Overview

Master manual memory management in C with proper allocation, deallocation, pointer handling, and techniques to avoid memory leaks and corruption.

Overview

C requires manual memory management through explicit allocation and deallocation. Understanding pointers, the heap, and proper memory handling is crucial for writing safe and efficient C programs.

Installation and Setup

Compiler and Tools

# Install GCC compiler
# macOS
xcode-select --install

# Linux (Ubuntu/Debian)
sudo apt-get install build-essential

# Check installation
gcc --version

# Memory debugging tools
# Install Valgrind (Linux)
sudo apt-get install valgrind

# Install Address Sanitizer (built into GCC/Clang)
gcc -fsanitize=address -g program.c -o program

Compilation Flags

# Basic compilation
gcc program.c -o program

# With warnings and debugging
gcc -Wall -Wextra -g program.c -o program

# With Address Sanitizer
gcc -fsanitize=address -g program.c -o program

# With optimization
gcc -O2 -Wall program.c -o program

Core Patterns

1. Dynamic Memory Allocation

// malloc - allocate memory
#include <stdlib.h>
#include <string.h>

int* allocate_array(size_t size) {
    int* arr = malloc(size * sizeof(int));
    if (arr == NULL) {
        return NULL;  // Allocation failed
    }
    return arr;
}

// calloc - allocate and zero-initialize
int* allocate_zeroed_array(size_t size) {
    int* arr = calloc(size, sizeof(int));
    if (arr == NULL) {
        return NULL;
    }
    return arr;
}

// realloc - resize allocation
int* resize_array(int* arr, size_t old_size, size_t new_size) {
    int* new_arr = realloc(arr, new_size * sizeof(int));
    if (new_arr == NULL && new_size > 0) {
        // Reallocation failed, original array still valid
        return NULL;
    }
    return new_arr;
}

// free - deallocate memory
void cleanup_array(int** arr) {
    if (arr != NULL && *arr != NULL) {
        free(*arr);
        *arr = NULL;  // Prevent dangling pointer
    }
}

2. Pointer Basics

// Pointer declaration and usage
void pointer_basics() {
    int value = 42;
    int* ptr = &value;  // ptr points to value
    
    printf("Value: %d\n", value);
    printf("Address: %p\n", (void*)&value);
    printf("Pointer: %p\n", (void*)ptr);
    printf("Dereferenced: %d\n", *ptr);
    
    *ptr = 100;  // Modify through pointer
    printf("New value: %d\n", value);
}

// Null pointers
void null_pointer_check(int* ptr) {
    if (ptr == NULL) {
        printf("Null pointer\n");
        return;
    }
    printf("Valid pointer: %d\n", *ptr);
}

// Pointer arithmetic
void pointer_arithmetic() {
    int arr[] = {10, 20, 30, 40, 50};
    int* ptr = arr;
    
    for (int i = 0; i < 5; i++) {
        printf("%d ", *(ptr + i));  // Same as ptr[i]
    }
    printf("\n");
}

3. Dynamic Strings

#include <string.h>

// Create string copy
char* string_duplicate(const char* str) {
    if (str == NULL) {
        return NULL;
    }
    
    size_t len = strlen(str);
    char* copy = malloc(len + 1);  // +1 for null terminator
    
    if (copy != NULL) {
        strcpy(copy, str);
    }
    
    return copy;
}

// String concatenation
char* string_concat(const char* s1, const char* s2) {
    if (s1 == NULL || s2 == NULL) {
        return NULL;
    }
    
    size_t len1 = strlen(s1);
    size_t len2 = strlen(s2);
    
    char* result = malloc(len1 + len2 + 1);
    if (result == NULL) {
        return NULL;
    }
    
    strcpy(result, s1);
    strcat(result, s2);
    
    return result;
}

// Safe string functions
char* safe_string_copy(const char* src, size_t max_len) {
    if (src == NULL) {
        return NULL;
    }
    
    size_t len = strnlen(src, max_len);
    char* dest = malloc(len + 1);
    
    if (dest != NULL) {
        memcpy(dest, src, len);
        dest[len] = '\0';
    }
    
    return dest;
}

4. Structures and Memory

// Structure with dynamic members
typedef struct {
    char* name;
    int* scores;
    size_t num_scores;
} Student;

