C Undefined Behavior Taxonomy & Detection

When to use

• Writing new C code that must be correct across compilers and optimization levels.
• Reviewing C code for UB that a compiler "happens" to accept.
• Diagnosing code that works on -O0 but breaks on -O2, or on GCC but not Clang.
• Interpreting an UBSan report or explaining why a check disappeared from asm.

When not to use

• Implementation-defined or unspecified behavior (those are NOT UB — different rules).
• C++ — use cpp-object-lifecycle / cpp-move-semantics.
• Rust — use rust-unsafe-reasoning.

What the agent often gets wrong