TL;DR
Xr0 is a verifier for C that aims to detect and eliminate a range of undefined behaviours at compile time using C-like annotations. The project is a work in progress that currently targets a subset of C89, is written in C, and is available as open-source software.
What happened
Xr0 is an annotation-driven verifier designed to reduce common undefined behaviours in C programs. It can catch issues such as use-after-free, double free, null pointer dereferences, and accesses to uninitialized memory by checking code annotated with C-like contracts. The tool currently verifies a subset of the C89 language; notable verification features for loops and recursive functions have not yet been implemented and are handled via axiomatic annotations for now. The developers say Xr0 1.0.0 will aim to make it possible to program in C without undefined behaviour, but at present the verifier is intended for checking parts of programs rather than entire codebases. Xr0 is implemented in standard C and published as open source, with repositories hosted on GitHub and SourceHut. Documentation includes a tutorial, a debugger to inspect verification, theses explaining the approach, and a roadmap; the project also maintains community channels on Discord and by email.
Why it matters
- Undefined behaviour in C is a common root cause of crashes and security vulnerabilities; a verifier can reduce such risks before runtime.
- Compile-time verification of memory- and pointer-related bugs can lower the need for costly runtime checks and debugging cycles.
- Annotation-based verification can be adopted incrementally, allowing teams to verify critical sections without rewriting whole projects.
- Open-source tooling and documentation can accelerate experimentation and community-driven improvements in C verification techniques.
Key facts
- Xr0 is a verifier for C that uses C-like annotations to check code correctness.
- It targets a subset of the C89 standard.
- The verifier can eliminate or detect issues such as use-after-frees, double frees, null pointer dereferences, and uninitialized memory use.
- Verification for loops and recursive functions is not yet implemented; these are currently addressed with axiomatic annotations.
- The project is written in pure C and published as open-source software on GitHub and SourceHut.
- Developers provide a debugger, a tutorial, explanatory theses, and a roadmap as part of the project resources.
- Xr0 1.0.0 is described as aiming to enable programming in C without undefined behaviour, but that goal is not yet achieved.
- Community engagement is available via Discord and email contacts provided by the project.
What to watch next
- Progress toward Xr0 1.0.0, which the project says will enable writing C with no undefined behaviour (confirmed in the source).
- Implementation status for verification of loops and recursive functions, currently not implemented and bridged by axiomatic annotations (confirmed in the source).
- Expansion of language coverage beyond the current C89 subset: not confirmed in the source.
Quick glossary
- Undefined behaviour: Program operations for which the language specification does not prescribe a result, often causing crashes, security issues, or unpredictable behavior.
- Use-after-free: An error that occurs when code accesses memory after it has been released back to the system, potentially leading to corruption or arbitrary code execution.
- Annotation: A programmer-supplied comment or metadata attached to code that a verifier uses to express assumptions, contracts, or intended behavior.
- Verifier: A static analysis tool that checks source code against formal properties or annotations to find bugs or prove correctness without running the program.
- C89: An early ISO C standard (also known as ANSI C) that defines the language rules Xr0 currently targets a subset of.
Reader FAQ
What kinds of bugs can Xr0 detect?
According to the project, it can detect or eliminate use-after-frees, double frees, null pointer dereferences, and uses of uninitialized memory.
Which C standard does Xr0 support?
The verifier currently works on a subset of C89.
Can Xr0 verify loops and recursive functions?
Not yet; verification for loops and recursion has not been implemented and is currently handled via axiomatic annotations.
Where can I find the source code and documentation?
The project is open source with repositories on GitHub and SourceHut, and provides a tutorial, debugger, theses, and a roadmap.
Xr0 C But Safe Blog Theses Tutorial Try Xr0 is a verifier for C. It eliminates many stubborn instances of undefined behaviour, like use-after-frees, double frees, null pointer dereferences and the use…
Sources
- Xr0 verifier, guarantee the safety of C programs at compile time
- xr0-org/xr0: The Xr0 Verifier for C
- Is there a C++ implementation that detects all undefined …
- A framework for systematically addressing undefined …
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