25 years of MISRA C: making safety & security relevant for embedded developers

Embedded software failures, either through safety compromise or security breach, make headlines. At the heart of millions of embedded devices around the world lies the C programming language, running everything from medical products and industrial robots to automotive and smart home components. Through it all, the MISRA C guidelines have helped embedded software developers reduce the likelihood of safety and security risks in their systems.

Created and maintained by the MISRA Working Group, the MISRA C guidelines reduce critical risks in embedded systems through a set of rules and directives that restrict language use to a safe subset – thereby minimising unexpected and undefined application behaviours in areas such as memory allocation, pointer management, and buffer overflows. April 2023 marks the 25^th anniversary of the guidelines and the release of the MISRA C:2012 Amendment 4 (AMD4) and MISRA C:2023 editions.

The evolution of the MISRA C guidelines

To remain relevant alongside the evolution of the C language standard, the MISRA Working Group releases updates to the MISRA C guidelines that align with real-world software processes, practices, and architectures. Table 1 illustrates a timeline of the MISRA C releases over the past 25 years. 

Table 1: History of the MISRA C guidelines

MISRA C:2023 covers multithreading & atomic types

The MISRA Working Group released new guidelines in AMD4 and consolidated all prior editions in the MISRA C:2023 edition. These releases cover C multithreading and atomic types to address the growing reliance on concurrency and sophisticated inter-process techniques within embedded systems to meet the needs of connected devices and feature-rich user experiences.

Reducing risks in multithreading

MISRA C:2023 minimises potential concurrency issues by adding new guidelines to restrict multithreading features to a safe subset. These guidelines, which cover critical elements of thread usage, include:

• Restricting dynamic thread creation to enforce more deterministic approaches to concurrency  
• Ensuring the application creates threads before linking mutexes to them  
• Minimising the risk of process deadlocks and data races
• Managing the safe use of thread objects and thread identifiers 

Restricting use of C atomic types 

C11 introduced atomic types and operations to minimise the risk of inter-process interference between data objects and the likelihood of data races. MISRA C:2023 includes guidelines that address undefined behaviors in atomic types that may compromise the system. These guidelines include:

• Ensuring the application configures atomic types correctly 
• Preventing the unintended removal of atomicity when referencing atomic types through pointers  
• Restricting the use of multiple atomic types in the same statement 

The path forward with MISRA C

MISRA C:2023 includes other guidelines to cover potentially problematic C language features, such as restricting the use of small integer macros and variables that the application declares but never references. 

An embedded development process that includes MISRA C:2023 not only reduces the likelihood of unpredictable code behaviors but also supports the assessment and improvement of software quality over time. Given the complexity and pace of development, many teams automate MISRA compliance using static analysis tools – a technology recommended by the MISRA C guidelines.

As the C language evolves and developers find new ways to maximise its capabilities, the MISRA C guidelines will keep pace. The latest editions of the guidelines represent code-safety and -security best practices that development teams should take advantage of through a well-defined MISRA C process and tool stack.