Struct packing and alignment in Embedded C - Mini Project: Build & Apply

Struct packing and alignment

📖 Scenario: You are working on a small embedded device that communicates sensor data. To save memory and send data efficiently, you need to understand how struct packing and alignment affect the size of your data structures.

🎯 Goal: You will create a struct representing sensor data, then apply packing to reduce its size, and finally print the size of the struct before and after packing.

📋 What You'll Learn

Create a struct called SensorData with three fields: char id, int value, and char status.

Create a variable called data of type SensorData.

Create a packed version of the struct called PackedSensorData using __attribute__((packed)).

Print the size of SensorData and PackedSensorData using sizeof.

💡 Why This Matters

🌍 Real World

Embedded systems often have limited memory and bandwidth. Understanding struct packing helps save memory and send data efficiently over networks.

💼 Career

Many embedded software jobs require knowledge of memory layout and optimization techniques like struct packing to write efficient low-level code.

1

Create the SensorData struct

Create a struct called SensorData with these exact fields in this order: char id, int value, and char status. Then declare a variable called data of type SensorData.

Embedded C

// Define struct SensorData and declare variable data
// Your code here

Need a hint?

Remember to define the struct with the fields in the exact order and then declare a variable of that struct type.

2

Create a packed version of the struct

Create a packed struct called PackedSensorData with the same fields and order as SensorData. Use __attribute__((packed)) after the struct definition. Then declare a variable called packedData of type PackedSensorData.

Embedded C

struct SensorData {
    char id;
    int value;
    char status;
};

struct SensorData data;

// Define packed struct PackedSensorData and declare variable packedData
// Your code here

Need a hint?

Use __attribute__((packed)) right after the closing brace of the struct definition.

3

Calculate sizes of structs

Create two variables: sizeNormal and sizePacked. Assign sizeof(data) to sizeNormal and sizeof(packedData) to sizePacked.

Embedded C

struct SensorData {
    char id;
    int value;
    char status;
};

struct SensorData data;

struct PackedSensorData {
    char id;
    int value;
    char status;
} __attribute__((packed));

struct PackedSensorData packedData;

// Create sizeNormal and sizePacked variables and assign sizes
// Your code here

Need a hint?

Use sizeof operator on the variables data and packedData.

4

Print the sizes of both structs

Use printf to print the sizes stored in sizeNormal and sizePacked. Print exactly these two lines:
Size of SensorData: X
Size of PackedSensorData: Y
where X and Y are the values of sizeNormal and sizePacked respectively.

Embedded C

struct SensorData {
    char id;
    int value;
    char status;
};

struct SensorData data;

struct PackedSensorData {
    char id;
    int value;
    char status;
} __attribute__((packed));

struct PackedSensorData packedData;

unsigned int sizeNormal = sizeof(data);
unsigned int sizePacked = sizeof(packedData);

// Print the sizes of both structs
// Your code here

Need a hint?

Use printf with %u to print unsigned integers and include a newline \n.