Manpages - zmq.7

The 0MQ context keeps the list of sockets and manages the async I/O thread and internal queries.

Before using any 0MQ library functions you must create a 0MQ context. When you exit your application you must destroy the context. These functions let you work with contexts:

Create a new 0MQ context

*zmq_ctx_new*(3)

Work with context properties

*zmq_ctx_set*(3) *zmq_ctx_get*(3)

Destroy a 0MQ context

*zmq_ctx_shutdown*(3) *zmq_ctx_term*(3)

Thread safety

A 0MQ context is thread safe and may be shared among as many application threads as necessary, without any additional locking required on the part of the caller.

Individual 0MQ sockets are not thread safe except in the case where full memory barriers are issued when migrating a socket from one thread to another. In practice this means applications can create a socket in one thread with zmq_socket() and then pass it to a newly created thread as part of thread initialisation, for example via a structure passed as an argument to pthread_create().

Multiple contexts

Multiple contexts may coexist within a single application. Thus, an application can use 0MQ directly and at the same time make use of any number of additional libraries or components which themselves make use of 0MQ as long as the above guidelines regarding thread safety are adhered to.

A 0MQ message is a discrete unit of data passed between applications or components of the same application. 0MQ messages have no internal structure and from the point of view of 0MQ itself they are considered to be opaque binary data.

The following functions are provided to work with messages:

Initialise a message

*zmq_msg_init*(3) *zmq_msg_init_size*(3) *zmq_msg_init_buffer*(3) *zmq_msg_init_data*(3)

Sending and receiving a message

*zmq_msg_send*(3) *zmq_msg_recv*(3)

Release a message

*zmq_msg_close*(3)

Access message content

*zmq_msg_data*(3) *zmq_msg_size*(3) *zmq_msg_more*(3)

Work with message properties

*zmq_msg_gets*(3) *zmq_msg_get*(3) *zmq_msg_set*(3)

Message manipulation

*zmq_msg_copy*(3) *zmq_msg_move*(3)

0MQ sockets present an abstraction of an asynchronous message queue, with the exact queueing semantics depending on the socket type in use. See *zmq_socket*(3) for the socket types provided.

The following functions are provided to work with sockets:

Creating a socket

*zmq_socket*(3)

Closing a socket

*zmq_close*(3)

Manipulating socket options

*zmq_getsockopt*(3) *zmq_setsockopt*(3)

Establishing a message flow

*zmq_bind*(3) *zmq_connect*(3)

Sending and receiving messages

*zmq_msg_send*(3) *zmq_msg_recv*(3) *zmq_send*(3) *zmq_recv*(3) *zmq_send_const*(3)

Monitoring socket events

*zmq_socket_monitor*(3)

Input/output multiplexing. 0MQ provides a mechanism for applications to multiplex input/output events over a set containing both 0MQ sockets and standard sockets. This mechanism mirrors the standard poll() system call, and is described in detail in *zmq_poll*(3). This API is deprecated, however.

There is a new DRAFT API with multiple zmq_poller_* function, which is described in *zmq_poller*(3).

A 0MQ socket can use multiple different underlying transport mechanisms. Each transport mechanism is suited to a particular purpose and has its own advantages and drawbacks.

The following transport mechanisms are provided:

Unicast transport using TCP

*zmq_tcp*(7)

Reliable multicast transport using PGM

*zmq_pgm*(7)

Local inter-process communication transport

*zmq_ipc*(7)

Local in-process (inter-thread) communication transport

*zmq_inproc*(7)

Virtual Machine Communications Interface (VMC) transport

*zmq_vmci*(7)

Unreliable unicast and multicast using UDP

*zmq_udp*(7)

0MQ provides proxies to create fanout and fan-in topologies. A proxy connects a frontend socket to a backend socket and switches all messages between the two sockets, opaquely. A proxy may optionally capture all traffic to a third socket. To start a proxy in an application thread, use *zmq_proxy*(3).

A 0MQ socket can select a security mechanism. Both peers must use the same security mechanism.

The following security mechanisms are provided for IPC and TCP connections:

Null security

*zmq_null*(7)

Plain-text authentication using username and password

*zmq_plain*(7)

Elliptic curve authentication and encryption

*zmq_curve*(7)

Generate a CURVE keypair in armored text format

*zmq_curve_keypair*(3)

Derive a CURVE public key from a secret key: *zmq_curve_public*(3)

Converting keys to/from armoured text strings

*zmq_z85_decode*(3) *zmq_z85_encode*(3)