compare · C
fastC vs C
C is the language fastC compiles to. fastC is a thin safety wrapper around C — every row is a structural guarantee fastC adds on top of C's unchecked-but-free semantics.
C is the language fastC compiles to. fastC is a thin safety wrapper around C — most rows compare fastC's structural guarantees to C's "unchecked but free" semantics. The point of this comparison is not to declare a winner; the point is to show what fastC's wedge looks like when the baseline is the language under it.
Row by row
| Dimension | fastC | C | Note |
|---|---|---|---|
| Memory safety without GC | Yes — runtime traps + contracts | No — undefined behavior on every UB site | fastC's traps fire deterministically; C's UB does not. |
| Capability-typed I/O | Yes — finite named set, minted in main | No — every fn can call open() / socket() / fork() | fastC's structural wedge over C. |
| Contracts on public APIs | Yes — @requires / @ensures, three-tier discharge | No — assert() is runtime-only, no static surface | Documentation conventions are not enforcement. |
| No executable build scripts | Yes — declarative manifest only | Depends on toolchain — Makefile is structural | Make + autoconf are mostly imperative. |
| Stripped 'hello' binary | 53 KB | 33 KB | C wins; fastC adds a runtime-trap shim. |
| Compile speed | ~30% faster than full Rust release | Fastest of all — clang -O0 on a small TU is sub-100 ms | C is the floor; fastC is close to it because we emit C. |
| Cross-compile setup | 8 presets via `zig cc` | Depends on toolchain — sysroots per target | fastC's preset list is small but works out of the box. |
| Toolchain breadth | One host (Rust 1.85+); 8 cross targets | Every embedded device on Earth has a C compiler | C wins decisively here. |
| Ecosystem size | 11 curated fastc-core packages | The entire ecosystem of every Unix and every libc | C wins by orders of magnitude. |
| Readability | Slightly more verbose — explicit casts, named caps in signatures | More terse — implicit conversions, ambient I/O | Trade-off, not strict win for either. |
| Maturity | 2026 v1.0 | 50+ years of production deployment | C wins; fastC is brand new. |
| Provenance on releases | Sigstore + SLSA L3 from day one | Project-by-project; not enforced ecosystem-wide | fastC structural. |
| Token efficiency for agents | Larger source (explicit conversions, cap params) | Smaller surface, but agents make more UB mistakes | Different failure modes; fastC trades verbosity for fewer silent-wrong outputs. |
| Audit-by-disassembly | Yes — 53 KB stripped, no GC, no runtime | Yes — same property C has always had | Roughly tied; both are bytes-down-to-the-syscall. |
Where fastC wins
fastC wins on safety, capability typing, contracts, and supply-chain provenance. The structural answers it adds are all things C's design predates. For new code in 2026 where memory safety and capability auditing matter, fastC is the better choice.
Where C wins
C wins on ubiquity, simplicity, toolchain breadth, and the maturity of its tooling. C is the right answer when you need to ship to a platform fastC does not have a target for, when binary size below 53 KB is load-bearing, or when you are extending an existing C codebase rather than starting fresh.
The honest framing
fastC's emit-to-C model means every fastC program is also a C program. The choice is whether you want fastC's safety wedge on top, or whether you are confident your C-only program is correct. For most production code in 2026 the answer is fastC; for kernel modules and embedded firmware where every byte counts, it is still C.