ZMQ(7)

💖 ZMQ 💖

okay so bestie, welcome to the zmq(7) manual — the 0MQ lightweight messaging kernel page. yes the page is literally named zmq, no notes, it's giving main character energy. sit down, grab a strawberry 🍓, and let's get into it.

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✨ NAME ✨

zmq — the 0MQ lightweight messaging kernel. that's it, that's the name. iconic. 💅

🌸 SYNOPSIS 🌸

#include <zmq.h>

cc [flags] files -lzmq [libraries]

🦋 DESCRIPTION 🦋

so the 0MQ lightweight messaging kernel is a library that takes your standard socket interfaces and extends them with features that used to be the whole personality of specialised messaging middleware products. 0MQ sockets hand you an abstraction of asynchronous message queues, multiple messaging patterns, message filtering (subscriptions), seamless access to multiple transport protocols and more. like... it's giving everything, fr fr. 💜

this documentation is basically the group chat: it 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. and when you exit your application you must destroy the context. no skipping, no cap. 😭

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. slay. 👑

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(). periodt.

Multiple contexts

multiple contexts may coexist within a single application. so 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. sharing is caring but read the rules bestie. 🫶

🍓 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. mysterious. elusive. 💀

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. 🙈 heads up bestie, it's still documented but it is deprecated, so don't build your whole future on it.

there is a new DRAFT API with multiple zmq_poller_* function, which is described in zmq_poller(3). new era unlocked. 🌈

🌈 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. pick your fighter.

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). the way it just... moves messages between sockets. obsessed. 😭

🔐 Security 🔐

a 0MQ socket can select a security mechanism. both peers must use the same security mechanism — it's a matching outfits situation, no solo fits allowed.

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. so check your returns bestie, errno is not gonna announce itself. 💅

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). translating error codes into actual human words, we love to see it. 🫶

👑 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. community-built and thriving, we love a collaborative moment. 🌸

💖 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]>

📜 LICENSE 📜

free use of this software is granted under the terms of the Mozilla Public License Version 2.0 (MPL-2.0). for details see the file LICENSE included with the 0MQ distribution.

🗒️ NOTES 🗒️

1. zeromq-dev@lists.zeromq.org — mailto:zeromq-dev@lists.zeromq.org

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and that's a wrap bestie 🎀 this was ZMQ(7) from the 0MQ 4.3.5 manual, dated 10/09/2023. go forth, make sockets, stay thread safe, and remember: check your return values or the errno will humble you. mwah. 💖✨