Header-Only Arena Allocator in C

An efficient, portable, and easy-to-use header-only arena allocator library written in pure C.

TL;DR

What is it? A high-performance, portable, header-only arena allocator for C.

Key Features:

• O(log n) Free List: Uses a Left-Leaning Red-Black Tree for efficient block reuse and low fragmentation.
• Minimal Overhead: Only 32 bytes of metadata per block (on 64-bit systems), achieved with pointer tagging and binary-compatible struct layouts.
• Nested Arenas: Supports hierarchical memory management with zero-cost sub-arenas.
• Safe by Design: Features compile-time assertions for configuration and runtime checks for pointer validity and magic number verification.
• 100% Test Coverage: Rigorously tested across multiple architectures and OSs.

Why use it? To get fast, controlled, and reliable memory management in performance-critical applications like game engines, embedded systems, or network servers.

How to use it? #define ARENA_IMPLEMENTATION in one .c file, then just #include "arena.h".

This library serves as the core memory manager for the Zen Framework - a lightweight, modular framework for building console applications in C.

Overview

This library provides a header-only implementation of an arena allocator in C. Arena allocation is a memory management technique that allocates memory in large chunks and then subdivides them for application use, offering significant performance benefits over standard malloc/free.

This implementation focuses on performance, memory efficiency, and robustness, providing advanced features like nested arenas within a portable package.

Key Benefits:

• High Performance: Fast allocations with O(log n) complexity for finding free blocks (via an LLRB-Tree) and O(1) for tail allocations.
• Memory Efficiency: Minimized metadata overhead and optimized block merging to combat fragmentation.
• Flexible Creation: Supports both Static Arenas (using pre-allocated buffers/stack) and Dynamic Arenas (heap-based).
• Nested Arenas: Allows creating sub-pools within a parent arena for scoped memory management.
• Fine-grained Control: Unlike many simple arenas, this one supports freeing individual blocks (arena_free_block) for memory reuse without resetting the entire arena.
• Source-Agnostic API: Operates on an Arena* handle, making the allocation logic independent of whether the memory source is static or dynamic.

Recommended Use Cases

• High-throughput Allocations: Systems performing frequent allocations where malloc overhead is too high.
• Controlled Memory Lifecycles: Grouping objects into scopes and deallocating them all at once (e.g., per-frame data, per-request state).
• Fragmentation-Sensitive Apps: Long-running applications where standard allocator fragmentation becomes a problem over time.
• Simplified Multithreading: Using a "one-allocator-per-thread" model. arena_new_nested makes this pattern safe and easy to implement.

Features

• Extreme Portability: Header-only and tested on Linux, macOS, and Windows; x86, ARM64, and ARM32; and both Little Endian and Big Endian orders.
• Full C++ Compatibility: Wrapped in extern "C" for seamless integration.
• Advanced Allocation: General-purpose free-list allocator with O(log n) performance and block merging.
• Optimized Tail Allocation: Most allocations at the end of the arena are O(1).
• Memory Correctness: Automatic 16-byte alignment by default (optimized for SIMD).
• Minimal Metadata: Optimized struct layout using pointer tagging to keep headers small.
• Full Control: Standard-like interface (arena_alloc, arena_calloc, arena_free_block) plus fast arena_reset.
• Powerful Debugging: Colorized terminal visualization of memory state via print_fancy.

Architectural Philosophy

Memory management involves a trade-off between cache locality, pointer stability, and resizability. You can only pick two.

This library prioritizes locality and stability.

Principle 1: Pointers are Stable (No realloc)

Once memory is allocated, its address never changes. This allows you to safely store pointers to allocated objects without worrying about invalidation. Consequently, there is no realloc; resizing requires allocating a new block and copying.

Principle 2: Memory is Local (Performance by Default)

Allocations happen in contiguous chunks. This dramatically improves cache performance compared to the "scattered" nature of standard malloc.

Getting Started

1. Include and Implement

In one .c file:

#define ARENA_IMPLEMENTATION
#include "arena.h"

Alternative Integration for Large Projects

In complex projects with intricate include hierarchies, ensuring that ARENA_IMPLEMENTATION is defined in exactly one .c file can be challenging. An alternative approach is to compile the header file directly into its own object file.

