Module 12: Dynamic Memory Allocation (Heap Management)

1. Stack vs Heap 1 / 4

Stack Memory vs Heap Memory Dynamics

Automatic scoped allocation vs Manual runtime management

Feature	Stack Memory	Heap Memory
Management	Automatic by CPU / Compiler	Manual by Programmer (`malloc` / `free`)
Allocation Speed	Ultra-fast (Single CPU instruction pointer decrement)	Moderate (Searches free memory chunk lists)
Capacity Limit	Small (e.g. 8 MB per thread on macOS/Linux)	Large (Limited only by physical RAM + Swap)
Lifetime	Destroyed when function exits	Persists until explicitly `free()`d or process exits

2. Allocation Functions 2 / 4

malloc, calloc, realloc, and free APIs

Managing heap allocations in <stdlib.h>

heap_apis.c

// 1. malloc: Allocates uninitialized raw bytes (contains garbage!)
int *m_arr = malloc(100 * sizeof(int));

// 2. calloc: Allocates memory and sets every single byte to ZERO
int *c_arr = calloc(100, sizeof(int));

// 3. Safe realloc pattern:
int *temp = realloc(m_arr, 200 * sizeof(int));
if (temp != NULL) {
    m_arr = temp; // Only update original pointer if realloc succeeded!
}

// 4. Free memory and neutralize pointer:
free(m_arr);
m_arr = NULL; // Prevents Dangling Pointer and Double-Free bugs

3. Memory Bugs 3 / 4

Common Heap Memory Bugs & Exploits

Memory Leaks, Double Free, Use-After-Free, and Buffer Overflows

Bug Name	Cause	Consequence
Memory Leak	Allocating heap memory but losing pointer reference without calling `free()`	Process RAM usage grows until Out-Of-Memory crash
Double Free	Calling `free(ptr)` twice on the exact same address	Corrupts heap allocator metadata tables (Crash / Security vulnerability)
Use-After-Free	Accessing `*ptr` after calling `free(ptr)`	Reads garbage or exploits hijacked memory pages

4. Memory Debugging 4 / 4

Debugging with AddressSanitizer & Valgrind

Automated memory leak and buffer overrun detection

Terminal Commands for Memory Analysis

# Method 1: Clang / GCC AddressSanitizer (Built-in runtime instrumentor)
gcc -fsanitize=address -g main.c -o myapp
./myapp  # Instantly prints stack trace on any memory leak or invalid access!

# Method 2: Valgrind Memcheck (Linux dynamic binary analysis)
valgrind --leak-check=full --show-leak-kinds=all ./myapp

Module 12: Dynamic Memory Allocation (Heap Management)

Stack Memory vs Heap Memory Dynamics

malloc, calloc, realloc, and free APIs

Common Heap Memory Bugs & Exploits

Debugging with AddressSanitizer & Valgrind

1. Why You Must Always Check for NULL

2. Practice Exercises & Solutions