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15_3_Receiving_Bitcoind_Notifications_with_C.md
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# 15.3 Receiving Notifications in C with ZMQ Libraries
> **NOTE:** This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
> :information_source: **NOTE:** This section has been recently added to the course and is an early draft that may still be awaiting review. Caveat reader.
[§15.1](15_1_Accessing_Bitcoind_with_C.md) and [§15.2](15_2_Programming_Bitcoind_with_C.md) introduced RPC and JSON libraries for C, and in doing so showed one of the advantages of accessing Bitcoin's RPC commands through a programming language: the ability to reasonably create much more complex programs. This chapter introduces a third library, for [ZMQ](http://zeromq.org/), and in doing so reveals another advantage: the ability to monitor for notifications. It will use that for coding a blockchain listener.
> :book: ***What is ZMQ?*** ZeroMQ (ZMQ)is a high-performance asynchronous messaging library that provides a message queue. ZeroMQ supports common messaging patterns (pub/sub, request/reply, client/server, and others) over a variety of transports (TCP, in-process, inter-process, multicast, WebSocket, and more), making inter-process messaging as simple as inter-thread messaging. You can find more details about ZMQ notifications and others kind of messages in [this repo](https://github.com/Actinium-project/ChainTools/blob/master/docs/chainlistener.md).
## Setting Up ZMQ
## Set Up ZMQ
Before you can create a blockchain listener, you will need to configure `bitcoind` to allow ZMQ notifications, and then you'll need to install a ZMQ library to take advantage of those notifications.
### Configuring `bitcoind` for ZMQ
### Configure `bitcoind` for ZMQ
Bitcoin Core is ZMQ-ready, but you must specify ZMQ endpoints. ZeroMQ publish-sockets prepend each data item with an arbitrary topic
prefix that allows subscriber clients to request only those items with a matching prefix. There are currently four topics supported by `bitcoind`:
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```
Your `bitcoind` will now issue ZMQ notifications
### Installing ZMQ
### Install ZMQ
To take advantage of those notifications, you need a ZMQ library to go with C; we'll thus be using a new ZMQ library instead of the `libbitcoinrpc` library in this section, but when you're experimenting in the future, you'll of course be able to combine them.
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```
You're now ready to code!
## Writing Your Notification Program
## Write Your Notification Program
The following C program is a simple client that subscribes to a ZMQ connection point served by `bitcoind` and reads incoming messages.
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}
```
### Testing the Notification Code
### Test the Notification Code
The source code is in the [src directory](src/15_3_chainlistener.c) as usual. You should compile it:
```