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Project: Memory Allocator

Implement a custom memory allocator that replaces the system malloc. This is the ultimate C project - you’ll understand exactly how dynamic memory works at the lowest level.

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Allocator Design

1

System Interface

Get memory from the kernel with sbrk/mmap
2

Block Management

Track free and allocated blocks
3

Fragmentation

Handle splitting and coalescing
4

Performance

Optimize for speed and memory efficiency

​
Block Structure

// malloc.h
#ifndef MY_MALLOC_H
#define MY_MALLOC_H

#include <stddef.h>
#include <stdint.h>
#include <stdbool.h>

// Block header - stored before each allocation
typedef struct block_header {
    size_t size;                  // Size of user data (not including header)
    bool is_free;                 // Is this block free?
    struct block_header *next;    // Next block in global list
    struct block_header *prev;    // Previous block in global list
    uint32_t magic;               // For corruption detection
} block_header_t;

#define HEADER_SIZE sizeof(block_header_t)
#define BLOCK_MAGIC 0xDEADBEEF
#define MIN_BLOCK_SIZE 16

// Alignment for all allocations
#define ALIGNMENT 16
#define ALIGN(size) (((size) + (ALIGNMENT-1)) & ~(ALIGNMENT-1))

// Public interface
void *my_malloc(size_t size);
void my_free(void *ptr);
void *my_calloc(size_t nmemb, size_t size);
void *my_realloc(void *ptr, size_t size);

// Debug
void my_malloc_stats(void);

#endif

​
Basic Implementation

// malloc.c
#define _GNU_SOURCE
#include "malloc.h"
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <stdio.h>
#include <pthread.h>

// Global state
static block_header_t *heap_start = NULL;
static block_header_t *heap_end = NULL;
static pthread_mutex_t malloc_lock = PTHREAD_MUTEX_INITIALIZER;

// Statistics
static size_t total_allocated = 0;
static size_t total_freed = 0;
static size_t num_allocations = 0;
static size_t num_frees = 0;

// Get more memory from the kernel
static void *request_memory(size_t size) {
    // For small allocations, use sbrk
    // For large allocations, use mmap
    
    size_t total = HEADER_SIZE + size;
    
    if (total < 128 * 1024) {  // < 128KB: use sbrk
        void *ptr = sbrk(total);
        if (ptr == (void*)-1) return NULL;
        return ptr;
    } else {
        // Large allocation: use mmap
        void *ptr = mmap(NULL, total,
                         PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS,
                         -1, 0);
        if (ptr == MAP_FAILED) return NULL;
        return ptr;
    }
}

// Find a free block of sufficient size
static block_header_t *find_free_block(size_t size) {
    block_header_t *current = heap_start;
    
    // First-fit strategy
    while (current) {
        if (current->is_free && current->size >= size) {
            return current;
        }
        current = current->next;
    }
    
    return NULL;
}

// Split a block if it's much larger than needed
static void split_block(block_header_t *block, size_t size) {
    size_t remaining = block->size - size - HEADER_SIZE;
    
    // Only split if remaining is large enough
    if (remaining < MIN_BLOCK_SIZE) return;
    
    // Create new block after this one
    block_header_t *new_block = (block_header_t*)((char*)block + HEADER_SIZE + size);
    new_block->size = remaining;
    new_block->is_free = true;
    new_block->magic = BLOCK_MAGIC;
    new_block->next = block->next;
    new_block->prev = block;
    
    if (block->next) {
        block->next->prev = new_block;
    } else {
        heap_end = new_block;
    }
    
    block->next = new_block;
    block->size = size;
}

// Coalesce adjacent free blocks
static void coalesce(block_header_t *block) {
    // Coalesce with next block
    if (block->next && block->next->is_free) {
        block->size += HEADER_SIZE + block->next->size;
        block->next = block->next->next;
        if (block->next) {
            block->next->prev = block;
        } else {
            heap_end = block;
        }
    }
    
