65. Pointer-iii

A pointer is a variable that stores the memory address of another variable.

int x = 10;
int *ptr = &x;   // ptr holds address of x

Pointers are core to C programming — especially in firmware, where direct access to memory, registers, and hardware resources is essential.

Why Pointers Matter in Embedded

• Access memory-mapped hardware registers
• Manage arrays and buffers efficiently
• Implement pass-by-reference in functions
• Optimize memory usage with minimal overhead
• Enable modular design using a pointer to a function and a struct
   

Declaring & Using Pointers

int x = 25;
int *p = &x;
printf("%d", *p);   // dereferencing → prints 25

• &x → “address of x”
• *p → “value at address stored in p” (dereference)
   

Memory Addressing Basics

If x is at 0x2000, and int = 4 bytes:

Common Pointer Operations

Pointers and Arrays

int arr[3] = {10, 20, 30};
int *p = arr;

printf("%d", *(p + 2));  // prints 30
In C, arr[i] is equivalent to *(arr + i)

Null Pointers

int *ptr = NULL;

• NULL pointer points to nothing
• Always check before dereferencing to avoid a crash
   

Pointer to Pointer

int x = 10;
int *p = &x;
int **pp = &p;

Used when you want to modify the original pointer itself (e.g., inside a function).

const and Pointers

const int *p1;     // pointer to constant data (can’t modify *p1)
int *const p2;     // constant pointer (can’t change p2)
const int *const p3; // const pointer to const data

Useful when:

• Reading from memory-mapped registers (read-only)
• Protecting data from accidental modification
   

Void Pointers (void *)

A void * is a generic pointer — it can point to any type but must be cast before dereferencing.

void *ptr;
int x = 10;
ptr = &x;

printf("%d", *(int *)ptr);  // cast required

Common in:

• Generic buffer handling (e.g., memcpy)
• Protocol stacks
• Register abstraction layers
   

Pass by Reference (Passing Pointers)

You can pass a pointer to allow a function to modify variables.

void update(int *p) {
    *p = 99;
}

int main() {
    int val = 10;
    update(&val);
    // val now = 99
}

Used for:

• Updating config
• Writing return values
• Avoiding global variables

Function pointers let you store and call functions dynamically.

void blink() { printf("LED Blink\n"); }
void (*func_ptr)() = blink;
func_ptr();  // calls blink

Example 

#include <stdio.h>
// Normal function
int add(int a, int b) {
    return a + b;
}

int main() {
    // Declare a function pointer
    int (*fp)(int, int) = add;

    // Call function using pointer
    int result = fp(5, 3);
    printf("%d\n", result);   // Output: 8

    return 0;
}

Used in:

• State machines
• Callback drivers
• Event handlers
   

What is Pointer Arithmetic?

Pointer arithmetic means performing arithmetic operations like +, -, ++, or -- on pointers, to move them through memory.

int arr[3] = {10, 20, 30};
int *p = arr;

p++;   // now p points to arr[1]

In embedded systems, pointer arithmetic is widely used to:

• Walk through arrays or buffers
• Scan memory blocks
• Handle communication frames (UART, SPI, etc.)
• Replace array indexing for optimization
   

How It Works

When you do p + 1, it doesn’t move 1 byte — it moves to the next element based on the pointer’s type:

So:

• p++ means: go to the next element of type (not next byte)
   

Example: 

int vs char

int nums[3] = {1, 2, 3};
char bytes[3] = {'a', 'b', 'c'};

int *iptr = nums;
char *cptr = bytes;

iptr++;  // jumps 4 bytes → now points to nums[1]
cptr++;  // jumps 1 byte  → now points to bytes[1]

Accessing via Pointer

int arr[5] = {5, 10, 15, 20, 25};
int *p = arr;

printf("%d", *(p + 3));   // prints 20
Same as arr[3] — this works with any type.

Pointer Loop Example

uint8_t data[4] = {1, 2, 3, 4};
uint8_t *ptr = data;

for (int i = 0; i < 4; i++) {
    printf("%d ", *(ptr + i));  // pointer arithmetic
}

Pointer Relevance in Firmware

Pointers are essential in:

• Peripheral access: Registers are accessed via pointer to fixed address
• Data buffers: Efficient traversal using pointer arithmetic
• Struct access: Dynamically organized configuration or state
• Low-level drivers: Direct hardware interactions 

Common Pitfalls (Practical Tips)

• Uninitialized pointer → Can crash or corrupt memory
• Forgetting to dereference → Logic error (*ptr)
• Overstepping array bounds → Memory overwrite
• Wrong pointer type → Use correct types (uint8_t*, int32_t*, etc.)
• Confusing pointer to value vs pointer to pointer

Pointer Arithmatic in Embedded

• Walk through frame buffers
• Iterate over I2C/UART/SPI data blocks
• Scan sensor values, logs, circular queues
• Implement low-level string/byte parsing 

You often use uint8_t *ptr = (uint8_t *)base_addr for hardware access.

Common Pitfalls (Practical Tips)

• ❌ Don’t add/subtract bytes manually — rely on type-based movement.
• ❌ Don’t mix pointer arithmetic with incorrect types (e.g., char* to int)
• ❌ Never cross array bounds → causes undefined behavior
• ✅ Use pointer arithmetic for efficient loops in memory-critical code
• ✅ Cast to uint8_t* if you need byte-wise access