You can achieve this by adding a specific build rule to your build system (e.g., Makefile, CMake). Here's an example using gcc:

# Example Makefile rule
arena.o: arena.h
	gcc -x c -DARENA_IMPLEMENTATION -c arena.h -o arena.o

Explanation:

• gcc -x c: This flag tells gcc to treat the input file (arena.h) as a C source file, even though it has a .h extension.
• -DARENA_IMPLEMENTATION: This defines the necessary macro to include the function implementations.
• -c: This tells gcc to compile the source file into an object file (arena.o) without linking.

Then, simply link the resulting arena.o object file with the rest of your project. This isolates the implementation into a single compilation unit, avoiding potential conflicts in large codebases.

2. Standard Usage

// Create a 1MB dynamic arena with default 16-byte alignment
Arena *arena = arena_new_dynamic(1024 * 1024);

// Standard allocation (returns 16-byte aligned pointer)
int *data = (int *)arena_alloc(arena, sizeof(int) * 100);

// Zero-initialized allocation
Point *pts = (Point *)arena_calloc(arena, 10, sizeof(Point));

// Free individual block (optional, allows internal reuse)
arena_free_block(data);

// Reset arena (clears all blocks in O(1), memory remains allocated)
arena_reset(arena);

// Full cleanup
arena_free(arena);

3. Custom Baseline Alignment & Static Memory

You can initialize an arena with a custom baseline alignment. All subsequent standard arena_alloc calls will automatically respect this alignment.

// Use pre-allocated buffer with a strict 64-byte baseline alignment (e.g., for AVX-512)
char buffer[4096];
Arena *static_arena = arena_new_static_custom(buffer, sizeof(buffer), 64);

// This pointer is guaranteed to be 64-byte aligned
void *p = arena_alloc(static_arena, 256);

4. Overriding Alignment per Allocation

If you need a specific block to have stricter alignment than the arena's baseline (e.g., aligning a single buffer to a page boundary), use arena_alloc_custom.

Arena *arena = arena_new_dynamic(1024 * 1024); // Baseline is 16

// Standard allocation (16-byte aligned)
void *p1 = arena_alloc(arena, 100);

// Specific allocation with 128-byte alignment
void *p2 = arena_alloc_custom(arena, 512, 128);

5. Nested Arenas (Scoped Memory)

Nested arenas allow you to create temporary sub-pools. Freeing a nested arena instantly returns its entire memory block to the parent.

void process_request(Arena *main_arena) {
    // Create a 64KB sub-arena from the main arena
    Arena *request_scope = arena_new_nested(main_arena, 1024 * 64);
    
    // All allocations in this scope are freed at once below
    void *tmp = arena_alloc(request_scope, 1024);
    
    arena_free(request_scope); 
}

Configuration

Define these macros before including arena.h to customize behavior:

Macro	Default	Description
ARENA_DEFAULT_ALIGNMENT	16	Minimum allocation alignment (must be power of two).
ARENA_MIN_BUFFER_SIZE	16	Minimum size of a free block split.
ARENA_POISONING	Auto	Fills freed memory with 0xDD in DEBUG builds.
ARENA_NO_MALLOC	Unset	Disables malloc/free dependencies (for static-only use).

Build Status & Portability

OS	Status
Ubuntu
macOS
Windows

By Compiler

Compiler	Status
GCC
GCC (MinGW)
Clang
MSVC

By Architecture

Architecture	Endianness	Status
x86_64	Little
x86_32	Little
AArch64	Little
ARMv7	Little
s390x	Big

C Standards Compliance

Standard	Status
C99 / C11 / C17 / C23

Hardware Verification (Bare Metal)

This library has been verified to run correctly on embedded hardware without standard library dependencies (ARENA_NO_MALLOC).

Architecture	Device	Status
ARM Cortex-M0+	Raspberry Pi Pico (RP2040)
Xtensa LX6	ESP32-WROOM

Why All This?

idk, i was bored

License

MIT License. See LICENSE for details.