ØMQ Manual - ØMQ/4.2.2


zmq - ØMQ lightweight messaging kernel


#include <zmq.h>

cc [flags] files -lzmq [libraries]


The ØMQ lightweight messaging kernel is a library which extends the standard socket interfaces with features traditionally provided by specialised messaging middleware products. ØMQ 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 ØMQ concepts, describes how ØMQ abstracts standard sockets and provides a reference manual for the functions provided by the ØMQ library.


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

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

Create a new ØMQ context
Work with context properties
zmq_ctx_set(3) zmq_ctx_get(3)
Destroy a ØMQ context
zmq_ctx_shutdown(3) zmq_ctx_term(3)

Thread safety

A ØMQ 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 ØMQ 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 ØMQ directly and at the same time make use of any number of additional libraries or components which themselves make use of ØMQ as long as the above guidelines regarding thread safety are adhered to.


A ØMQ message is a discrete unit of data passed between applications or components of the same application. ØMQ messages have no internal structure and from the point of view of ØMQ 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_data(3)
Sending and receiving a message
zmq_msg_send(3) zmq_msg_recv(3)
Release a message
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)


ØMQ sockets present an abstraction of a 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
Closing a socket
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

Input/output multiplexing

ØMQ provides a mechanism for applications to multiplex input/output events over a set containing both ØMQ sockets and standard sockets. This mechanism mirrors the standard poll() system call, and is described in detail in zmq_poll(3).


A ØMQ 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
Reliable multicast transport using PGM
Local inter-process communication transport
Local in-process (inter-thread) communication transport
Virtual Machine Communications Interface (VMC) transport
Unreliable unicast and multicast using UDP


ØMQ 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 ØMQ 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
Plain-text authentication using username and password
Elliptic curve authentication and encryption
Generate a CURVE keypair in armored text format

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 ØMQ library functions handle errors using the standard conventions found on POSIX systems. Generally, this means that upon failure a ØMQ 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 ØMQ-specific error codes into error message strings; for details refer to zmq_strerror(3).


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)


The following miscellaneous functions are provided:

Report ØMQ library version

Language bindings

The ØMQ 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 ØMQ 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 ØMQ community and pointers can be found on the ØMQ website.


This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at


Main web site:

Report bugs to the ØMQ development mailing list: <gro.qmorez.stsil|ved-qmorez#gro.qmorez.stsil|ved-qmorez>


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 ØMQ distribution.