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Magic-number audit (fork-added C surfaces) — 2026-05-30

Scope

Sweep fork-added C code for unnamed numeric literals that have semantic meaning (CERT C INT07-C, MISRA C:2012 rule 4.10).

Method

  1. Enumerate suspicious literals with the deferred-tool grep pattern \b[0-9]{2,}[uUlL]?\b in core/src/**.c and core/src/**.h, excluding third-party / iqa / 3rdparty trees and copyright headers.
  2. Triage by fork-added directory:
Directory Raw literal count (post-exclusions)
core/src/dnn 152
core/src/mcp 80
core/src/cuda 30
core/src/hip 16
core/src/metal 11
core/src/sycl 5
  1. Re-triage to drop:
  2. IEEE-754 / float16 bit-field shifts (0x7f800000u, << 23) — these are width-encoded constants, not magic numbers; the comment header on each function already documents the IEEE-754 reference.
  3. JSON-RPC standard error codes (already in a named enum in dispatcher.c).
  4. In-tree constants that are already symbolic (VMAF_CUDA_DRAIN_BATCH_MAX, VMAF_CUDA_THREADS_PER_WARP, VMAF_MCP_SSE_BODY_MAX, etc.).

  5. Buffer caps already explained by an inline comment that derives the value from a sibling constant (e.g. cap = VMAF_MCP_MAX_LINE_BYTES + 256u).

  6. Promote the highest-impact residue to named #defines.

Findings (pass 1)

File Literal New name Reason kept after triage
core/src/mcp/mcp.c listen(fd, 16) × 2 VMAF_MCP_LISTEN_BACKLOG Cross-cutting SOMAXCONN approximation; visible in two TUs
core/src/mcp/mcp.c (unsigned)transport > 31u × 2 VMAF_MCP_TRANSPORT_BITMASK_MAX Encodes the implicit sizeof(unsigned) * CHAR_BIT - 1 shift safety per CERT INT34-C
core/src/mcp/mcp.c cfg->max_drain_per_frame > 64u VMAF_MCP_MAX_DRAIN_PER_FRAME Public config-validation predicate; reviewer needs to know "why 64"
core/src/mcp/mcp.c len > 256u (SSE path) VMAF_MCP_SSE_PATH_MAX HTTP-route cap
core/src/mcp/mcp.c path_len >= 100u (UDS) VMAF_MCP_UDS_PATH_MAX Cross-POSIX sun_path headroom
core/src/mcp/compute_vmaf.c wv < 8.0 \|\| ... > 8192.0 VMAF_MCP_PIC_DIM_{MIN,MAX} YUV dimension bounds for the MCP tool surface
core/src/mcp/compute_vmaf.c bitdepth in {8,10,12,16} VMAF_MCP_BITDEPTH_{8,10,12,16} Pixel-format whitelist
core/src/mcp/transport_sse.c char hdr[256] VMAF_MCP_SSE_STATUS_BUF HTTP-status scratch
core/src/mcp/transport_sse.c cap = LINE + 256u VMAF_MCP_SSE_LINE_HEADROOM Document the headroom delta
core/src/mcp/transport_sse.c budget = 64u VMAF_MCP_SSE_FIELD_SCAN_BUDGET Bounded-scan safety
core/src/mcp/transport_sse.c tv_usec = 200 * 1000 VMAF_MCP_SSE_POLL_USEC Worker heartbeat
core/src/mcp/transport_sse.c char drain[64] VMAF_MCP_SSE_DRAIN_BUF Inbound discard scratch
core/src/mcp/transport_sse.c char method[16] VMAF_MCP_SSE_METHOD_BUF HTTP-method scratch
core/src/mcp/transport_sse.c v = v * 10 + ... VMAF_MCP_SSE_DECIMAL_RADIX Decimal-radix annotation
core/src/picture.c bpc < 8 \|\| bpc > 16 VMAF_PIC_BPC_{MIN,MAX} Cross-backend bit-depth bound
core/src/picture.c w > 32768u \|\| h > 32768u VMAF_PIC_DIM_MAX CERT INT30-C derivation
core/src/cuda/picture_cuda.c bpc < 8 \|\| bpc > 16 × 2 VMAF_CUDA_PIC_BPC_{MIN,MAX} Same as picture.c, kept local to TU
core/src/libvmaf.c char fallback[32] VMAF_DNN_NAME_FALLBACK_BUF Documented derivation from output%zu worst case
core/src/libvmaf.c char suffix[48] VMAF_DNN_NAME_DEDUP_BUF Documented dedup-loop suffix bound
core/src/libvmaf.c strnlen(*, 1024u) × 2 VMAF_DNN_NAME_STRNLEN_CAP CERT STR06-C cap

Total: 26 call-site rewrites, ~20 named constants.

Out of scope (pass 2+ candidates)

  • core/src/dnn/tensor_io.c IEEE-754 bit shifts (0x7f800000, << 23, 0x1fu, >> 13). These encode the IEEE-754 binary32/binary16 layout directly; renaming them risks obscuring the standard reference and would need an IEEE754_* header that none of the touched TUs share. Defer to a dedicated float16 ADR if maintainers want a tighter close-out.
  • predict.c percentile / bootstrap statistics literals (2.5, 97.5, 0.01). These are upstream Netflix code; renaming would create an unnecessary rebase delta against upstream/master.
  • mcp/dispatcher.c JSON-RPC error codes — already in a named enum.

Verification

  • CPU-only build: meson setup build-magic core -Denable_cuda=false -Denable_sycl=false && ninja -C build-magic — clean.
  • Unit tests: meson test -C build-magic — 63/63 pass.
  • Bit-exactness: no numeric values changed; Netflix golden gate preserved by construction.
  • scripts/ci/assertion-density.sh: pass (161 asserts across 99 fork functions).

References

  • CERT C INT07-C, INT30-C, INT34-C, STR06-C.
  • MISRA C:2012 rule 4.10.
  • ADR-0874.