zmq - 0MQ lightweight messaging kernel


#include <zmq.h>

cc [flags] files -lzmq [libraries]


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.


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


Work with context properties

zmq_ctx_set zmq_ctx_get

Destroy a 0MQ context


Monitor a 0MQ context


These deprecated functions let you create and destroy contexts:

Initialise 0MQ context


Terminate 0MQ context


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 initialization, 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 zmq_msg_init_size zmq_msg_init_data

Sending and receiving a message

zmq_msg_send zmq_msg_recv

Release a message


Access message content

zmq_msg_data zmq_msg_size zmq_msg_more

Work with message properties

zmq_msg_get zmq_msg_set

Message manipulation

zmq_msg_copy zmq_msg_move


0MQ sockets present an abstraction of a asynchronous message queue, with the exact queueing semantics depending on the socket type in use. See zmq_socket 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 zmq_setsockopt

Establishing a message flow

zmq_bind zmq_connect

Sending and receiving messages

zmq_msg_send zmq_msg_recv zmq_send zmq_recv

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.


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


Reliable multicast transport using PGM


Local inter-process communication transport


Local in-process (inter-thread) communication transport



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.


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.

The zmq_strerror() function is provided to translate 0MQ-specific error codes into error message strings; for details refer to zmq_strerror.


The following miscellaneous functions are provided:

Report 0MQ library version



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.


This 0MQ manual page was written by Martin Sustrik <>, Martin Lucina <>, and Pieter Hintjens <>.


Main web site:

Report bugs to the 0MQ development mailing list: <>


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.