Starting with C/C++

In this section we will learn how to build a firmware from scratch, using C/C++ API. For this, we'll use command line rather than Web UI.

We will build an app that uses DHT22 temperature/humidity sensor, and allows to read sensor values remotely. Start terminal if you are on Mac or Linux. If you're on Windows, start command prompt, and make a copy of the empty Mongoose OS app, which is a bare-bones app used as an app template. Here and below we assume Mac/Linux system:

Clone template project

git clone my-app
Cloning into 'my-app'...
remote: Counting objects: 68, done.
remote: Total 68 (delta 0), reused 0 (delta 0), pack-reused 68
Unpacking objects: 100% (68/68), done.
cd my-app

See what files we have:

find .

The two important files we'll be dealing with is mos.yml which describes how to build an app and src/main.c which is going to have our C code.

Add necessary libraries

Open mos.yml file in your favorite editor and add support for DHT sensor:

  - origin:
  - origin:
  - origin:
  - origin:
  - origin:  # <-- Add this line!

Note - full list of available libraries is here. Now, open src/main.c, you'll see the following skeleton code which initialises an app that does nothing:

#include "mgos.h"

enum mgos_app_init_result mgos_app_init(void) {

Create custom configuration section

Let's add a code that reads from a DHT temperature sensor every second. The pin to which a sensor is attached, we make configurable by editing a config_schema: section in the mos.yml, to have it like this:

 - ["app", "o", {title: "My app custom settings"}]
 - ["", "i", 5, {title: "GPIO pin a sensor is attached to"}]

This custom configuration section will allow us to change sensor pin at run time, without recompiling firmware. That could be done programmatically or via the mos tool, e.g. mos config-set

Add code that uses sensor library

Then, edit src/main.c, add a timer (see timer api docs) that reads DHT and logs the value (error handling is intentionally omitted):

#include "mgos.h"
#include "mgos_dht.h"

static void timer_cb(void *dht) {
  LOG(LL_INFO, ("Temperature: %lf", mgos_dht_get_temp(dht)));

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_dht *dht = mgos_dht_create(mgos_sys_config_get_app_pin(), DHT22);
  mgos_set_timer(1000, true, timer_cb, dht);

Build the app and test sensor reading

Connect DHT sensor to pin 5. The sensor itself has following pins:

This is an example with ESP8266 NodeMCU. Red connector is VCC 3.3 volts, black connector is ground GND, and yellow is data, connected to pin 5:

Build, flash the firmware, and attach the console to see device logs. Assume we're working with ESP8266:

mos build --arch esp8266
Connecting to, user test
Uploading sources (1506 bytes)
Firmware saved to build/
mos flash
Loaded my-app/esp8266 version 1.0 (20170809-062613/???)
Using port /dev/cu.SLAB_USBtoUART
Opening /dev/cu.SLAB_USBtoUART @ 115200...
Connecting to ESP8266 ROM, attempt 1 of 10...
Running flasher @ 1500000...
  Flasher is running
Flash size: 4194304, params: 0x0240 (dio,32m,40m)
     2656 @ 0x0 -> 0
     4096 @ 0x3fb000
      128 @ 0x3fc000
Booting firmware...
All done!
mos console
[Aug  9 08:35:52.871] mgos_init            Init done, RAM: 53184 total, 46248 free, 45696 min free
[Aug  9 08:35:52.880] mongoose_poll        New heap free LWM: 45440
[Aug  9 08:35:53.889] timer_cb             Temperature: 18.6

Console log shows sensor reading appearing in 1-second intervals. Press Ctrl-C to quit the console.

Add RPC service to read sensor data remotely

Now let's use the cornerstone of Mongoose OS remote management capabilities. We can make any hardware function be remotely accessible. This is done by creating an RPC service. Read more about it in the Overview and Core libraries sections, and here we jump straight to it. Looking at MG-RPC API doc, add RPC service Temp.Read:

#include "mgos.h"
#include "mgos_dht.h"
#include "mgos_rpc.h"

static void timer_cb(void *dht) {
  LOG(LL_INFO, ("Temperature: %lf", mgos_dht_get_temp(dht)));

static void rpc_cb(struct mg_rpc_request_info *ri, void *cb_arg,
                   struct mg_rpc_frame_info *fi, struct mg_str args) {
  mg_rpc_send_responsef(ri, "{value: %lf}", mgos_dht_get_temp(cb_arg));
  (void) fi;
  (void) args;

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_dht *dht = mgos_dht_create(mgos_sys_config_get_app_pin(), DHT22);
  mgos_set_timer(1000, true, timer_cb, dht);
  mg_rpc_add_handler(mgos_rpc_get_global(), "Temp.Read", "", rpc_cb, dht);

Build and test the final app

Build and flash the app:

mos build --arch esp8266 && mos flash

And now, call the device's RPC service:

mos call Temp.Read
{"value": 18.6}

This call could be performed over the serial connection as well as over network connection - see RPC section to learn more.

To summarise, this section demonstrates how to use C/C++ API, libraries, custom configuration, and MG-RPC remote management mechanism.

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