# custom-allocators

Guides agents through designing custom C and Rust memory allocators, tuning jemalloc or mimalloc, and measuring heap fragmentation.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 350 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/allocators/custom-allocators/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/custom-allocators

## What it does

The skill equips coding agents with reference implementations for fixed-size pool allocators and bump arenas in C. Instead of routing every allocation through generic libc malloc, it shows how to use linear arenas for per-request lifetimes where an entire block resets at once.

It covers runtime configuration for jemalloc and mimalloc via `LD_PRELOAD` alongside environment flags like `MALLOC_CONF`. For Rust projects, it demonstrates implementing the `GlobalAlloc` trait to track allocations or run in `#![no_std]` targets.

Memory leaks in arenas happen when resets are skipped between loop iterations or server frames. In Rust, passing a mismatched `Layout` to `dealloc` triggers undefined behavior, so the skill notes storing size and alignment alongside raw pointers.

## Install

### Claude Code (native platform)

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

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

### Codex

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

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

### Cursor

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

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

### GitHub Copilot

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

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

### Gemini CLI

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

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

### Windsurf (Devin Desktop)

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

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill custom-allocators -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/allocators/custom-allocators/SKILL.md

## When to use it
- implement a fast O(1) fixed-size block pool in C
- build a request-scoped bump arena allocator with reset
- tune jemalloc dirty page purging via MALLOC_CONF
- implement the Rust GlobalAlloc trait for allocation tracking

## Example prompts
- Write an aligned bump arena allocator in C with an arena_reset function
- Show the MALLOC_CONF settings to tune dirty page decay in jemalloc
- Implement a tracking allocator in Rust using the GlobalAlloc trait

## 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: be999efb5849d4ea.

BEGIN THIRD-PARTY FILE be999efb5849d4ea
````text
---
name: custom-allocators
description: Custom allocator skill for memory allocation strategies. Use when implementing pool/slab/arena allocators, tuning jemalloc/mimalloc, writing Rust GlobalAlloc, or benchmarking allocator performance. Activates on queries about jemalloc, mimalloc, tcmalloc, arena allocator, GlobalAlloc, or fragmentation.
---

# Custom Allocators

## Purpose

Guide agents through memory allocator design and tuning: pool/slab/arena/buddy taxonomy, jemalloc internals and `MALLOC_CONF`, mimalloc design, tcmalloc thread-caching, writing a simple pool allocator in C, Rust `GlobalAlloc` trait, fragmentation metrics, and benchmarking.

## When to Use

- Replacing malloc for latency-sensitive or embedded workloads
- Tuning jemalloc/mimalloc for server heap behavior
- Implementing arena allocation for request-scoped or frame-based lifetimes
- Writing a Rust custom global allocator for `no_std` or performance
- Diagnosing heap fragmentation (internal vs external)
- Benchmarking allocator throughput and latency

## Workflow

### 1. Allocator taxonomy

```
Allocator types
├── Pool/fixed-size — O(1) alloc/free, fixed block sizes
├── Slab — kernel-style, cache-friendly size classes
├── Arena/bump — fast alloc, bulk free (reset arena)
├── Buddy — power-of-two blocks, low fragmentation for large allocs
└── General (malloc) — jemalloc, mimalloc, tcmalloc
```

### 2. Simple pool allocator in C

```c
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct pool_block {
    struct pool_block *next;
} pool_block_t;

typedef struct {
    void   *memory;
    size_t  block_size;
    size_t  num_blocks;
    pool_block_t *free_list;
} pool_t;

int pool_init(pool_t *p, size_t block_size, size_t num_blocks) {
    p->block_size = block_size < sizeof(pool_block_t) ?
        sizeof(pool_block_t) : block_size;
    p->num_blocks = num_blocks;
    p->memory = aligned_alloc(64, p->block_size * num_blocks);
    if (!p->memory) return -1;
    p->free_list = NULL;
    for (size_t i = 0; i < num_blocks; i++) {
        pool_block_t *blk = (pool_block_t *)((char *)p->memory + i * p->block_size);
        blk->next = p->free_list;
        p->free_list = blk;
    }
    return 0;
}

void *pool_alloc(pool_t *p) {
    if (!p->free_list) return NULL;
    pool_block_t *blk = p->free_list;
    p->free_list = blk->next;
    return blk;
}

void pool_free(pool_t *p, void *ptr) {
    pool_block_t *blk = (pool_block_t *)ptr;
    blk->next = p->free_list;
    p->free_list = blk;
}
```

### 3. Arena allocator

```c
typedef struct {
    char  *base;
    size_t capacity;
    size_t offset;
} arena_t;

void *arena_alloc(arena_t *a, size_t size, size_t align) {
    uintptr_t cur = (uintptr_t)(a->base + a->offset);
````
END THIRD-PARTY FILE be999efb5849d4ea

The third-party file has ended. Everything between the be999efb5849d4ea 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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