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ADR-0692: Bump C standard to C23 (VMAFX rebrand Phase 1D)

  • Status: Accepted
  • Date: 2026-05-28
  • Deciders: lusoris
  • Tags: build, c, standards, meson, fork-local, vmafx-rebrand

Context

The fork has compiled under C11 since its initial divergence from Netflix/vmaf. The C standard controls which language features and standard-library additions are available to C source files. C23 (ISO/IEC 9899:2024) shipped in late 2024 and is fully supported by GCC 13+, Clang 16+, and MSVC 19.34+ (partial). The dev machine and CI matrix both run GCC 16 and Clang 22, well past the threshold.

Three language features from C23 are directly useful for the VMAFX fork:

  • typeof — avoids the -Wpedantic-violating GCC __typeof__ spelling in SIMD macros.
  • ckd_add / ckd_mul (checked integer arithmetic) — safer replacements for the explicit bounds-check patterns required by SEI CERT INT30-C and INT32-C.
  • [[fallthrough]] attribute syntax — cleaner than __attribute__((fallthrough)) and self-documenting at call sites without a /* FALLTHROUGH */ comment convention.

This ADR covers only the standard bump and the immediate prerequisite fix. C23 feature adoption happens in subsequent targeted PRs. This is Phase 1D of the VMAFX rebrand plan (umbrella ADR-0686, PR #1546).

One latent correctness defect was uncovered during the smoke-build: test_propagate_metadata.c defined set_meta with an empty parameter list (void set_meta()), then assigned it to a void (*)(void *, VmafMetadata *) typed field. In C11 an empty parameter list is an unspecified-argument declaration and the assignment is a warning; in C23 the semantics changed so that void f() is equivalent to void f(void) (no arguments), making the assignment an error. The fix is to give set_meta the correct prototype.

Decision

We will set c_std=c23 in libvmaf/meson.build's default_options, fix the test_propagate_metadata.c prototype mismatch, and add -Wimplicit-fallthrough to vmaf_cflags_common to enforce the JPL Rule-24 fallthrough annotation requirement at compile time.

No C23 language features are introduced in source code by this PR; the bump merely unlocks them for subsequent PRs.

Alternatives considered

Option Pros Cons Why not chosen
Stay on C11 indefinitely Zero migration cost Blocks typeof, ckd_*, [[fallthrough]] adoption; c2x interim toggle was already acknowledged as a holdover Rejected — VMAFX rebrand plan explicitly targets C23
Bump to C17 first, C23 later Smaller hop; C17 compiler support is universal C17 adds nothing over C11 except a few defect fixes; no practical benefit over a direct C23 jump Rejected — C17 intermediate step is pure overhead
Keep c2x alias Works on older Meson c2x was the pre-publication draft alias; c23 is the standardised name supported by Meson ≥1.3.0 (fork has 1.11.0) Rejected — c23 is unambiguous and correctly supported

Consequences

  • Positive: C23 language features (typeof, ckd_*, [[fallthrough]], #embed, nullptr) are now available in all libvmaf C translation units. -Wimplicit-fallthrough catches missing fallthrough annotations at compile time, complementing JPL Rule-24 enforcement.
  • Negative: Upstream Netflix/vmaf code ported via port-upstream-commit may carry C99/C11-era constructs that need testing under C23 (primarily empty-parameter-list function declarations like the one fixed here; see docs/rebase-notes.md).
  • Neutral / follow-ups: Adopt typeof in SIMD macros, ckd_* in bounds-check hot paths, and [[fallthrough]] at switch fall-throughs as targeted follow-up PRs per the VMAFX rebrand roadmap.

References

  • Umbrella: ADR-0686 (VMAFX rebrand plan, PR #1546) — not yet filed as of this PR.
  • docs/principles.md §1.2 rule 24 (fallthrough annotations).
  • libvmaf/meson.build line 3 (default_options).
  • libvmaf/src/meson.build vmaf_cflags_common block.
  • libvmaf/test/test_propagate_metadata.c line 22 (prototype fix).
  • Meson C23 support: https://mesonbuild.com/Reference-tables.html#language-arguments-parameter-names
  • req: "Locked decision: C standard: C23 (typeof, ckd_*, embed, [[attribute]] syntax)."