C Memory Management Internals

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C provides low-level control over memory, allowing programmers to manage how memory is allocated, used, and released.
Understanding memory internals helps in writing efficient and safe programs.

Memory Layout of a C Program

When a C program is executed, memory is divided into several segments:

+-------------------+
|   Stack           |
+-------------------+
|   Heap            |
+-------------------+
|   BSS Segment     |
+-------------------+
|   Data Segment    |
+-------------------+
|   Text Segment    |
+-------------------+

1. Text Segment (Code Segment)

• Contains compiled program instructions

• Read-only

• Prevents accidental modification of code

2. Data Segment

Stores global and static variables that are initialized.

int x = 10;   // Stored in data segment

3. BSS Segment

Stores uninitialized global and static variables.

int y;        // Stored in BSS segment

• Automatically initialized to zero

4. Heap

• Used for dynamic memory allocation

• Memory allocated using malloc(), calloc(), realloc()

• Grows upward

int *p = malloc(sizeof(int));

5. Stack

• Stores local variables, function parameters, return addresses

• Memory is automatically managed

• Follows LIFO order

• Grows downward

void func() {
    int a;    // Stored on stack
}

Stack vs Heap

Memory Allocation Internals

malloc()

• Allocates a block of memory

• Does not initialize memory

calloc()

• Allocates multiple blocks

• Initializes memory to zero

realloc()

• Resizes allocated memory

free()

• Releases allocated memory back to heap

Common Memory Problems

1. Memory Leak

• Allocated memory not freed

int *p = malloc(10 * sizeof(int));
// missing free(p)

2. Dangling Pointer

• Pointer refers to freed memory

free(p);
p = NULL;

3. Buffer Overflow

• Writing beyond allocated memory

4. Double Free

• Freeing the same memory twice

Best Practices

• Always check malloc() return value

• Free memory after use

• Set pointers to NULL after freeing

• Avoid accessing freed memory

Summary

• C memory is divided into text, data, BSS, heap, and stack

• Stack is automatic, heap is manual

• Dynamic memory must be managed carefully

• Poor memory handling leads to bugs and crashes