Reducing undefined behavior in the C language

Undefined behavior in C is being reduced, which means your code will behave more predictably across different compilers and versions.

Martin Uecker, a biomedical engineering professor and MRI software developer, presented at Kernel Recipes 2026 on reducing undefined behavior in the C programming language and its potential for memory safety. Despite C's age, Uecker highlighted its continued strengths: portability, long-term stability, fast compilation, and predictable execution. He detailed how C's early standards, designed for diverse hardware, introduced complexities leading to undefined behavior, giving compiler implementers significant freedom. This freedom, while enabling optimizations and hardware interaction, also allows compilers to produce unexpected results or ignore errors when programs deviate from the standard's defined behavior. Uecker cited examples like division by zero and struct padding issues to illustrate how compilers might exploit undefined behavior, leading to disagreements on language semantics and potential bugs. He noted that recent standards, like C23 and the upcoming C2y, are actively removing undefined behaviors and introducing features to enhance safety and predictability.

The C committee is actively addressing undefined behavior by forming study groups focused on memory safety and removing problematic instances from the standard, with the C2y draft already eliminating 45 such cases. Concurrently, a growing ecosystem of tools, including enhanced compiler warnings, static analyzers integrated into compilers like GCC, and runtime sanitizers, are becoming more effective at detecting and preventing undefined behavior. These advancements aim to improve type safety, spatial memory safety through bounds checking, and temporal memory safety to mitigate use-after-free bugs, signaling a concerted effort to make C a more robust and secure language.

While C will not achieve full memory safety in the immediate future, the ongoing standardization efforts and tool development suggest a path towards greater predictability and fewer security vulnerabilities. The combination of restricted language subsets and formal verification tools is expected to yield the most complete results in the near term. Future C standards are set to introduce features like bit-precise integer types, checked integer operations, and improved array length determination, further solidifying C's role as a continually evolving and improving language. Interested developers are encouraged to participate in the working groups to influence these changes.