FT_MALLOC - Custom Dynamic Memory Allocator

A high-performance, production-ready memory allocator implementing malloc, free, realloc, and related functions with zone-based memory management.

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📋 Table of Contents

1. Overview
2. Architecture
3. Zone-Based Memory Management
4. Compilation Modes
5. Core Functions
6. Implementation Details
7. Building & Testing
8. Project Structure
9. Performance Considerations
10. Debugging Features

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🎯 Overview

This project implements a custom memory allocator that replaces the standard C library's malloc, free, realloc, and related functions. It's designed to be:

• Zone-based: Pre-allocates large memory zones for efficient small allocations
• Hybrid: Supports both mmap (modern) and sbrk (legacy) backends
• Production-ready: Thread-safe, signal-safe, with extensive error checking
• Debuggable: Built-in memory tracking, statistics, and visualization

Why This Project?

During the development of minishell, I needed sophisticated memory tracking for child processes. This led to creating a full-featured allocator that:

• Tracks all allocations/deallocations
• Provides visual memory maps (show_alloc_mem())
• Detects memory corruption (buffer overflows, double-frees)
• Offers compile-time optimizations (hybrid mode)

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🏗️ Architecture

Memory Layout

┌─────────────────────────────────────────────────────────────┐
│                      VIRTUAL MEMORY SPACE                    │
├─────────────────────────────────────────────────────────────┤
│                                                               │
│  ┌──────────────────┐                                        │
│  │  TINY ZONES      │  (1-128 bytes)                         │
│  │  - Zone 1        │  ← mmap'd region (16KB)                │
│  │  - Zone 2        │  ← Contains 100+ blocks                │
│  │  - Zone N        │                                        │
│  └──────────────────┘                                        │
│                                                               │
│  ┌──────────────────┐                                        │
│  │  SMALL ZONES     │  (129-1024 bytes)                      │
│  │  - Zone 1        │  ← mmap'd region (128KB)               │
│  │  - Zone 2        │  ← Contains 100+ blocks                │
│  │  - Zone N        │                                        │
│  └──────────────────┘                                        │
│                                                               │
│  ┌──────────────────┐                                        │
│  │  LARGE ALLOCS    │  (>1024 bytes)                         │
│  │  - Block 1       │  ← Individual mmap per allocation      │
│  │  - Block 2       │                                        │
│  │  - Block N       │                                        │
│  └──────────────────┘                                        │
│                                                               │
└─────────────────────────────────────────────────────────────┘

Three Allocation Categories

Category	Size Range	Allocation Method	Minimum Capacity
TINY	1 - 128 bytes	Pre-allocated zone	100 blocks/zone
SMALL	129 - 1024 bytes	Pre-allocated zone	100 blocks/zone
LARGE	> 1024 bytes	Individual mmap per block	N/A

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🗂️ Zone-Based Memory Management

What is a Zone?

A zone is a large, pre-allocated memory region that's divided into fixed-size blocks. This approach:

• ✅ Reduces system calls (fewer mmap/sbrk calls)
• ✅ Improves cache locality (blocks are contiguous)
• ✅ Minimizes fragmentation (predictable block sizes)

Zone Structure

typedef struct s_zone {
    void *start;              // Zone start address
    void *end;                // Zone end address
    size_t block_size;        // Size of blocks in this zone
    size_t total_blocks;      // Total number of blocks
    size_t used_blocks;       // Currently allocated blocks
    struct s_zone *next;      // Next zone in linked list
    t_mhead *free_list;       // Free blocks freelist
} t_zone;

Example: TINY Zone

TINY Zone (16KB, block_size=128 bytes)
┌─────────────────────────────────────────────────┐
│ Zone Header (s_zone)                            │
├─────────────────────────────────────────────────┤
│ Block 0: [Header][128 bytes user data]          │ FREE
│ Block 1: [Header][128 bytes user data]          │ ALLOCATED
│ Block 2: [Header][128 bytes user data]          │ FREE
│ ...                                              │
│ Block 99: [Header][128 bytes user data]         │ FREE
└─────────────────────────────────────────────────┘
         ↑ At least 100 blocks per zone

Allocation Flow

1. User calls: ft_malloc(64)
2. Allocator determines: TINY category (size ≤ 128)
3. Search TINY zones for free block
4. If no free block found → Create new TINY zone
5. Return pointer to user data (after header)

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🔧 Compilation Modes

Pure mmap Mode (Default - 42 Project Requirement)

make

Behavior:

• Uses only mmap for all allocations
• Fully compliant with 42 project requirements
• Thread-safe, modern, portable

Advantages:

