Hacker NewsThursday · May 21, 2026FREE

Everything in C is undefined behavior

The blog post, titled 'Everything in C is undefined behavior,' explores the pervasive nature of undefined behavior within the C programming language. Published on May 20, 2026, the article, shared on Hacker News, posits that even seemingly straightforward C constructs can fall into categories where the language standard does not dictate specific outcomes. This ambiguity, the author argues, is not merely an academic concern but a practical challenge for software development. It enables compilers to make assumptions that, while valid according to the standard, can lead to code behaving differently across various compilers, optimization levels, or even different versions of the same compiler. This variability complicates debugging efforts significantly, as a bug might only manifest under specific compilation conditions. The author delves into examples where common C idioms, such as certain pointer manipulations, integer overflows, or uninitialized variable access, are technically undefined. This widespread presence of undefined behavior complicates the task of writing robust and secure C applications, as developers must not only understand the explicit rules of C but also anticipate the implicit assumptions compilers might make when encountering undefined scenarios. Furthermore, undefined behavior is a frequent source of security vulnerabilities, as unexpected program states can be exploited. The article serves as a critical examination of C's design philosophy and its real-world implications for reliability, predictability, and security in software engineering, urging developers to be acutely aware of these often-overlooked language nuances.