c-memory-management

Original source / Raw SKILL.md

---
name: c-memory-management
description: Use when managing memory in C programs with malloc/free, pointers, and avoiding common memory safety pitfalls.
allowed-tools:
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---

# C Memory Management

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

```bash
# 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

```bash
# 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

```c
// 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

```c
// 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

```c
#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

```c
// 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

```c
// 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

```c
// 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

```c
// 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

```c
// 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

```c
// 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

```c
#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

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

## Common Pitfalls

1. **Memory leaks** - Forgetting to free allocated memory
2. **Double free** - Freeing same pointer twice
3. **Use after free** - Accessing freed memory
4. **Buffer overflow** - Writing beyond allocated bounds
5. **Dangling pointers** - Using pointers after free
6. **Null pointer dereference** - Not checking for NULL
7. **sizeof mistakes** - Using wrong size calculations
8. **Stack overflow** - Large stack allocations
9. **Uninitialized memory** - Reading uninitialized data
10. **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

## Resources

- [The C Programming Language (K&R)](https://www.amazon.com/Programming-Language-2nd-Brian-Kernighan/dp/0131103628)
- [C Memory Management](https://en.cppreference.com/w/c/memory)
- [Valgrind Documentation](https://valgrind.org/docs/manual/manual.html)
- [Modern C by Jens Gustedt](https://modernc.gforge.inria.fr/)
- [C Standard Library Reference](https://en.cppreference.com/w/c)