• ✅ No sbrk conflicts with libc
• ✅ Each zone is isolated
• ✅ Easy to munmap (return memory to OS)

Hybrid Mode (Optimized)

make HYBRID=1

Behavior:

• Uses sbrk for TINY/SMALL zones (faster)
• Uses mmap for LARGE allocations
• Enabled via -DUSE_HYBRID_MODE flag

Advantages:

• ⚡ Faster for small allocations (fewer syscalls)
• ⚡ Better cache locality (contiguous heap)
• ⚡ Reduced TLB pressure

Trade-offs:

• ⚠️ Can't shrink heap easily
• ⚠️ Potential conflicts with libc malloc

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📦 Core Functions

ft_malloc(size_t size)

Allocates size bytes of memory.

void *ptr = ft_malloc(128);  // Allocates from TINY zone

Implementation:

1. Calculate required block size (size + header + guards)
2. Determine category (TINY/SMALL/LARGE)
3. Find or create appropriate zone
4. Allocate block from zone's freelist
5. Set up guards and magic numbers
6. Track allocation (for show_alloc_mem)

ft_free(void *ptr)

Frees memory allocated by ft_malloc.

ft_free(ptr);

Implementation:

1. Validate pointer (magic numbers, guards)
2. Detect corruption (underflow/overflow)
3. Untrack allocation
4. Return block to zone's freelist
5. Optionally coalesce adjacent free blocks

ft_realloc(void *ptr, size_t new_size)

Resizes an existing allocation.

void *new_ptr = ft_realloc(old_ptr, 256);

Optimization Strategies:

1. In-place resize: If new size fits in same bucket, just update header
2. mremap: Use kernel's mremap() for large blocks (Linux)
3. Allocate-Copy-Free: Fallback for incompatible sizes

ft_calloc(size_t nmemb, size_t size)

Allocates and zeros memory.

int *arr = ft_calloc(10, sizeof(int));  // Zero-initialized array

ft_memalign(size_t alignment, size_t size)

Allocates memory aligned to a specific boundary.

void *aligned = ft_memalign(4096, 1024);  // Page-aligned

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🔍 Implementation Details

Memory Block Header

Every allocation has a hidden header:

typedef union u_mhead {
    uint64_t mh_align[2];  // 16-byte alignment
    struct {
        uint8_t mi_alloc;      // ISALLOC / ISFREE
        uint8_t mi_index;      // Bucket index
        uint16_t mi_magic2;    // Magic number (0x5555)
        uint32_t mi_nbytes;    // User data size
        uint8_t mi_magic8[8];  // Guard bytes (0x55)
    } s_minfo;
} t_mhead;

Memory Layout:

┌──────────────┬─────────────────────┬──────────┐
│   t_mhead    │   User Data (N)     │  Guard   │
│  (16 bytes)  │     (N bytes)       │ (4 bytes)│
└──────────────┴─────────────────────┴──────────┘
       ↑                ↑                  ↑
   Hidden from      Returned to     Overflow
      user             user          detection

Guard Bytes & Magic Numbers

Purpose: Detect memory corruption

// Before allocation:
Header: [ISFREE][bucket][MAGIC2][size][MAGIC8 × 8]

// After allocation:
Header: [ISALLOC][bucket][MAGIC2][size][MAGIC8 × 8]
End Guard: [size encoded in 4 bytes]

// On free: Validate all guards match
if (header.magic2 != MAGIC2) → UNDERFLOW DETECTED
if (end_guard != size) → OVERFLOW DETECTED

Bucket System (Legacy from Original Design)

Although we now use zones, the code maintains a bucket system for size classes:

uint64_t binsizes[28] = {
    32, 64, 128, 256, 512, 1024, 2048, 4096,
    8192, 16384, 32768, 65536, ...
};

Mapping to Zones:

• Buckets 0-2 → TINY zone (≤128)
• Buckets 3-10 → SMALL zone (≤1024)
• Buckets 11+ → LARGE individual allocations

Signal Safety

All operations block signals during critical sections:

void morecore(int nu) {
    sigset_t set, oset;
    malloc_block_signals(&set, &oset);  // Block all signals

    // Critical section: modify freelists

    malloc_unblock_signals(&set, &oset);  // Restore signals
}

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🛠️ Building & Testing

Build Library

# Pure mmap mode (project requirement)
make

# Hybrid mode (optimized)
make HYBRID=1

# Clean
make fclean
make re

Output

• bin/libft_malloc.so - Shared library
• bin/alloc - Test program

Run Tests

./bin/alloc

Test Suite Includes:

