How to build a custom memory allocator in C from scratch

Memory management is one of the most foundational yet least understood layers of software. Behind every dynamic allocation in C—whether you call malloc, calloc, realloc, or free—lies an intricate system that balances speed, fragmentation, and safety. While most developers treat these functions as black boxes, building a minimal memory allocator from scratch exposes the trade-offs that define performance across platforms.

This project began as an experiment: to implement a cross-platform memory allocator in C that mirrors the behavior of standard malloc, but with full control over how memory is requested, tracked, and reused. The goal wasn’t to replace the system allocator, but to uncover the architectural decisions that make modern allocators fast, safe, and portable.

Why Most Tutorials Miss the Point

Many educational resources introduce memory allocation by explaining what it does—allocating and freeing blocks—but rarely address why allocators are designed the way they are. Classic tutorials often rely on sbrk, a legacy system call that moves a global pointer to expand or shrink the heap. While simple in concept, sbrk introduces fragility:

• It assumes exclusive control over a contiguous memory region.
• It cannot be safely used across threads or libraries.
• It isn’t available on Windows, making cross-platform code difficult.

Instead of sbrk, this implementation uses mmap on Linux and VirtualAlloc on Windows—system calls that reserve independent, isolated pages of virtual memory. This approach avoids global bottlenecks and ensures consistent behavior across operating systems.

Design Goals: Portability and Modularity

The allocator was built with four non-negotiable principles:

• Cross-platform: Works identically on Linux and Windows without platform-specific branches in the logic.
• Page-backed: Requests memory from the OS in fixed-size pages (typically 4KB), enabling efficient reuse and resizing.
• Explicit free-list: Uses a linked list to track free memory blocks, allowing intelligent reuse, splitting, and merging.
• Modular: Separates memory acquisition logic from allocation algorithms through a clean abstraction layer.

This is not a slab allocator optimized for fixed-size objects, nor is it a high-performance arena allocator for game engines. It’s a teaching tool—a minimal, functional malloc that demonstrates the core mechanics behind dynamic memory management.

A Layered Architecture for Clarity

To keep the code maintainable and portable, the allocator is split into three logical layers:

1. Allocator Logic: The heart of the system, responsible for block management, free-list traversal, splitting, and coalescing.
2. OS Abstraction: A thin interface that isolates platform-specific calls (mmap/munmap on Linux, VirtualAlloc/VirtualFree on Windows).
3. Utilities: Helper functions for alignment, metadata handling, and mathematical operations.

// os.h — the minimal platform abstraction
void *os_alloc(size_t size);
void os_free(void *ptr, size_t size);

This separation ensures that core allocation logic remains unchanged while only the low-level memory acquisition adapts to the OS. The architecture mirrors modern allocators like jemalloc and mimalloc, which also isolate platform-specific behavior behind clean interfaces.

Free-List Management: The Heart of the Allocator

At runtime, the allocator maintains a linked list of free memory blocks. When a request comes in, it searches for a suitable block using one of two strategies:

• First-fit: Return the first free block large enough to satisfy the request.
• Best-fit: Search for the smallest block that can hold the request, minimizing waste.

Once a block is selected, it may be:

• Split: If the block is larger than needed, the excess is carved off and added back to the free list.
• Coalesced: When adjacent free blocks exist, they are merged to form larger contiguous regions, reducing fragmentation.

This dynamic approach allows memory to be reused efficiently, even after frequent allocations and deallocations.

Modern Allocators: Lessons from Production Systems

While this project is educational, modern allocators like glibc malloc, jemalloc, and tcmalloc employ advanced strategies:

• jemalloc uses per-thread arenas to reduce lock contention in multi-threaded applications.
• mimalloc maps memory in fixed-size chunks to improve cache locality and prevent fragmentation.
• tcmalloc caches small allocations locally to avoid global synchronization.

These systems prioritize scalability and predictability over simplicity. By contrast, our implementation focuses on clarity and portability, making it an ideal starting point for deeper study.

What’s Next: Expanding the Allocator

This first phase establishes the foundation—page-based allocation, free-list management, and cross-platform support. Future iterations could explore:

• Thread safety through mutexes or lock-free structures.
• Support for alignment guarantees beyond the system page size.
• Integration with real-world benchmarks to measure fragmentation and performance.

Ultimately, this project isn’t just about building a malloc clone—it’s about understanding the invisible systems that power every program. By peeling back the layers of memory management, we gain insight into performance, reliability, and the hidden costs of dynamic allocation.