    // Coalesce with previous block
    if (block->prev && block->prev->is_free) {
        block->prev->size += HEADER_SIZE + block->size;
        block->prev->next = block->next;
        if (block->next) {
            block->next->prev = block->prev;
        } else {
            heap_end = block->prev;
        }
    }
}

void *my_malloc(size_t size) {
    if (size == 0) return NULL;
    
    // Align size
    size = ALIGN(size);
    
    pthread_mutex_lock(&malloc_lock);
    
    // Try to find a free block
    block_header_t *block = find_free_block(size);
    
    if (block) {
        // Found a free block
        block->is_free = false;
        split_block(block, size);
    } else {
        // Need to request more memory
        block = request_memory(size);
        if (!block) {
            pthread_mutex_unlock(&malloc_lock);
            return NULL;
        }
        
        block->size = size;
        block->is_free = false;
        block->magic = BLOCK_MAGIC;
        block->next = NULL;
        block->prev = heap_end;
        
        if (heap_end) {
            heap_end->next = block;
        }
        heap_end = block;
        
        if (!heap_start) {
            heap_start = block;
        }
    }
    
    // Update stats
    total_allocated += block->size;
    num_allocations++;
    
    pthread_mutex_unlock(&malloc_lock);
    
    // Return pointer to user data (after header)
    return (void*)((char*)block + HEADER_SIZE);
}

void my_free(void *ptr) {
    if (!ptr) return;
    
    pthread_mutex_lock(&malloc_lock);
    
    // Get block header
    block_header_t *block = (block_header_t*)((char*)ptr - HEADER_SIZE);
    
    // Validate block
    if (block->magic != BLOCK_MAGIC) {
        fprintf(stderr, "my_free: corrupt heap detected!\n");
        pthread_mutex_unlock(&malloc_lock);
        return;
    }
    
    if (block->is_free) {
        fprintf(stderr, "my_free: double free detected!\n");
        pthread_mutex_unlock(&malloc_lock);
        return;
    }
    
    // Mark as free
    block->is_free = true;
    
    // Update stats
    total_freed += block->size;
    num_frees++;
    
    // Coalesce adjacent free blocks
    coalesce(block);
    
    pthread_mutex_unlock(&malloc_lock);
}

void *my_calloc(size_t nmemb, size_t size) {
    // Check for overflow
    size_t total = nmemb * size;
    if (nmemb != 0 && total / nmemb != size) {
        return NULL;  // Overflow
    }
    
    void *ptr = my_malloc(total);
    if (ptr) {
        memset(ptr, 0, total);
    }
    return ptr;
}

void *my_realloc(void *ptr, size_t size) {
    if (!ptr) return my_malloc(size);
    if (size == 0) {
        my_free(ptr);
        return NULL;
    }
    
    block_header_t *block = (block_header_t*)((char*)ptr - HEADER_SIZE);
    
    // If current block is big enough, just return it
    if (block->size >= size) {
        return ptr;
    }
    
    // Need to allocate new block
    void *new_ptr = my_malloc(size);
    if (!new_ptr) return NULL;
    
    memcpy(new_ptr, ptr, block->size);
    my_free(ptr);
    
    return new_ptr;
}

void my_malloc_stats(void) {
    pthread_mutex_lock(&malloc_lock);
    
    printf("=== Malloc Statistics ===\n");
    printf("Total allocated: %zu bytes\n", total_allocated);
    printf("Total freed: %zu bytes\n", total_freed);
    printf("In use: %zu bytes\n", total_allocated - total_freed);
    printf("Allocations: %zu\n", num_allocations);
    printf("Frees: %zu\n", num_frees);
    
    // Walk the heap
    size_t free_blocks = 0, used_blocks = 0;
    size_t free_bytes = 0, used_bytes = 0;
    
    block_header_t *block = heap_start;
    while (block) {
        if (block->is_free) {
            free_blocks++;
            free_bytes += block->size;
        } else {
            used_blocks++;
            used_bytes += block->size;
        }
        block = block->next;
    }
    
    printf("\nHeap blocks:\n");
    printf("  Free: %zu blocks, %zu bytes\n", free_blocks, free_bytes);
    printf("  Used: %zu blocks, %zu bytes\n", used_blocks, used_bytes);
    printf("  Fragmentation: %.2f%%\n", 
           free_bytes > 0 ? (double)free_blocks / (free_blocks + used_blocks) * 100 : 0);
    
    pthread_mutex_unlock(&malloc_lock);
}

​
Advanced: Best-Fit and Segregated Lists

// advanced_malloc.c
#include "malloc.h"

