What is c99-opinionated-guide?

A coding guide for systems and embedded C99 projects. Embedded software runs on specialised hardware, and this guide sets rules for memory ownership, data layout, builds and testing.

Use it when editing systems or embedded C files, especially for caller-owned buffers, data-oriented layouts, SIMD variants, handles, warnings and sanitiser checks.

It reduces inconsistency in low-level code where hidden allocations, unsafe memory use or build warnings can cause difficult failures.

c99-opinionated-guide — Skill for Claude Code by xonovex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add xonovex/platform --skill c99-opinionated-guide

Clone the repo

git clone --depth 1 https://github.com/xonovex/platform

Made for: Claude Code.

Or install xonovex-skill-c99-opinionated, the plugin that ships this one along with the rest of its 1 skill.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/xonovex/platform/c99-opinionated-guide.svg)](https://agentmods.dev/skills/xonovex/platform/c99-opinionated-guide)

Your own site

<a href="https://agentmods.dev/skills/xonovex/platform/c99-opinionated-guide"><img src="https://agentmods.dev/badge/skills/xonovex/platform/c99-opinionated-guide.svg" alt="Measured on agentmods" height="20"></a>

Per session 142 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,761 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00142	$0.01761
Opus 5	$0.00071	$0.00881
Sonnet 5	$0.00028	$0.00352
Haiku 4.5	$0.00014	$0.00176

Measured 8d ago against content hash a4ebe3a97b10, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-08, from the pricing page.

Security

Grade A, and why

c99-opinionated-guide scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

packages/skill/skill-c99-opinionated/c99-opinionated-guide/SKILL.md · 61 lines

How it starts

The opening of the file, as written. The whole thing — 61 lines — stays where its author put it; the contents beside it link to each section on GitHub.

C99 Opinionated Guidelines (Systems/Game/Embedded)

Essentials

Overlay on c99-guide - This guide carries only the opinionated decisions. For the shared C99 idioms: const-correctness, designated initializers (ZII), inline-over-macros, compound literals, fixed-width types, value-oriented APIs, and baseline error/return patterns, follow c99-guide
Memory management - This style's defining choice is caller-owned storage with no hidden library allocation; apply memory-management-guide, then enforce the C99 buffer boundary in references/safety-validations.md
Data-oriented design - Layout/cache is a core choice of this style: apply data-oriented-design-guide
Quality - Pin strict C99, -Werror for correctness but relax the unused-symbol family for libraries, keep an ASan/UBSan debug build, run static analysis, see references/build-warnings-policy.md

Architecture

Implementation variants - Ship scalar → AoS → SoA → SIMD with _* suffixes + parity tests; layout rationale in data-oriented-design-guide, see references/implementation-variants.md
Handles & indices - Apply data-oriented-design-guide for stable-handle layout and iteration tradeoffs; enforce C99 check ordering and pointer lifetime in references/safety-validations.md
Alignment - Strict C99 has no portable over-alignment syntax; isolate a compiler-specific alignment macro and use an explicitly exposed platform allocator, see references/alignment.md; the why is in data-oriented-design-guide (SIMD) and lock-free-guide (false sharing)
Composability - Composable stages/primitives over a uniform currency, explicit caller-wired composition, see references/composability.md
Hot reload - Reloadable native modules via API/function-pointer tables + host-owned state, see references/hot-reload.md
Physical design - Headers don't include headers; one header = one system's interface; opaque handles + forward declarations; acyclic deps and fast incremental builds, see references/physical-design.md
Plugin architecture - Small plugins talking through a string-keyed registry of plain-C function-pointer interfaces; runtime discovery, lean core, see references/plugin-architecture.md
Cross-language APIs - Plain-C portable-subset surface, flat data over pointers, call-scoped pointers, spec-generated bindings, see references/cross-language-api.md
File naming - *_type.h, *_impl.h, *_aos.h, *_soa.h, *_simde.h, see references/file-naming.md

Read the full file on GitHub · 61 lines

Files

What ships with it

14 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

8d ago First seen · 61 lines · 142 tokens per session scan A a4ebe3a97b10

Subscribe to this mod's changes

c99-opinionated-guide is a skill published in the GitHub repository xonovex/platform (5 stars, last pushed 2d ago), licensed MIT. It adds 142 tokens to every session and 1,761 once invoked, about $0.0007 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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