Undefined Behavior¶

A programming-language case for which the governing specification imposes no requirements on implementation behavior.

Version: v1 · 2026-10-04 · History
Domain-specific #: 13778
Domain group: Applied Sciences & Engineering
Origin domain: Computer Science & Software Engineering
Subdomains: Programming Languages, Language Standards → Computer Science & Software Engineering
Aliases: UB, Undefined program behavior

Core Idea¶

Undefined behavior is a case for which a programming-language specification imposes no requirements on an implementation. Under the cited C++ working draft, an execution that triggers such a condition has no standard-mandated result for that case. A compiler can optimize defined executions on the assumption that those conditions are avoided.[^{ref-cea316ed7f5b][}ref-70deb5df5b0b]

Scope of Application¶

The exact boundary depends on the language and version. The C++ draft classifies an unrepresentable signed arithmetic result and a shift count outside the allowed range as UB. It separately defines unspecified, implementation-defined and erroneous behavior. Processor “UNPREDICTABLE” language and LLVM IR poison have their own rules, not automatic aliases.[^{ref-cea316ed7f5b-4][}ref-cea316ed7f5b-5][^ref-cea316ed7f5b-3]

Clarity¶

“Undefined” is not merely a result the programmer does not know. Unspecified behavior is bounded by allowed outcomes; implementation-defined behavior must be documented; recent C++ erroneous behavior is well-defined but recommended to diagnose. UB is the no-requirements category.[^{ref-cea316ed7f5b][}ref-cea316ed7f5b-2][^ref-cea316ed7f5b-3]

Manages Complexity¶

The category marks cases a standard does not constrain, allowing compiler assumptions for defined executions. That shifts prevention onto program authors and tools; a sanitizer can detect selected reached cases but does not itself define the language rule.[^{ref-70deb5df5b0b][}ref-21d8517fea28]

Abstract Reasoning¶

Name the standard version and clause, determine whether a specific execution reaches its undefined condition, and avoid inferring portable behavior from one observed run. The latest C++ draft has more precise defined-prefix wording than the blanket slogan that absolutely nothing before a UB event can be constrained.[^ref-cea316ed7f5b-2]

Knowledge Transfer¶

The contract question transfers across languages and intermediate representations, but the classifications and consequences must be rechecked per specification. Erroneous Program is a separate identity-review hold, not a UB alias; the generic missing-guarantee skeleton does not by itself make this a prime.

[^ref-cea316ed7f5b]: C++ standards draft editors, “C++ Working Draft Eelis Snapshot c7015b485cc3db8efaa9dfb9ff0809c5394a4ed1”, unofficial eel.is HTML rendering of “Working Draft: Programming Languages — C++” generated 2026-08-23, [defns.undefined] definition; not an ISO publication. [^ref-cea316ed7f5b-2]: C++ standards draft editors, “C++ Working Draft Eelis Snapshot c7015b485cc3db8efaa9dfb9ff0809c5394a4ed1”, unofficial eel.is HTML rendering of “Working Draft: Programming Languages — C++” generated 2026-08-23, [intro.abstract] distinctions and defined-prefix wording; not an ISO publication. [^ref-cea316ed7f5b-3]: C++ standards draft editors, “C++ Working Draft Eelis Snapshot c7015b485cc3db8efaa9dfb9ff0809c5394a4ed1”, unofficial eel.is HTML rendering of “Working Draft: Programming Languages — C++” generated 2026-08-23, [defns.erroneous] definition; not an ISO publication. [^ref-cea316ed7f5b-4]: C++ standards draft editors, “C++ Working Draft Eelis Snapshot c7015b485cc3db8efaa9dfb9ff0809c5394a4ed1”, unofficial eel.is HTML rendering of “Working Draft: Programming Languages — C++” generated 2026-08-23, [expr.pre] arithmetic rule; not an ISO publication. [^ref-cea316ed7f5b-5]: C++ standards draft editors, “C++ Working Draft Eelis Snapshot c7015b485cc3db8efaa9dfb9ff0809c5394a4ed1”, unofficial eel.is HTML rendering of “Working Draft: Programming Languages — C++” generated 2026-08-23, [expr.shift] shift-count rule; not an ISO publication. [^ref-70deb5df5b0b]: LLVM compiler author's UB explanation. [^ref-21d8517fea28]: Clang UndefinedBehaviorSanitizer documentation.

Neighborhood in Abstraction Space¶

Undefined Behavior sits in a sparse region of the domain-specific corpus (88^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Program Scope & Nesting Disciplines (10 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-08