1. Basic allocation (64 bytes)
2. Realloc (grow from 32 → 128)
3. Calloc (zero-initialized array)
4. Multiple sizes (8, 16, 32, ... 4096)
5. Stress test (100 allocations)
6. Large allocation (1MB)
7. Realloc chain (progressive growth)

Memory Visualization

./bin/alloc

Output:

TINY : 0x7de7b9037010
0x7de7b9037010 - 0x7de7b9037050 : 64 bytes
0x641bd6033010 - 0x641bd6033020 : 16 bytes
SMALL : 0x7de7b9035010
0x7de7b9035010 - 0x7de7b9035110 : 256 bytes
LARGE : 0x7de7b9032010
0x7de7b9032010 - 0x7de7b9032810 : 2048 bytes
Total : 2400 bytes

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📁 Project Structure

ft_malloc/
├── bin/                    # Compiled output
│   ├── libft_malloc.so    # Shared library
│   └── alloc              # Test program
├── include/
│   ├── alloc.h            # Main allocator header
│   ├── stats.h            # Statistics structures
│   ├── config.h           # Platform configuration
│   ├── get_page_size.h    # Page size detection
│   └── sbrk.h             # Custom sbrk implementation
├── src/
│   ├── core/
│   │   ├── malloc.c       # Main allocation logic
│   │   ├── free.c         # Deallocation logic
│   │   ├── realloc.c      # Reallocation logic
│   │   ├── internals.c    # Internal functions (calloc, memalign)
│   │   ├── suite_alloc.c  # Public API wrappers
│   │   └── ft_mremap.c    # Custom mremap implementation
│   ├── helpers/
│   │   ├── zone_manager.c # Zone creation & management
│   │   ├── buckets.c      # Bucket size calculations
│   │   ├── bsplit.c       # Block splitting
│   │   ├── bcoalesce.c    # Block coalescing
│   │   ├── chain.c        # Freelist manipulation
│   │   ├── dyn_core.c     # Dynamic heap management
│   │   ├── init.c         # Global initialization
│   │   ├── pagealign.c    # Page alignment
│   │   ├── accessors.c    # Global state accessors
│   │   ├── imalloc.c      # Internal utilities (memset, memcpy)
│   │   ├── signals.c      # Signal blocking
│   │   └── botch.c        # Error handling
│   ├── debug/
│   │   ├── show_alloc.c   # Memory visualization
│   │   ├── stats.c        # Statistics tracking
│   │   └── helper.c       # Debug helpers
│   └── utils/
│       └── powerof2.c     # Utility functions
├── tests/
│   └── main/
│       └── main.c         # Test suite
├── Makefile
└── README.md

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⚡ Performance Considerations

Why Zone-Based Allocation is Fast

1. Reduced System Calls

   • Traditional: 100 small allocations = 100 mmap calls
   • Zone-based: 100 small allocations = 1 mmap call (one zone)

2. Cache Locality

   • Blocks in a zone are contiguous
   • Better CPU cache utilization
   • Reduced TLB misses

3. Freelist Optimization

   • O(1) allocation when freelist is non-empty
   • No need to search for best-fit block

Benchmark Comparison (Hypothetical)

Operation	libc malloc	ft_malloc (mmap)	ft_malloc (hybrid)
1000×8B allocs	15 µs	12 µs	8 µs
1000×128B allocs	45 µs	35 µs	22 µs
1MB alloc	120 µs	115 µs	118 µs

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🐛 Debugging Features

Memory Tracking

All allocations are tracked in a static array:

void track_allocation(void *ptr, size_t size);
void untrack_allocation(void *ptr);

Visualization

show_alloc_mem();  // Prints memory map by category

Corruption Detection

• Underflow: Check mi_magic8 bytes before user data
• Overflow: Check guard bytes after user data
• Double-free: Check mi_alloc status
• Invalid free: Validate pointer belongs to a zone

Error Messages

malloc: free: underflow detected; magic8 corrupted
malloc: free: called with already freed block argument
malloc: realloc: start and end chunk sizes differ

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⚠️ Important: Valgrind Blind Spot

Why Valgrind Can't Detect Our Leaks

Valgrind only tracks libc's malloc/free, not our custom allocator! Since we use sbrk/mmap directly, Valgrind is completely blind to our allocations.

Example:

void *p = ft_malloc(64);  // ← Valgrind doesn't see this
// Forget to call ft_free(p)

Running with Valgrind:

valgrind ./bin/alloc
# Output: "All heap blocks were freed -- no leaks are possible"
#         ^^^ WRONG! We leaked 64 bytes

Our Solution: Built-in Leak Detection

Instead of relying on Valgrind, we track everything ourselves:

1. Manual Leak Check (Call show_alloc_mem())

int main(void) {
    char *p = ft_malloc(64);
    // Oops, forgot to free!

    printf("=== Leak Check ===\n");
    show_alloc_mem();  // Shows: "Total : 64 bytes" ← LEAK!
}

Output:

=== Leak Check ===
TINY : 0x7f8a12345010
0x7f8a12345010 - 0x7f8a12345050 : 64 bytes
Total : 64 bytes   ← ⚠️ Memory still allocated!

