Undefined Behaviors in C

Wanted to take some notes after watching a video on ub and optimizing c.

What is Undefined Behavior (UB)?

Undefined Behavior (UB) refers to code constructs in C for which the C standard does not prescribe any specific behavior. This means the compiler is free to handle these situations in any manner it sees fit, which can lead to unpredictable program behavior, including program crashes, data corruption, or even seemingly correct but unintended program operation. UB arises from situations like dividing by zero, accessing out-of-bounds array elements, or using uninitialized variables.

Key Points Discussed

1. Compatibility Between C and C++: While efforts are made to keep C and C++ compatible, they are distinct languages with their own standards. The talk primarily focuses on C, noting that many principles also apply to C++ due to shared compilers.

2. Depth of C: C’s simplicity belies its depth. Being an old language, much is known about C, providing a rich field of exploration, especially regarding compiler behavior and performance optimization.

3. Compiler Transformations and the “As-If” Rule: Compilers optimize code under the “as-if” rule, meaning they can transform code as long as the observable behavior remains unchanged. This leads to optimizations like dead code elimination and reordering of instructions.

4. The Importance of volatile Keyword: volatile indicates that a variable’s value may change in ways not explicitly specified by the program, preventing certain optimizations to ensure correct operation, especially in hardware interaction and multithreading contexts.

5. Atomic Operations and Thread Safety: The discussion includes atomicity, and the acquire-release semantic to ensure operations on shared data between threads are performed safely without unintended reordering or data races.

6. Undefined Behavior (UB): UB is a significant focus, explaining how it allows compilers to perform aggressive optimizations by assuming certain conditions are never met (e.g., division by zero or array bounds violations). However, this also means that seemingly harmless code can have unpredictable results if it invokes UB.

7. Survival Tips for C Programmers: The speaker concludes with practical advice for navigating the complexities of C programming. This includes avoiding certain features (e.g., variable-length arrays, bit fields), using tools like sanitizers, understanding memory models, and being cautious with optimizations that may rely on UB.