C3

C3 is a systems language in the C tradition, designed and led by Christoffer Lerno. Self-described as a "C alternative" rather than a C++/Rust competitor — keeps C's manual memory model and ABI compatibility, but makes opinionated changes to defaults that C's standardization process never could. Compiles via LLVM, links with C, can include C headers directly.

What's distinctive

The reason C3 ends up in this wiki isn't the syntax — it's the willingness to revisit C's defaults publicly when they don't hold up. The "unsigned sizes was a five year mistake" post is an example of the language's design conversations being conducted in writing, with admission of error.

Specific design decisions worth knowing:

• Modules and slices baked in, no preprocessor in the C sense.
• Macros are a separate hygienic system (@ prefix), not text substitution.
• Optionals and errors unified — return values can carry "fault" types, the ? and ! operators handle the propagation.
• Defer, like Go and Zig.
• Contracts — function-level pre/postcondition annotations.
• Compile-time evaluation via $ prefix — comptime in zig terms, but separated lexically from runtime code.

The signed-sizes change

After 5 years on unsigned-default sizes, C3 0.8.0 (2026) flipped the default. The post-mortem (unsigned-sizes-c3-mistake) argues the original convention — inherited from C's size_t — is the root cause of the well-known unsigned bug class, and that ergonomic patches don't reach it. Java and Go are cited as the languages that got this right.

The signed-size type was renamed isz → sz, with usz as the explicit-unsigned counterpart. Implicit signed↔unsigned conversion and mixed comparisons were dropped.

Blog

The C3 blog at c3-lang.org/blog is Lerno's writing surface for design rationale, retrospectives, and release notes. Unlike most language blogs, it's willing to publish "this default was wrong" posts.

Ingested articles

• unsigned-sizes-c3-mistake — May 2026 post on switching to signed sizes by default

See also

• zig — the other "let's redo C" project; differs on unsigned sizes (Zig kept them) and on metaprogramming model
• signed-vs-unsigned-sizes — the concept this language is currently the canonical worked example of
• type-systems-vocabulary — adjacent debate space about how language designers talk about types

External: c3-lang.org, github.com/c3lang/c3c