meshtastic/docs/development/device/module-api.mdx

---
id: module-api
title: Module API
sidebar_label: Module API
sidebar_position: 3
---

This is a tutorial on how to write small modules which run on the device. Modules are bits of regular 'Arduino' code that can send and receive packets to other Nodes/Apps/PCs using the Meshtastic mesh.

## Key concepts

All modules should be sub-classes of MeshModule. By inheriting from this class and creating an instance of your new module - your module will be automatically registered to receive packets.

Messages are sent to particular port numbers (similar to UDP networking). Your new module should eventually pick its own port number (see below). For development use, you can simply use `PRIVATE_APP` (which is the default).

Packets can be sent and received as either:

1. Raw binary structures
2. [Protobufs](https://developers.google.com/protocol-buffers).

## Class hierarchy

You will typically want to inherit from either SinglePortModule (if you are just sending/receiving raw bytes) or ProtobufModule (if you are sending/receiving protobufs). You'll implement your own handleReceived/handleReceivedProtobuf - probably based on the example code.

The relevant bits of the class hierarchy are as follows:

### First Level: MeshModule

- [src/mesh/MeshModule.h](http://github.com/meshtastic/firmware/tree/master/src/mesh/MeshModule.h) - you probably don't want to use this base-class directly.
  
### Second Level: SinglePortModule

- [src/mesh/SinglePortModule.h](http://github.com/meshtastic/firmware/tree/master/src/mesh/SinglePortModule.h) - for modules that send/receive from a single port number (the normal case).

### Third Level: ProtobufModule

- [src/mesh/ProtobufModule.h](http://github.com/meshtastic/firmware/tree/master/src/mesh/ProtobufModule.h) - for modules that send/receive a single particular Protobuf type. Inherit from this if you are using protocol buffers in your module.

## Startup Operations

If your module needs to perform any operations at startup you can override and implement the `setup()` method to run your code.

If you need to send a packet you can call `service.sendToMesh` with code like this (from the examples):

```cpp
MeshPacket *p = allocReply();
p->to = dest;

service.sendToMesh(p);
```

## Example Modules

A number of [key services](http://github.com/meshtastic/firmware/tree/master/src/modules) are implemented using the Module API, These modules are as follows:

- [TextMessageModule](http://github.com/meshtastic/firmware/tree/master/src/modules/TextMessageModule.h) - Receives text messages and displays them on the LCD screen/stores them in the local DB.

- [NodeInfoModule](http://github.com/meshtastic/firmware/tree/master/src/modules/NodeInfoModule.h) - Receives/sends User information to other nodes so that usernames are available in the databases.

- [RemoteHardwareModule](http://github.com/meshtastic/firmware/tree/master/src/modules/RemoteHardwareModule.h) - A module that provides easy remote access to device hardware (for things like turning GPIOs on or off). Intended to be a more extensive example and provide a useful feature of its own. See [remote-hardware](/docs/hardware/peripheral/#remote-hardware) for details.

- [ReplyModule](http://github.com/meshtastic/firmware/tree/master/src/modules/ReplyModule.h) - A simple module that just replies to any packet it receives (provides a 'ping' service).

## Getting started

The easiest way to get started is:

1. [Build the firmware](/docs/development/firmware/build) codebase.
2. Copy the [ReplyModule](http://github.com/meshtastic/firmware/tree/master/src/modules/ReplyModule.cpp) as a template into `src/modules/`. 
  ```sh
  cp src/modules/ReplyModule.* src/modules/YourModule.*
  ```
3. Change the port number from `PortNum_REPLY_APP` to `PortNum_PRIVATE_APP`.
4. Edit the `setupModules()` function located at `modules/Moduless.cpp` to add a call to create an instance of your module (see comment at head of that function).
5. Rebuild with your new module and install on the device.
6. Use the [Meshtastic Python CLI tool](https://github.com/meshtastic/Meshtastic-python) to send a packet to your board, for example:
   - `meshtastic --dest 1234 --sendping` where _1234_ is another mesh node to send the ping to.

## Threading

It is very common that you would like your module to be invoked periodically.
We use a crude/basic cooperative threading system to allow this on any of our supported platforms. Simply inherit from OSThread and implement `runOnce()`. See the OSThread [documentation](http://github.com/meshtastic/firmware/tree/master/src/concurrency/OSThread.h) for more details.

## Sending messages

If you would like to proactively send messages (rather than just responding to them), just call `service.sendToMesh()`. For an example of this, see [NodeInfoModule::sendOurNodeInfo(...)](http://github.com/meshtastic/firmware/tree/master/src/modules/NodeInfoModule.cpp).

## Picking a port number

See [Meshtastic Port Numbers](/docs/development/firmware/portnum)

## How to add custom protocol buffers

If you would like to use protocol buffers to define the structures you send over the mesh (recommended), here's how to do that.

1. Create a new `.proto` file in the protos directory.
2. Run `./bin/regen-protos.sh` to regenerate the C code for accessing the protocol buffers. If you don't have the required nanopb tool, follow the instructions printed by the script to get it.
3. Done! You can now use your new protobuf just like any of the existing protobufs in Meshtastic.