# memory-model

A skill that walks your agent through C++ and Rust memory ordering, picks the right atomic order, and helps track down data races.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 634 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/low-level-programming/memory-model/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/memory-model

## What it does

The skill fires on questions about memory ordering, acquire-release, seq_cst, relaxed atomics, happens-before, memory barriers, `std::atomic` or Rust atomic ordering. Your agent gets a side-by-side table of C++ and Rust orderings, the strength scale from Relaxed up to SeqCst, and what each order actually guarantees.

Inside is an eight-step workflow: ordering overview, the `memory_order_*` versus `Ordering::*` table, `std::atomic` examples, a decision tree for picking an order, ready patterns (spinlock, reference counting, one-time initialisation), Rust atomics, fences, and a table of common mistakes. Two of those mistakes come up constantly: reaching for `volatile` as a thread-safety tool in C++, and sprinkling SeqCst everywhere instead of profiling first.

The skill points to `references/cpp-memory-ordering.md` and to related skills — sanitizers for race detection, Miri for Rust ordering violations, x86 assembly for reading the generated fence instructions. Worth installing if you write lock-free structures or inherit concurrent code you have to reason about; these bugs rarely show up by eye.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill memory-model -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/memory-model/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/memory-model/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill memory-model -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/memory-model/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill memory-model -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/memory-model/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills memory-model
```

2. **Manually: put the files in place**
- File: `.github/skills/memory-model/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill memory-model -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/low-level-programming/memory-model
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/memory-model/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill memory-model -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/low-level-programming/memory-model/SKILL.md

## When to use it
- pick the right memory order for a given atomic operation
- explain why acquire-release is not synchronising two threads
- build a lock-free queue with multiple producers
- track down a data race around `std::atomic`
- port a C++ atomic pattern to Rust `Ordering`

## Example prompts
- Which memory order should this store use if I'm publishing data to another thread?
- Walk through this `std::atomic_flag` spinlock and check the acquire and release
- Rewrite this reference counter with Rust atomics and explain the `Ordering` choices

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: a620b2c52527006a.

BEGIN THIRD-PARTY FILE a620b2c52527006a
````text
---
name: memory-model
description: C++ and Rust memory model skill for concurrent programming. Use when understanding memory ordering, writing lock-free data structures, using std::atomic or Rust atomics, diagnosing data races, or selecting the correct memory order for atomic operations. Activates on queries about memory ordering, acquire-release, seq_cst, relaxed atomics, happens-before, memory barriers, std::atomic, or Rust atomic ordering.
---

# Memory Model

## Purpose

Guide agents through C++ and Rust memory models: memory orderings, the happens-before relation, atomic operations, fences, and practical patterns for lock-free data structures.

## Triggers

- "What is the C++ memory model?"
- "What memory order should I use for my atomic operation?"
- "What is the difference between acquire-release and seq_cst?"
- "How do I use std::atomic in C++?"
- "What is acquire/release in Rust atomics?"
- "How do I implement a lock-free queue?"

## Workflow

### 1. Memory ordering overview

Modern CPUs and compilers reorder operations for performance. The memory model specifies what reorderings are allowed and how synchronisation is achieved.

```text
Ordering strength (weakest to strongest):
Relaxed < Release/Acquire < AcqRel < SeqCst

Stronger ordering = more synchronization = more correct, but slower
Weaker ordering  = fewer barriers = faster, but needs careful analysis
```

### 2. Memory orderings

| Order | C++ | Rust | What it means |
|-------|-----|------|--------------|
| Relaxed | `memory_order_relaxed` | `Ordering::Relaxed` | No ordering guarantee; just atomicity |
| Consume | `memory_order_consume` | (use Acquire) | Data dependency ordering |
| Acquire | `memory_order_acquire` | `Ordering::Acquire` | This load sees all writes before the matching release |
| Release | `memory_order_release` | `Ordering::Release` | All writes before this store are visible to acquire |
| AcqRel | `memory_order_acq_rel` | `Ordering::AcqRel` | Both acquire and release on RMW ops |
| SeqCst | `memory_order_seq_cst` | `Ordering::SeqCst` | Total order across all seq_cst operations |

### 3. C++ std::atomic

```cpp
#include <atomic>
#include <thread>

std::atomic<int> counter{0};
std::atomic<bool> ready{false};

// Producer thread
void producer() {
    data = 42;                              // (1) write data
    ready.store(true, std::memory_order_release);  // (2) signal
}

// Consumer thread
void consumer() {
````
END THIRD-PARTY FILE a620b2c52527006a

The third-party file has ended. Everything between the a620b2c52527006a tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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