Design a code module

This article demonstrates how to design a new code module.

Header File

A header file:

  • Shall have #pragma once attribute (google it for the reason)
  • Shall NEVER have variables defined
//- Note: Remove all lines from your code that start with //-
//- Put this line as the very first line in your header module
#pragma once

//- #include all header files that THIS header needs
//- Do not include headers here that are not needed
//- For example, we do not need gpio.h file here, but maybe you can move this to switch_led.c
#include "gpio.h"
//- DO NOT put any variables here, like so:
static int do_not_do_this;
int definitely_do_not_do_this;

//- All functions without paramters should be marked as (void)
void switch_led_logic__initialize(void);

void switch_led_logic__run_once(void);

#pragma once is a replacement of

#ifndef YOUR_FILE_NAME__
#define YOUR_FILE_NAME__
void your_api(void);


Intent of #pragma once and #ifndef

  • When other code modules #include your header file, you only want functions to be declared once
  • The name of #ifndef can be anything unique, but must not conflict with other files
  • #include literally copies and pastes the contents of the file in the file wherever you have the #include 


Source File

A source file:

  • Shall have all variables defined as static; this will keep their visibility private to their file
//- Note: Remove all lines from your code that start with //-
//- Include the header file for which this code modules belongs to
#include "switch_led_logic.h"

//- Declare all variables as STATIC
static gpio_s my_led;

//- Define your public functions (part of this module's header file)
void switch_led_logic__initialize(void) {
  my_led = gpio__construct_as_output(GPIO__PORT_2, 0);

void switch_led_logic__run_once(void) {

Unit Test file

A unit-test file:

  • Shall #include the headers that you want (those that should not be "mocked")
  • Shall #include Mock headers to generate stubs (rather than the full implementation)


Useful stuff

Clang auto-formatter will format the source code for you. It will also sort the #includes, so it is recommended to put an empty line such that it sorts the #includes more elegantly. For example, you can separate the FreeRTOS includes, system includes, and other includes.

//- Note: Remove all lines from your code that start with //-

//- Include system includes first
#include <stdio.h>

//- FreeRTOS requires this header file inclusion before any of its soure code
//- This only applies to code included from FreeRTOS
#include "FreeRTOS.h"
#include "semphr.h"
#include "task.h"

//- Clang will sort these
#include "abc.h"
#include "def.h"


Try the following
  • Have two code modules, such as main.c and periodic_callbacks.c include a header file that does not have #pragma once and observe what happens when you compile
Back to top