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deepmind.google·6d ago
AI Engineering
Hacker News·May 20·8 min read
Everything in C is undefined behavior
TL;DR
C's specification leaves many operations intentionally undefined—meaning the compiler can optimize them away or produce unpredictable results—creating silent bugs that are hard to detect. The article examines how undefined behavior (UB) permeates C's design, from integer overflow to uninitialized variables to pointer arithmetic, showing concrete examples where standard-compliant code produces unexpected output.
✦ Why It Matters
Engineers must recognize undefined behavior in C to avoid silent correctness failures that testing may not catch.
Key Takeaways
- Understand that undefined behavior can occur even in simple operations.
- Review pointer alignments to avoid crashes on different architectures.
- Be cautious with type casting to prevent unexpected results.
- Regularly audit code for common sources of undefined behavior.
Full Summary
Undefined behavior (UB) in C and C++ occurs when the code does not adhere to the language's specifications, leading to unpredictable outcomes. Common examples include accessing uninitialized memory or dereferencing misaligned pointers.
The article argues that UB is not just a compiler optimization issue; it fundamentally means that the compiler assumes the code is valid, which can lead to serious errors. The author highlights that even seemingly innocuous operations, like casting types or using standard library functions, can introduce UB.
For instance, casting a pointer incorrectly or using a signed character can lead to crashes or unexpected behavior. This widespread issue suggests that all nontrivial C and C++ code is likely to contain UB, raising concerns about the reliability of software written in these languages.