// Segregated free lists for different size classes
#define NUM_SIZE_CLASSES 8
static block_header_t *free_lists[NUM_SIZE_CLASSES];

// Size class boundaries: 16, 32, 64, 128, 256, 512, 1024, >1024
static size_t size_class_boundaries[] = {16, 32, 64, 128, 256, 512, 1024, SIZE_MAX};

static int get_size_class(size_t size) {
    for (int i = 0; i < NUM_SIZE_CLASSES; i++) {
        if (size <= size_class_boundaries[i]) {
            return i;
        }
    }
    return NUM_SIZE_CLASSES - 1;
}

static void add_to_free_list(block_header_t *block) {
    int class = get_size_class(block->size);
    
    // Insert at head
    block->next = free_lists[class];
    if (free_lists[class]) {
        free_lists[class]->prev = block;
    }
    block->prev = NULL;
    free_lists[class] = block;
}

static void remove_from_free_list(block_header_t *block) {
    int class = get_size_class(block->size);
    
    if (block->prev) {
        block->prev->next = block->next;
    } else {
        free_lists[class] = block->next;
    }
    
    if (block->next) {
        block->next->prev = block->prev;
    }
}

static block_header_t *find_best_fit(size_t size) {
    int class = get_size_class(size);
    
    // Search in appropriate size class and larger
    for (int c = class; c < NUM_SIZE_CLASSES; c++) {
        block_header_t *best = NULL;
        block_header_t *current = free_lists[c];
        
        while (current) {
            if (current->size >= size) {
                if (!best || current->size < best->size) {
                    best = current;
                    // Exact fit - stop searching
                    if (best->size == size) break;
                }
            }
            current = current->next;
        }
        
        if (best) {
            remove_from_free_list(best);
            return best;
        }
    }
    
    return NULL;
}

​
Interposition: Replace System malloc

// Replace system malloc with our implementation
// Compile with: gcc -shared -fPIC -o libmymalloc.so malloc.c
// Use with: LD_PRELOAD=./libmymalloc.so ./your_program

// Override standard functions
void *malloc(size_t size) {
    return my_malloc(size);
}

void free(void *ptr) {
    my_free(ptr);
}

void *calloc(size_t nmemb, size_t size) {
    return my_calloc(nmemb, size);
}

void *realloc(void *ptr, size_t size) {
    return my_realloc(ptr, size);
}

// Also needed for C++ programs
void *__libc_malloc(size_t size) { return my_malloc(size); }
void __libc_free(void *ptr) { my_free(ptr); }

​
Testing

// test_malloc.c
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "malloc.h"

void test_basic(void) {
    printf("Testing basic allocation...\n");
    
    int *p = my_malloc(sizeof(int));
    assert(p != NULL);
    *p = 42;
    assert(*p == 42);
    my_free(p);
    
    printf("  PASS\n");
}

void test_multiple(void) {
    printf("Testing multiple allocations...\n");
    
    void *ptrs[100];
    for (int i = 0; i < 100; i++) {
        ptrs[i] = my_malloc(64);
        assert(ptrs[i] != NULL);
        memset(ptrs[i], i, 64);
    }
    
    // Verify contents
    for (int i = 0; i < 100; i++) {
        char *p = ptrs[i];
        for (int j = 0; j < 64; j++) {
            assert(p[j] == (char)i);
        }
    }
    
    // Free odd indices
    for (int i = 1; i < 100; i += 2) {
        my_free(ptrs[i]);
        ptrs[i] = NULL;
    }
    
    // Allocate again - should reuse freed blocks
    for (int i = 1; i < 100; i += 2) {
        ptrs[i] = my_malloc(64);
        assert(ptrs[i] != NULL);
    }
    