Student* create_student(const char* name, size_t num_scores) {
    Student* student = malloc(sizeof(Student));
    if (student == NULL) {
        return NULL;
    }
    
    student->name = string_duplicate(name);
    if (student->name == NULL) {
        free(student);
        return NULL;
    }
    
    student->scores = malloc(num_scores * sizeof(int));
    if (student->scores == NULL) {
        free(student->name);
        free(student);
        return NULL;
    }
    
    student->num_scores = num_scores;
    memset(student->scores, 0, num_scores * sizeof(int));
    
    return student;
}

void destroy_student(Student** student) {
    if (student == NULL || *student == NULL) {
        return;
    }
    
    free((*student)->name);
    free((*student)->scores);
    free(*student);
    *student = NULL;
}

5. Memory Pools

// Simple memory pool
typedef struct {
    void* pool;
    size_t block_size;
    size_t num_blocks;
    size_t next_free;
} MemoryPool;

MemoryPool* create_pool(size_t block_size, size_t num_blocks) {
    MemoryPool* pool = malloc(sizeof(MemoryPool));
    if (pool == NULL) {
        return NULL;
    }
    
    pool->pool = malloc(block_size * num_blocks);
    if (pool->pool == NULL) {
        free(pool);
        return NULL;
    }
    
    pool->block_size = block_size;
    pool->num_blocks = num_blocks;
    pool->next_free = 0;
    
    return pool;
}

void* pool_allocate(MemoryPool* pool) {
    if (pool == NULL || pool->next_free >= pool->num_blocks) {
        return NULL;
    }
    
    void* block = (char*)pool->pool + (pool->next_free * pool->block_size);
    pool->next_free++;
    
    return block;
}

void destroy_pool(MemoryPool** pool) {
    if (pool == NULL || *pool == NULL) {
        return;
    }
    
    free((*pool)->pool);
    free(*pool);
    *pool = NULL;
}

6. Reference Counting

// Reference counted string
typedef struct {
    char* data;
    size_t ref_count;
} RefString;

RefString* refstring_create(const char* str) {
    RefString* rs = malloc(sizeof(RefString));
    if (rs == NULL) {
        return NULL;
    }
    
    rs->data = string_duplicate(str);
    if (rs->data == NULL) {
        free(rs);
        return NULL;
    }
    
    rs->ref_count = 1;
    return rs;
}

RefString* refstring_retain(RefString* rs) {
    if (rs != NULL) {
        rs->ref_count++;
    }
    return rs;
}

void refstring_release(RefString** rs) {
    if (rs == NULL || *rs == NULL) {
        return;
    }
    
    (*rs)->ref_count--;
    
    if ((*rs)->ref_count == 0) {
        free((*rs)->data);
        free(*rs);
    }
    
    *rs = NULL;
}

7. Memory Debugging

// Debug wrapper for malloc
#ifdef DEBUG_MEMORY
typedef struct {
    void* ptr;
    size_t size;
    const char* file;
    int line;
} AllocationInfo;

static AllocationInfo allocations[1000];
static size_t num_allocations = 0;

void* debug_malloc(size_t size, const char* file, int line) {
    void* ptr = malloc(size);
    if (ptr != NULL && num_allocations < 1000) {
        allocations[num_allocations].ptr = ptr;
        allocations[num_allocations].size = size;
        allocations[num_allocations].file = file;
        allocations[num_allocations].line = line;
        num_allocations++;
    }
    return ptr;
}

void debug_free(void* ptr) {
    for (size_t i = 0; i < num_allocations; i++) {
        if (allocations[i].ptr == ptr) {
            allocations[i] = allocations[num_allocations - 1];
            num_allocations--;
            break;
        }
    }
    free(ptr);
}

void print_leaks() {
    printf("Memory leaks: %zu\n", num_allocations);
    for (size_t i = 0; i < num_allocations; i++) {
        printf("  %p (%zu bytes) at %s:%d\n",
               allocations[i].ptr,
               allocations[i].size,
               allocations[i].file,
               allocations[i].line);
    }
}

