Undefined behavior
Undefined behavior is program execution for which the applicable language or system specification imposes no requirements on the resulting behavior or effects.
100 C++ Mistakes and How to Avoid Them
Undefined behavior is a program operation whose result or behavior is not defined by the applicable language standard.
- How it is used
- Frames undefined behavior as a correctness mistake requiring cautious avoidance in production and legacy C++ code.
- How deep it goes
- Defines undefined behavior and explains several object-lifetime causes, but stops short of a complete language-standard taxonomy.
- Examples
- Uses uninitialized Person members, base-pointer deletion of Square, and returning a local concatenated string.
- What you can do after
- Diagnose/debug with it by recognizing unsafe reads, polymorphic deletion, and dangling returns, then apply initialization, virtual destructors, or value returns.
Read how 100 C++ Mistakes and How to Avoid Them explains undefined behavior →
Modern C
Undefined behavior is a C execution condition for which the language standard imposes no requirements on the program's behavior.
- How it is used
- Uses undefined behavior to explain why seemingly working C programs can fail or be optimized unexpectedly.
- How deep it goes
- Explains consequences through signed overflow and gives operational rules for pointers, formatting, aliasing, and thread access without fully developing concurrency ordering.
- Examples
- Contrasts signed and unsigned incrementing loops and identifies invalid pointer dereferences, mismatched formatted output, and conflicting thread accesses.
- What you can do after
- Evaluate/understand it by checking whether signed arithmetic, pointer use, formatted output, aliasing, or shared access can make execution undefined.
Modern C, Third Edition
Undefined behavior is a C execution condition for which the standard imposes no requirements on the program's subsequent behavior.
- How it is used
- Identifies operations whose execution invalidates assumptions about a C program’s continuation.
- How deep it goes
- Explains compiler assumptions and unreliable outcomes through signed arithmetic, while deferring a fuller treatment of program failure.
- Examples
- Compares signed and unsigned increment loops and examines mismatched printf format specifiers.
- What you can do after
- Diagnose/debug with it by identifying invalid operations rather than trusting apparently normal output.
Read how Modern C, Third Edition explains undefined behavior →
C++ Concurrency in Action, Second Edition
Undefined behavior is program behavior for which the C++ language standard imposes no requirements.
- How it is used
- Identifies program behavior the C++ standard leaves unconstrained, including data races and invalid parallel-algorithm calls.
- How deep it goes
- Defines the consequence and explains data-race and parallel-policy cases, without surveying undefined behavior exhaustively.
- Examples
- Uses a detached thread's local buffer and a lock-free stack pop on an empty stack as examples.
- What you can do after
- Diagnose data-race and parallel-policy cases, and identify atomic or mutex synchronization as ways to define shared access.
Read how C++ Concurrency in Action, Second Edition explains undefined behavior →
Refactoring to Rust
Undefined behavior is program execution whose effects are not constrained by the language or platform specification.
- How it is used
- Constrains raw-pointer handling at a Rust/C FFI boundary by requiring validation before dereference.
- How deep it goes
- Defines null-pointer dereference as undefined behavior and demonstrates a null check before an unsafe write, without covering broader cases.
- Examples
- Uses a C calculator's solution out-parameter and Rust's solve function, first producing an uninitialized result and then writing 1024.
- What you can do after
- Apply it via null checks and narrowly scoped unsafe blocks when writing through C-provided pointers.
Learn C++ by Example
Undefined behavior is program execution for which the C++ language imposes no requirements on the result.
- How it is used
- Uses integer-overflow experiments to reveal language limits and motivate validation with algorithms and assertions.
- How deep it goes
- Defines undefined behavior and demonstrates overflow detection, but stops short of cataloguing its causes or explaining compiler internals.
- Examples
- Uses positive-number row generation, overflowing int values, Visual Studio wraparound, and non-negative checks as practical examples.
- What you can do after
- Diagnose/debug with it by testing numeric assumptions and recognizing that observed overflow behavior is not portable.
Learn Rust in a Month of Lunches
Undefined behavior is program execution whose effects are not constrained by the language specification because a required safety invariant has been violated.
- How it is used
- Explains why unsafe Rust requires explicit responsibility for operations that can violate aliasing, race, or validity assumptions.
- How deep it goes
- Demonstrates UB through races and invalid unchecked unwrapping, but does not develop a formal language-level taxonomy.
- Examples
- Uses concurrent increments of static mut and invalid Option::unwrap_unchecked() calls.
- What you can do after
- Diagnose unsafe operations by reading compiler warnings and recognizing races or violated unchecked assumptions.
Read how Learn Rust in a Month of Lunches explains undefined behavior →
Related concepts
- data raceproduced by · A data race is a concurrent access to shared data in which at least one access is a write and the accesses are not adequately synchronized.
- numerical overflowproduced by · Numerical overflow is the condition in which an arithmetic result falls outside the range representable by its numeric type or format.
- signed arithmetic overflowproduced by · Signed arithmetic overflow is the condition in which an operation on a signed integer produces a result outside the range representable by its type.
- virtual destructoraddressed by · A virtual destructor is a destructor whose dynamic dispatch ensures that destroying an object through a base-class interface invokes the derived-class destructor.
- format stringproduced by · A format string is a character sequence containing literal text and directives that specify how supplied values are represented in formatted input or output.
- double delete errorproduced by · A double-delete error is a memory-management error in which the same allocated memory is deallocated more than once.
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