    // Free all
    for (int i = 0; i < 100; i++) {
        my_free(ptrs[i]);
    }
    
    printf("  PASS\n");
}

void test_large(void) {
    printf("Testing large allocations...\n");
    
    void *p = my_malloc(1024 * 1024);  // 1MB
    assert(p != NULL);
    memset(p, 0xAB, 1024 * 1024);
    my_free(p);
    
    printf("  PASS\n");
}

void test_realloc(void) {
    printf("Testing realloc...\n");
    
    char *p = my_malloc(10);
    strcpy(p, "hello");
    
    p = my_realloc(p, 100);
    assert(strcmp(p, "hello") == 0);
    strcat(p, " world");
    
    my_free(p);
    
    printf("  PASS\n");
}

void test_calloc(void) {
    printf("Testing calloc...\n");
    
    int *arr = my_calloc(100, sizeof(int));
    for (int i = 0; i < 100; i++) {
        assert(arr[i] == 0);
    }
    my_free(arr);
    
    printf("  PASS\n");
}

int main(void) {
    printf("=== Memory Allocator Tests ===\n\n");
    
    test_basic();
    test_multiple();
    test_large();
    test_realloc();
    test_calloc();
    
    printf("\n=== All tests passed! ===\n");
    my_malloc_stats();
    
    return 0;
}

​
Performance Benchmarks

// benchmark.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "malloc.h"

#define ITERATIONS 100000

void benchmark_malloc(const char *name, void *(*alloc)(size_t), void (*dealloc)(void*)) {
    void *ptrs[ITERATIONS];
    
    clock_t start = clock();
    
    for (int i = 0; i < ITERATIONS; i++) {
        ptrs[i] = alloc(64);
    }
    
    for (int i = 0; i < ITERATIONS; i++) {
        dealloc(ptrs[i]);
    }
    
    clock_t end = clock();
    double seconds = (double)(end - start) / CLOCKS_PER_SEC;
    
    printf("%s: %.4f seconds (%d ops/sec)\n", 
           name, seconds, (int)(ITERATIONS * 2 / seconds));
}

int main(void) {
    printf("=== Malloc Benchmark ===\n");
    
    benchmark_malloc("System malloc", malloc, free);
    benchmark_malloc("My malloc", my_malloc, my_free);
    
    return 0;
}

​
Debugging Features

// Add memory debugging features

typedef struct {
    const char *file;
    int line;
    size_t size;
} alloc_info_t;

// Track allocations for leak detection
#define MAX_TRACKED 10000
static alloc_info_t tracked[MAX_TRACKED];
static size_t tracked_count = 0;

void *my_malloc_debug(size_t size, const char *file, int line) {
    void *ptr = my_malloc(size);
    if (ptr && tracked_count < MAX_TRACKED) {
        tracked[tracked_count].file = file;
        tracked[tracked_count].line = line;
        tracked[tracked_count].size = size;
        tracked_count++;
    }
    return ptr;
}

#define my_malloc(size) my_malloc_debug(size, __FILE__, __LINE__)

void my_malloc_check_leaks(void) {
    printf("=== Memory Leak Report ===\n");
    for (size_t i = 0; i < tracked_count; i++) {
        printf("Leaked %zu bytes at %s:%d\n",
               tracked[i].size, tracked[i].file, tracked[i].line);
    }
    if (tracked_count == 0) {
        printf("No leaks detected!\n");
    }
}

​
Learning Outcomes

  • How heap memory actually works
  • Block headers and metadata
  • Memory fragmentation and coalescing
  • Multiple allocation strategies (first-fit, best-fit, segregated lists)
  • Thread-safe allocator design
  • System calls: sbrk and mmap
  • Performance optimization techniques
  • Memory debugging and leak detection

​
Extensions

Arena Allocator

Fast bump allocation with bulk free

Pool Allocator

Fixed-size allocations for objects

Thread-Local Caching

Per-thread free lists like tcmalloc

Memory Pools

Pre-allocated pools for game engines

​
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