Enforcing bounds safety for C — Clang 23.0.0git documentation
Hacker News
February 16, 2026
AI-Generated Deep Dive Summary
Clang 23.0.0 introduces the `-fbounds-safety` extension, designed to enhance C programming safety by preventing out-of-bounds (OOB) memory accesses—a leading cause of security vulnerabilities. This feature leverages bounds annotations on pointers to enforce checks at compile or runtime, ensuring memory access stays within specified limits. If a check fails, the program deterministically traps before accessing invalid memory, eliminating OOB bugs.
The extension allows programmers to annotate pointers with `__counted_by(N)`, indicating that a pointer has `N` valid elements. For example:
```void foo(int *__counted_by(N) ptr, size_t N);```
This annotation enables the compiler to insert bounds checks on every dereference. It also integrates seamlessly with existing C code while maintaining ABI compatibility, making it easier for developers to adopt incrementally. The extension uses implicit wide pointers ("fat" pointers) to carry bounds information without requiring annotations for local variables.
`-fbounds-safety` addresses several challenges that have hindered safer C dialects by offering a conforming extension with low adoption costs. It allows partial implementation across projects, providing immediate safety benefits while enabling gradual migration. This approach is particularly valuable in large-scale systems where full adoption is impractical.
This innovation matters because OOB errors remain a critical security risk, especially in high-stakes applications like operating systems and firmware. By reducing these vulnerabilities, `-fbounds-safety` helps improve software reliability and security without disrupting existing workflows or binary compatibility. Its proven success in production environments underscores its practicality and potential to transform C
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Originally published on Hacker News on 2/16/2026