Your First Zephyr Application

Let’s build and run your first Zephyr application to verify your setup is working.

Building the Sample

Zephyr includes a hello_world sample that we’ll use to test your environment.

cd ~/zephyrproject

# Build for QEMU (no hardware needed)
west build -b qemu_cortex_m3 zephyr/samples/hello_world

Build Output

-- west build: generating a build system
Loading Zephyr default modules (Zephyr base).
...
[1/108] Preparing syscall dependency handling
...
[108/108] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       12544 B       256 KB      4.79%
            SRAM:        4160 B        64 KB      6.35%
        IDT_LIST:          0 GB         2 KB      0.00%

Running in QEMU

# Run the built application in QEMU
west build -t run

Expected Output

*** Booting Zephyr OS build v3.6.0 ***
Hello World! qemu_cortex_m3

Press Ctrl+A then X to exit QEMU.

Understanding the Sample

Let’s examine what makes up a Zephyr application:

Directory Structure

samples/hello_world/
├── CMakeLists.txt    # Build configuration
├── prj.conf          # Kconfig options
├── README.rst        # Documentation
└── src/
    └── main.c        # Application code

CMakeLists.txt

# Minimum CMake version
cmake_minimum_required(VERSION 3.20.0)

# Find Zephyr package (sets up build system)
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})

# Project name
project(hello_world)

# Add source files
target_sources(app PRIVATE src/main.c)

prj.conf

# Project configuration (empty for hello_world)
# Add Kconfig options here
# