45. Serialized Data Buffer
In firmware, you often need to assemble a communication packet as a byte array before sending it over UART/SPI.
You are given the following fields:
| Field | Size | Description |
| Start | 1 byte | Always 0xA5 |
| Command | 1 byte | User input |
| Value | 2 byte | 16-bit data (uint16_t) |
| Status | 1 byte | Flags (0 or 1) |
| Checksum | 4 byte | 32-bit checksum (uint32_t) |
| End | 1 byte | Always 0x5A |
Your task is to:
- Fill a uint8_t buffer[10] with the data in this order
- Use pointer casting or byte manipulation
- Print the entire buffer contents as space-separated integers
Example-1
Input:
Command = 0x01
Value = 0x1234
Status = 1
Checksum = 0xAABBCCDD
Output:
165 1 52 18 1 221 204 187 170 90
Example-2
Input:
Command = 0xFF
Value = 0x00FF
Status = 0
Checksum = 0x01020304
Output:
165 255 255 0 0 4 3 2 1 90
What is an Array?
An array in C is a contiguous block of memory that stores multiple elements of the same data type. It allows you to:
- Group variables together under a single name
- Access each item using an index
- Efficiently perform operations like iteration, sum, reverse, etc.
Core Array Syntax and Usage in C
Array Declaration
int numbers[10]; // Array of 10 integers
uint8_t buffer[32]; // 32-byte buffer (commonly used in firmware)
Declares a fixed-size array — memory is allocated statically.
Array Initialization
int values[4] = {10, 20, 30, 40}; // full initialization
int empty[4] = {0}; // all elements = 0
char message[] = "Hi"; // string-style initIf you skip the size (like message[ ]) , the compiler counts the size automatically.
Accessing Array Elements
int arr[3] = {5, 10, 15};
printf("%d", arr[1]); // prints 10
arr[2] = 20; // modify element at index 2The index starts from 0. Always ensure you stay within 0 to n-1.
Array Size with sizeof()
int arr[5];
int total_bytes = sizeof(arr); // e.g., 20 if int = 4 bytes
int element_bytes = sizeof(arr[0]); // gives size of int i.e. 4 bytes
int element_count = sizeof(arr) / sizeof(arr[0]); // gives 5This only works within the same scope where the array is declared (not when passed to a function).
Memory Layout (Example)
int arr[4] = {10, 20, 30, 40};
Assuming the starting address is 0x2000, the memory looks like:
| Address | Value |
|---|---|
| 0x2000 | 10 |
| 0x2004 | 20 |
| 0x2008 | 30 |
| 0x200C | 40 |
Each int = 4 bytes (on most systems).
Where Arrays Are Used in Firmware
In embedded systems, arrays are used for:
- Data buffers (UART, SPI, ADC)
- Sensor samples
- Circular queues
- Protocol packets/frames
- Bitmaps/lookup tables
Relevance in Embedded
- Arrays = lightweight, deterministic memory structures
- You work without dynamic memory, so statically allocated arrays are essential
- Used in interrupt routines, communication, and data processing
Common Pitfalls (Practical Tips)
- Off-by-one errors: If n = 5, then valid indices are 0 to 4 (not 5).
- Wrong type for storage: If you’re reading bytes, use uint8_t[] not int[].
- Forgetting to initialize: Especially in statically defined buffers.
- Overwriting data in fixed-size buffers — always check bounds.
- Mixing pointers and arrays incorrectly — be clear whether you’re passing array base or pointer offset.