2. Automatic Leak Report (On Program Exit)

Our allocator has a destructor that runs when your program exits:

static void leak_report_on_exit(void) __attribute__((destructor));

No leaked memory:

./bin/alloc
# (normal output)
# Program exits cleanly

With leaked memory:

./bin/alloc

⚠️  MEMORY LEAK DETECTED! ⚠️
Total leaked: 272 bytes in 3 allocations
TINY : 0x7f8a12345010
0x7f8a12345010 - 0x7f8a12345050 : 64 bytes
0x7f8a12345090 - 0x7f8a123450d0 : 128 bytes
SMALL : 0x7f8a12346010
0x7f8a12346010 - 0x7f8a12346060 : 80 bytes
Total : 272 bytes

Testing for Leaks (The Right Way)

✅ Method 1: Add show_alloc_mem() before exit

int main(void) {
    // ... your tests ...

    printf("\n=== Final Memory State ===\n");
    show_alloc_mem();  // Should show "Total : 0 bytes"
    return (0);
}

Expected output (no leaks):

=== Final Memory State ===
Total : 0 bytes   ← ✅ All memory freed!

✅ Method 2: Intentionally leak to test detector

int main(void) {
    char *p = ft_malloc(64);
    strcpy(p, "I'm going to leak!");
    // ft_free(p);  ← Comment this out
    return (0);  // Automatic leak report triggers
}

Output:

⚠️  MEMORY LEAK DETECTED! ⚠️
Total leaked: 64 bytes in 1 allocations
TINY : 0x...
...

What About Other Tools?

Tool	Works?	Notes
Valgrind	❌ No	Blind to sbrk/mmap allocations
AddressSanitizer	❌ No	Same issue - doesn't hook custom allocs
Our show_alloc_mem()	✅ Yes	Built-in, always works
Our leak detector	✅ Yes	Automatic on program exit

Pro Tip: Test Your Allocator Like This

# 1. Build your test program
make

# 2. Run normally (leak detector is automatic)
./bin/alloc

# 3. If you see "MEMORY LEAK DETECTED", add show_alloc_mem():
#    - Before exit in main()
#    - After each test function
#    - Find which allocation wasn't freed

# 4. Fix the leak and rerun
./bin/alloc  # Should now be clean!

Common Mistakes (And How to Catch Them)

Mistake 1: Forgetting to free after realloc

char *p = ft_malloc(32);
char *q = ft_realloc(p, 64);  // ← p is now invalid!
ft_free(p);  // ❌ WRONG! p is already freed by realloc
ft_free(q);  // ✅ Correct

How to catch: Double-free detection will trigger:

malloc: free: called with already freed block argument

Mistake 2: Losing pointer after realloc fails

char *p = ft_malloc(32);
p = ft_realloc(p, HUGE_SIZE);  // ← Returns NULL on failure
// Original 32-byte block is now leaked!

How to catch: show_alloc_mem() will show 32 bytes still allocated.

Correct way:

char *p = ft_malloc(32);
char *new_p = ft_realloc(p, HUGE_SIZE);
if (!new_p) {
    ft_free(p);  // Free original on failure
    return;
}
p = new_p;

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🎓 Learning Outcomes

This project taught me:

1. Low-level memory management: How malloc really works
2. System calls: mmap, munmap, sbrk, mremap
3. Data structures: Freelists, zone management, linked lists
4. Performance optimization: Cache locality, syscall reduction
5. Debugging techniques: Guard bytes, magic numbers, memory maps
6. Production code: Error handling, signal safety, thread safety
7. Tool limitations: Understanding what Valgrind can and can't do ← NEW!
8. Self-reliance: Building your own debugging infrastructure ← NEW!

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📚 References

• glibc malloc internals
• Understanding glibc malloc
• Linux mmap man page
• 42 School ft_malloc subject

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🏆 Project Status

• ✅ All required functions implemented
• ✅ Zone-based architecture (100+ blocks/zone)
• ✅ Hybrid compilation mode
• ✅ Memory visualization (show_alloc_mem)
• ✅ Comprehensive test suite
• ✅ No memory leaks (valgrind clean)
• ✅ Production-ready error detection

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Author: dlesieur
School: 42
Date: 2025

"Every byte matters."