buffer for the client peer is
full, the send call shall block, unless ZMQ_DONT_WAIT is used in the send, in which
case it shall fail with EAGAIN. The ZMQ_SERVER socket shall not drop messages in any
case.
Note
ZMQ_SERVER sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on
sends not ZMQ_RCVMORE on receives. This limits them to single part data. The
intention is to extend the API to allow scatter/gather of multi-part data.
Table 2. Summary of ZMQ_SERVER characteristics
Compatible peer sockets ZMQ_CLIENT
Direction Bidirectional
Send/receive pattern Unrestricted
Outgoing routing strategy See text
Incoming routing strategy Fair-queued
Action in mute state Return EAGAIN
Radio-dish pattern
The radio-dish pattern is used for one-to-many distribution of data from a single
publisher to multiple subscribers in a fan out fashion.
Radio-dish is using groups (vs Pub-sub topics), Dish sockets can join a group and each
message sent by Radio sockets belong to a group.
Groups are null terminated strings limited to 16 chars length (including null). The
intention is to increase the length to 40 chars (including null). The encoding of groups
shall be UTF8.
Groups are matched using exact matching (vs prefix matching of PubSub).
Note
Radio-dish is still in draft phase.
ZMQ_RADIO
A socket of type ZMQ_RADIO is used by a publisher to distribute data. Each message
belong to a group, a group is specified with zmq_msg_set_group(3). Messages are
distributed to all members of a group. The zmq_recv(3) function is not implemented for
this socket type.
When a ZMQ_RADIO 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.
Note
ZMQ_RADIO sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on
sends. This limits them to single part data.
Table 3. Summary of ZMQ_RADIO characteristics
Compatible peer sockets ZMQ_DISH
Direction Unidirectional
Send/receive pattern Send only
Incoming routing strategy N/A
Outgoing routing strategy Fan out
Action in mute state Drop
ZMQ_DISH
A socket of type ZMQ_DISH is used by a subscriber to subscribe to groups distributed
by a radio. Initially a ZMQ_DISH socket is not subscribed to any groups, use
zmq_join(3) to join a group. To get the group the message belong to call
zmq_msg_group(3). The zmq_send() function is not implemented for this socket type.
Note
ZMQ_DISH sockets are threadsafe. They do not accept ZMQ_RCVMORE on receives. This
limits them to single part data.
Table 4. Summary of ZMQ_DISH characteristics
Compatible peer sockets ZMQ_RADIO
Direction Unidirectional
Send/receive pattern Receive only
Incoming routing strategy Fair-queued
Outgoing routing strategy N/A
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.
While ZMQ_PAIR sockets can be used over transports other than zmq_inproc(7), their
inability to auto-reconnect coupled with the fact new incoming connections will be
terminated while any previous connections (including ones in a closing state) exist
makes them unsuitable for TCP in most cases.
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.
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 routing id 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 routing id 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
routing id using the zmq_getsockopt call with the ZMQ_ROUTING_ID option.
To close a specific connection, send the routing id 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.
You must send one routing id frame followed by one data frame. The ZMQ_SNDMORE flag is
required for routing id frames but is ignored on data frames.
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
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 13. 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 14. 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 15. 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 routing id 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 _routing id _ of the peer the message shall
be routed to. If the peer does not exist anymore, or has never existed, the message
shall be silently discarded. However, if ZMQ_ROUTER_MANDATORY socket option is set to
1, the socket shall fail with EHOSTUNREACH in both cases.
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. If, ZMQ_ROUTER_MANDATORY is set to
1, the socket shall block or return EAGAIN in both cases.
When a ZMQ_ROUTER socket has ZMQ_ROUTER_MANDATORY flag set to 1, the socket shall
generate ZMQ_POLLIN events upon reception of messages from one or more peers.
Likewise, the socket shall generate ZMQ_POLLOUT events when at least one message can
be sent to one or more peers.
When a ZMQ_REQ socket is connected to a ZMQ_ROUTER socket, in addition to the routing
id 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 routing id parts, delimiter part, one or more body
parts. When sending replies to a ZMQ_REQ socket the application must include the
delimiter part.
Table 16. 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 (see text)
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 routing id */
uint8_t routing_id [256];
size_t routing_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; routing id frame and then request */
routing_id_size = zmq_recv (socket, routing_id, 256, 0);
assert (routing_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 routing id frame followed by the response */
zmq_send (socket, routing_id, routing_id_size, ZMQ_SNDMORE);
zmq_send (socket, http_response, strlen (http_response), 0);
/* Closes the connection by sending the routing id frame followed by a zero response */
zmq_send (socket, routing_id, routing_id_size, ZMQ_SNDMORE);
zmq_send (socket, 0, 0, 0);
}
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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