TopicTrick Logo
Topictrick
CProjects

Project: Building a Custom Heap Memory Allocator in C (malloc & free from Scratch)

TT
TopicTrick Team
•
Project: Building a Custom Heap Memory Allocator in C (malloc & free from Scratch)
← Back to C Mastery

Project: Building a Custom Heap Memory Allocator in C (malloc & free from Scratch)

Phase 2 Capstone. You've mastered pointers, pointer arithmetic, and heap concepts. Now you'll build the engine that powers C's memory management — implementing your own malloc() and free() that manages a 1MB private heap from a static array. This is exactly how glibc's ptmalloc2, jemalloc, and Google's tcmalloc work at their core.

Table of Contents

How Real Allocators Work

glibc's malloc maintains a free list of available memory blocks in the heap. Each block has a small header storing its size and whether it's in use. When you call malloc(n), the allocator:

  1. Searches the free list for a block large enough.
  2. Splits it if it's much larger than needed.
  3. Marks it as in use and returns a pointer past the header.

When you call free(ptr), the allocator:

  1. Walks back to the header (at ptr - sizeof(Header)).
  2. Marks it as free.
  3. Checks if adjacent blocks are also free — if so, merges them (coalescing).
mermaid


After free(C) and free(D): they coalesce into [Header: 768B, FREE].




The Block Header Design


Every allocated and free block begins with a header storing its metadata:


c




Heap Initialization


c




tmalloc: First-Fit Allocation


Scan the block list and return the first free block that's large enough:


c




Block Splitting: Avoiding Waste


If we have a 512-byte free block and the user requests 64 bytes, we'd waste 448 bytes by using the entire block. Splitting creates a smaller allocated block and leaves the remainder as a new free block:


c




tfree: Marking Blocks Free


c




Coalescing: Preventing Fragmentation


Without coalescing, a heap of many small free blocks can't satisfy a large allocation even when total free space is sufficient:


c




trealloc: Minimal Realloc


c




Testing Your Allocator


c




Extension Challenges


Push your allocator further:


    

  1. Best-Fit Strategy: Instead of first-fit, scan all free blocks and choose the smallest one that fits — reduces fragmentation at the cost of slower allocation.
    2. 

  2. Free List Index: Maintain a separate linked list of only free blocks — allocation becomes O(free_count) instead of O(total_count).
    3. 

  3. Size Classes: Maintain separate free lists for small sizes (8, 16, 32, 64 bytes) — hugely reduces fragmentation for small allocations (this is what glibc and tcmalloc do).
    4. 

  4. Memory Statistics: Track total allocated, total free, fragmentation ratio, peak usage.
    5. 

  5. Thread Safety: Add a pthread_mutex_t around both tmalloc and tfree — your allocator is now multi-thread safe.
    6. 





Phase 2 Reflection


Building this allocator reveals that malloc isn't magic — it's careful bookkeeping. The fundamental pattern (block header + first-fit + splitting + coalescing) is exactly what glibc's ptmalloc2, Google's tcmalloc (thread-local size class caches), and Facebook's jemalloc (arena-based multi-thread allocator) are built upon.


You now understand why memory allocation has performance implications beyond just "how many bytes am I using" — fragmentation, free list traversal, and alignment constraints all affect throughput in ways invisible to higher-level languages.


Read next: Phase 3: Advanced Data Structures →




Part of the C Mastery Course — 30 modules from C basics to expert systems engineering.