ZMQ(7) 0MQ Manual ZMQ(7)
NAME
zmq - 0MQ lightweight messaging kernel
SYNOPSIS
#include <zmq.h>
cc [flags] files -lzmq [libraries]
DESCRIPTION
The 0MQ lightweight messaging kernel is a library which extends the
standard socket interfaces with features traditionally provided by
specialised messaging middleware products. 0MQ sockets provide an
abstraction of asynchronous message queues, multiple messaging
patterns, message filtering (subscriptions), seamless access to
multiple transport protocols and more.
This documentation presents an overview of 0MQ concepts, describes how
0MQ abstracts standard sockets and provides a reference manual for the
functions provided by the 0MQ library.
Context
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.
Messages
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)
Sockets
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).
Transports
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)
Proxies
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).
Security
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)
ERROR HANDLING
The 0MQ library functions handle errors using the standard conventions
found on POSIX systems. Generally, this means that upon failure a 0MQ
library function shall return either a NULL value (if returning a
pointer) or a negative value (if returning an integer), and the actual
error code shall be stored in the errno variable.
On non-POSIX systems some users may experience issues with retrieving
the correct value of the errno variable. The zmq_errno() function is
provided to assist in these cases; for details refer to zmq_errno(3).
The zmq_strerror() function is provided to translate 0MQ-specific error
codes into error message strings; for details refer to zmq_strerror(3).
UTILITY
The following utility functions are provided:
Working with atomic counters
zmq_atomic_counter_new(3) zmq_atomic_counter_set(3)
zmq_atomic_counter_inc(3) zmq_atomic_counter_dec(3)
zmq_atomic_counter_value(3) zmq_atomic_counter_destroy(3)
MISCELLANEOUS
The following miscellaneous functions are provided:
Report 0MQ library version
zmq_version(3)
LANGUAGE BINDINGS
The 0MQ library provides interfaces suitable for calling from programs
in any language; this documentation documents those interfaces as they
would be used by C programmers. The intent is that programmers using
0MQ from other languages shall refer to this documentation alongside
any documentation provided by the vendor of their language binding.
Language bindings (C++, Python, PHP, Ruby, Java and more) are provided
by members of the 0MQ community and pointers can be found on the 0MQ
website.
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.
RESOURCES
Main web site: http://www.zeromq.org/
Report bugs to the 0MQ development mailing list:
<zeromq-dev@lists.zeromq.org[1]>
COPYING
Free use of this software is granted under the terms of the GNU Lesser
General Public License (LGPL). For details see the files COPYING and
COPYING.LESSER included with the 0MQ distribution.
NOTES
1. zeromq-dev@lists.zeromq.org
mailto:zeromq-dev@lists.zeromq.org
0MQ 4.3.4 01/18/2023 ZMQ(7)
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