MemoryManager — Custom Memory Allocator in C++

A low-level memory allocator built from scratch using sbrk() system calls, a doubly-linked list of memory blocks, and configurable allocation strategies (best-fit and worst-fit). Includes hole coalescing, bitmap memory visualization, and Valgrind-verified leak-free operation.

What This Is

A userspace memory manager that replaces malloc/free with custom allocation logic. It manages a contiguous block of memory obtained via sbrk(), tracks allocated and free blocks using a doubly-linked list, and implements two allocation strategies for studying memory fragmentation behavior.

How It Works

graph LR
    Init["initialize(size)<br/><i>sbrk() to get<br/>memory from OS</i>"] --> Pool["Memory Pool<br/><i>Doubly-linked list<br/>of blocks</i>"]

    Pool --> Alloc["allocate(size)"]
    Pool --> Free["free(pointer)"]

    Alloc --> BF["Best Fit<br/><i>Smallest sufficient<br/>hole</i>"]
    Alloc --> WF["Worst Fit<br/><i>Largest hole</i>"]

    Free --> Coal["Coalesce<br/><i>Merge adjacent<br/>free blocks</i>"]

    Pool --> Bitmap["getBitmap()<br/><i>Bit-level memory<br/>status map</i>"]
    Pool --> Dump["dumpMemoryMap()<br/><i>Write layout<br/>to file</i>"]

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Technical Highlights

• sbrk() Memory Acquisition — Requests raw memory from the OS using the sbrk() system call, managing it entirely in userspace with no reliance on the standard allocator.
• Best-Fit / Worst-Fit Strategies — Two configurable allocation algorithms. Best-fit minimizes wasted space per allocation; worst-fit reduces fragmentation by keeping large holes available.
• Hole Coalescing — When a block is freed, the allocator checks adjacent blocks and merges contiguous free blocks to prevent fragmentation buildup.
• Memory Alignment — Handles proper alignment requirements so allocated blocks respect CPU word boundaries.
• Bitmap Visualization — getBitmap() returns a bit-level representation of memory status (allocated vs. free), useful for debugging and understanding fragmentation patterns.
• Memory Map Dump — dumpMemoryMap() writes the complete block list (addresses, sizes, status) to a file for analysis.
• Valgrind Verified — Tested with Valgrind to confirm zero memory leaks and no invalid memory access.

Core API

void  initialize(unsigned int size, bool bestFit);  // Get memory from OS
void* allocate(unsigned int size);                   // Allocate a block
void  free(void* pointer);                           // Free a block (+ coalesce)
vector<bool> getBitmap();                            // Bit-level memory status
vector<pair<void*, unsigned int>> getList();          // List of free holes
void  dumpMemoryMap(string filename);                // Write layout to file

Building & Running

make              # Build with g++
./memmanager      # Run
make valgrind     # Run with Valgrind leak checking

What I Learned

Building a memory allocator from scratch gives you an appreciation for what malloc does behind the scenes — and why fragmentation, alignment, and coalescing matter in systems programming. It's one of the best exercises for understanding how operating systems manage the resource that every program depends on.