Custom-Malloc-Memory-Allocator

A custom memory allocator built in C that implements malloc and free using a doubly linked list. The project manages heap memory with sbrk/brk, supports block splitting and merging, heap expansion/shrinking, and basic thread safety, while exploring low-level memory management and OS internals.

Custom Memory Allocator in C

A custom implementation of malloc() and free() built from scratch in C to explore low-level memory management, heap allocation, and operating system internals.

This project manually manages heap memory using sbrk() and brk(), maintains memory blocks through a doubly linked list, and demonstrates how memory allocators split, merge, expand, and shrink heap regions dynamically.

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Features

• Custom malloc and free implementation
• Heap management using sbrk() and brk()
• Doubly linked list based block management
• Best-fit allocation strategy
• Block splitting during allocation
• Block merging (coalescing) during free
• Heap expansion and shrinking
• Basic thread safety using a simple lock
• Memory/page inspection utilities
• Automated tests using assert

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Memory Layout

+----------------------+
| Allocator Header     |
|----------------------|
| magic bytes          |
| lock                 |
| amount of blocks     |
| amount of pages      |
+----------------------+

+----------------------+
| Block Header         |
|----------------------|
| marker               |
| prev pointer         |
| next pointer         |
| length               |
| in_use flag          |
+----------------------+
| User Memory          |
+----------------------+

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How It Works

Allocation (an_malloc)

• Searches for the smallest available free block
• Splits blocks if extra space remains
• Expands the heap using sbrk() if required
• Returns a pointer to usable memory

Free (an_free)

• Marks the block as free
• Merges adjacent free blocks
• Shrinks the heap when possible
• Clears released memory

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Concepts Explored

• Heap memory management
• Program break (brk / sbrk)
• Virtual memory pages
• Pointer arithmetic
• Raw memory traversal
• Linked lists
• Memory fragmentation
• Thread synchronization
• Linux process memory maps

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Project Structure

allocator.c
│
├── an_malloc()        -> custom malloc implementation
├── an_free()          -> custom free implementation
├── add_used_block()  -> allocation logic
├── reduce_heap_size_if_possible()
├── Heap/Page Experiments
└── Automated Tests

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Build & Run

Compile

gcc allocator.c -o allocator

Run

./allocator

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Example Usage

char *ptr = (char *)an_malloc(100);

ptr[0] = 'A';

an_free(ptr);

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Tests Included

• Basic allocation tests
• Large allocation tests
• Block splitting tests
• Block merging tests
• Heap growth/shrinking tests
• Complex allocation/free scenarios

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Limitations

This allocator is designed for educational purposes and is not production-ready.

Missing features include:

• Proper memory alignment
• calloc
• realloc
• Advanced synchronization primitives
• Double-free protection
• Optimized allocation bins
• Metadata protection

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Learning Goals

This project was built to better understand:

• How malloc() and free() work internally
• How operating systems manage heap memory
• Low-level systems programming concepts
• Memory safety and fragmentation
• Runtime allocator design

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