A South Plugin Example In C/C++: the DHT11 Sensor

Using the same example as before, the DHT11 temperature and humidity sensor, let’s look at how to create the plugin in C/C++.

The Software

For this plugin we use the wiringpi C library to connect to the hardware of the Raspberry Pi

$ sudo apt-get install wiringpi
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
wiringpi
...
$

The Plugin

This is the code for the plugin.cpp file that provides the plugin API:

/*
 * Fledge south plugin.
 *
 * Copyright (c) 2018 OSisoft, LLC
 *
 * Released under the Apache 2.0 Licence
 *
 * Author: Amandeep Singh Arora
 */
#include <dht11.h>
#include <plugin_api.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <string>
#include <logger.h>
#include <plugin_exception.h>
#include <config_category.h>
#include <rapidjson/document.h>
#include <version.h>

using namespace std;
#define PLUGIN_NAME "dht11_V2"

/**
 * Default configuration
 */
const static char *default_config = QUOTE({
                "plugin" : {
                        "description" : "DHT11 C south plugin",
                        "type" : "string",
                        "default" : PLUGIN_NAME,
                        "readonly": "true"
                        },
                "asset" : {
                        "description" : "Asset name",
                        "type" : "string",
                        "default" : "dht11",
                        "order": "1",
                        "displayName": "Asset Name",
                        "mandatory" : "true"
                        },
                "pin" : {
                        "description" : "Rpi pin to which DHT11 is attached",
                        "type" : "integer",
                        "default" : "7",
                        "displayName": "Rpi Pin"
                        }
                });


/**
 * The DHT11 plugin interface
 */
extern "C" {

/**
 * The plugin information structure
 */
static PLUGIN_INFORMATION info = {
        PLUGIN_NAME,              // Name
        VERSION,                  // Version
        0,                        // Flags
        PLUGIN_TYPE_SOUTH,        // Type
        "1.0.0",                  // Interface version
        default_config            // Default configuration
};

/**
 * Return the information about this plugin
 */
PLUGIN_INFORMATION *plugin_info()
{
        return &info;
}

/**
 * Initialise the plugin, called to get the plugin handle
 */
PLUGIN_HANDLE plugin_init(ConfigCategory *config)
{
        unsigned int pin;

        if (config->itemExists("pin"))
        {
                pin = stoul(config->getValue("pin"), nullptr, 0);
        }

        DHT11 *dht11= new DHT11(pin);

        if (config->itemExists("asset"))
                dht11->setAssetName(config->getValue("asset"));
        else
                dht11->setAssetName("dht11");

        Logger::getLogger()->info("m_assetName set to %s", dht11->getAssetName());

        return (PLUGIN_HANDLE)dht11;
}

/**
 * Poll for a plugin reading
 */
Reading plugin_poll(PLUGIN_HANDLE *handle)
{
        DHT11 *dht11 = (DHT11*)handle;
        return dht11->takeReading();
}

/**
 * Reconfigure the plugin
 */
void plugin_reconfigure(PLUGIN_HANDLE *handle, string& newConfig)
{
ConfigCategory        conf("dht", newConfig);
DHT11 *dht11 = (DHT11*)*handle;

        if (conf.itemExists("asset"))
                dht11->setAssetName(conf.getValue("asset"));
        if (conf.itemExists("pin"))
        {
                unsigned int pin = stoul(conf.getValue("pin"), nullptr, 0);
                dht11->setPin(pin);
        }
}

/**
 * Shutdown the plugin
 */
void plugin_shutdown(PLUGIN_HANDLE *handle)
{
        DHT11 *dht11 = (DHT11*)handle;
        delete dht11;
}
};

The full source code, including the DHT11 class can be found in GitHub https://github.com/fledge-iot/fledge-south-dht

Building Fledge and Adding the Plugin

If you have not built Fledge yet, follow the steps described here. After the build, you can optionally install Fledge following these steps.

• Clone the fledge-south-dht repository

$ git clone https://github.com/fledge-iot/fledge-south-dht.git
...
$

• Set the environment variable FLEDGE_ROOT to the directory in which you built Fledge

$ export FLEDGE_ROOT=~/fledge
$

• Go to the location in which you cloned the fledge-south-dht repository and create a build directory and run cmake in that directory

$ cd ~/fledge-south-dht
$ mkdir build
$ cd build
$ cmake ..
...
$

• Now make the plugin

$ make
$

• If you have started Fledge from the build directory, copy the plugin into the destination directory

$ mkdir -p $FLEDGE_ROOT/plugins/south/dht
$ cp libdht.so $FLEDGE_ROOT/plugins/south/dht
$

• If you have installed Fledge by executing sudo make install, copy the plugin into the destination directory

$ sudo mkdir -p /usr/local/fledge/plugins/south/dht
$ sudo cp libdht.so /usr/local/fledge/plugins/south/dht
$

Note

If you have installed Fledge using an alternative DESTDIR, remember to add the path to the destination directory to the cp command.

• Add service

$ curl -sX POST http://localhost:8081/fledge/service -d '{"name": "dht", "type": "south", "plugin": "dht", "enabled": true}'

You may now use the C/C++ plugin in exactly the same way as you used a Python plugin earlier.