How to Make Embedded C Code Reliable?

C is a powerful language that can be used to write high-performance embedded systems, but writing reliable C code can be challenging due to its low-level nature and potential for bugs and memory-related errors. In this blog post, we’ll cover some best practices for writing reliable C code, with examples to illustrate each point.

1. Write clean and well-structured code

Clean and well-structured code is the foundation for reliable code. It makes it easier for you and other developers to understand what your code does, and it reduces the risk of introducing bugs.

Here’s an example of clean and well-structured C code:

This code defines an array to hold the states of a set of LEDs and three functions:

• init_leds to initialize the LED states to off,

• toggle_led to invert the state of a specific LED, and

• print_led_states to print the current state of each LED to the console.

The main function calls these three functions in sequence to initialize the LED states, toggle some of the LED states, and print the current LED states.

The code is clean and well-structured, with clear function and variable names, and comments explaining what each part of the code does. The use of constants and a static array to hold the LED states also helps to keep the code organized and easy to read.

This code can be tested on a PC by compiling and running it as a standalone program. It toggles the states of some of the LEDs, prints the current LED states to the console, and demonstrates how to write reliable C code for an embedded system that can be tested on a PC.

2. Use defensive programming techniques

Defensive programming means writing code with the assumption that things will go wrong and designing your code to handle errors and unexpected input. Use error-checking functions like assert() and handle input validation properly.

Here’s an example of defensive programming in C:

This code defines a buffer to hold a set of items, and two functions:

• add_item to add an item to the buffer, and

• remove_item to remove the last item from the buffer and return its value.

The code uses defensive programming techniques to prevent errors that could occur if the buffer is full or empty.

The add_item function first checks if the buffer is already full before attempting to add an item. If the buffer is full, it prints an error message and returns without modifying the buffer. Similarly, the remove_item function first checks if the buffer is empty before attempting to remove an item. If the buffer is empty, it prints an error message and returns a default value of zero.

This code demonstrates how to use defensive programming techniques to prevent errors in embedded C code. By checking for errors before attempting to modify the buffer, the code can avoid common pitfalls such as buffer overflows or underflows.

3. Use proper memory management

Memory management is a common source of bugs and crashes in C code. Make sure to allocate and deallocate memory correctly, and avoid buffer overflows.

Here’s an example of proper memory management in C:

In this example, we use the malloc() function to allocate memory for an array of integers. We check if the memory allocation was successful and handle the error if it failed. We then fill the array with values and print them out. Finally, we use the free() function to deallocate the memory.

4. Use a version control system

Version control is a must-have tool for any software development project, including C code. Use a version control system like Git to keep track of changes to your code and collaborate with other developers.

Here’s an example of using Git for version control:

Let's say we're working on firmware for an embedded system with a team of developers. We want to use Git to manage changes to the code and ensure everyone is working from the latest version.

First, we create a Git repository for our project and clone it to our local machine. Then we create a new branch called feature-1 for our work.

We write some new code to implement the feature, commit our changes to the feature-1 branch, and continue to make changes and commit them as we work.

Meanwhile, other developers might be working on other features or bug fixes in their own branches. We use Git's merge command to bring these changes into our feature-1 branch and ensure our code is up-to-date.

Once our feature is complete, we merge it back into the master branch and push it to the Git repository for the rest of the team to access.

Using Git for an embedded C project can help prevent conflicts, ensure all developers are working from the latest version of the code, and make it easy to roll back to a previous version if needed.

5. Use asserts in software development

Asserts are a powerful tool for detecting errors and bugs in software code. They are particularly useful in embedded systems where issues can be difficult to diagnose due to the limited resources and real-time constraints of the system.

One common use case for assert statements is to validate input arguments and ensure that they meet the expected requirements. For example, if a function expects a positive integer value as an argument, an assert statement can be used to check that the value is indeed positive before proceeding with the function's execution. If the value is not positive, the assert will fail and report an error message, indicating that the function was called with invalid input.

Another use case for assert statements is to verify the correctness of calculations and program flow. For instance, if a program is expected to terminate under specific conditions, an assert statement can be used to ensure that these conditions are being met correctly. If the conditions are not being met, the assert will fail, and an error message will be generated, indicating that the program is not behaving as expected.

Here’s an example of using unit tests in C:

In this example, the factorial function calculates the factorial of a given integer. The assert statement at the beginning of the function checks that the input is non-negative. If the input is negative, the assert will fail and trigger an error message. The assert statement at the end of the main function checks that the result of the factorial function is correct. If the result is incorrect, the assert will fail and trigger an error message.

Overall, assert statements are a powerful tool for catching bugs and ensuring the correctness of your code. They should be used judiciously, however, as they can slow down the execution of the program and consume additional memory. It is also important to remember that assert statements are typically disabled in release builds of the software, so they should not be relied upon as the sole means of error checking and reporting.

6. Follow a coding standard like MISRA-C

MISRA-C is a widely-used coding standard for the C programming language, specifically designed for safety-critical and embedded systems. Following a coding standard like MISRA-C can help ensure the reliability, safety, and security of your C code.

Some key guidelines from MISRA-C include:

• Limiting the use of language features that can lead to undefined behavior or implementation-defined behavior

• Avoiding the use of global variables

• Using typedefs to improve code readability and maintainability

• Using explicit type casting to avoid implicit conversions

• Ensuring that all functions have a return type

• Using preprocessor macros carefully and avoiding complex macros

By following a coding standard like MISRA-C, you can improve the reliability and safety of your C code, reduce the risk of errors and vulnerabilities, and make it easier to maintain and update your code over time. For more details check this blog post.

Conclusion

In conclusion, writing reliable C code can be challenging due to its low-level nature and potential for bugs and memory-related errors. However, by following best practices like writing clean and well-structured code, using defensive programming techniques, proper memory management, version control systems, and assert statements, developers can minimize the risk of introducing errors and improve the reliability of their C code. By using these best practices, developers can write robust and secure C code, making it easier to test and maintain software applications. Developing reliable C code is essential in embedded systems, and implementing these best practices can make embedded systems more reliable and safe.