#define malloc(size) debug_malloc(size, __FILE__, __LINE__)
#define free(ptr) debug_free(ptr)
#endif

8. Stack vs Heap

// Stack allocation
void stack_example() {
    int local_var = 42;  // Stack
    char buffer[100];    // Stack
    
    // Automatically freed when function returns
}

// Heap allocation
void heap_example() {
    int* dynamic = malloc(sizeof(int));  // Heap
    if (dynamic != NULL) {
        *dynamic = 42;
        free(dynamic);  // Must manually free
    }
}

// Mixed allocation
typedef struct {
    int id;                  // Stack (part of struct)
    char* name;             // Heap (pointer to heap)
} Record;

Record* create_record(int id, const char* name) {
    Record* rec = malloc(sizeof(Record));  // Heap
    if (rec == NULL) {
        return NULL;
    }
    
    rec->id = id;  // Stack value
    rec->name = string_duplicate(name);  // Heap
    
    if (rec->name == NULL) {
        free(rec);
        return NULL;
    }
    
    return rec;
}

9. Double Free Prevention

// Safe free macro
#define SAFE_FREE(ptr) do { \
    if (ptr != NULL) { \
        free(ptr); \
        ptr = NULL; \
    } \
} while(0)

// Usage
void safe_cleanup() {
    int* arr = malloc(10 * sizeof(int));
    
    // ... use arr ...
    
    SAFE_FREE(arr);
    // arr is now NULL, can safely call again
    SAFE_FREE(arr);  // No-op, safe
}

// Reference clearing
void clear_reference(void** ref) {
    if (ref != NULL && *ref != NULL) {
        free(*ref);
        *ref = NULL;
    }
}

10. Memory Alignment

#include <stdalign.h>

// Aligned allocation
void* aligned_malloc(size_t size, size_t alignment) {
    void* ptr = NULL;
    
    #ifdef _WIN32
        ptr = _aligned_malloc(size, alignment);
    #else
        if (posix_memalign(&ptr, alignment, size) != 0) {
            return NULL;
        }
    #endif
    
    return ptr;
}

void aligned_free(void* ptr) {
    #ifdef _WIN32
        _aligned_free(ptr);
    #else
        free(ptr);
    #endif
}

// Structure alignment
typedef struct {
    alignas(16) double values[4];  // 16-byte aligned
} AlignedData;

Best Practices

Always check malloc return value - Handle allocation failures
Free all allocated memory - Prevent memory leaks
Set pointers to NULL after free - Avoid dangling pointers
Use sizeof with types - Ensure correct allocation size
Initialize allocated memory - Use calloc or memset
Match malloc/free calls - Every allocation needs deallocation
Use valgrind for testing - Detect memory errors
Avoid manual pointer arithmetic - Use array indexing when possible
Handle realloc failures - Keep original pointer valid
Document ownership - Clarify who frees memory

Common Pitfalls

Memory leaks - Forgetting to free allocated memory
Double free - Freeing same pointer twice
Use after free - Accessing freed memory
Buffer overflow - Writing beyond allocated bounds
Dangling pointers - Using pointers after free
Null pointer dereference - Not checking for NULL
sizeof mistakes - Using wrong size calculations
Stack overflow - Large stack allocations
Uninitialized memory - Reading uninitialized data
Memory fragmentation - Poor allocation patterns

When to Use

Systems programming requiring manual control
Embedded systems with limited resources
Performance-critical applications
Operating system development
Device drivers and kernel modules
Real-time systems
Legacy codebase maintenance
Interfacing with hardware
Memory-constrained environments
Low-level library development

📖 c-memory-management

Overview

Overview

Installation and Setup

Compiler and Tools

Compilation Flags

Core Patterns

1. Dynamic Memory Allocation

2. Pointer Basics

3. Dynamic Strings

4. Structures and Memory

5. Memory Pools

6. Reference Counting

7. Memory Debugging

8. Stack vs Heap

9. Double Free Prevention

10. Memory Alignment

Best Practices

Common Pitfalls

When to Use

Resources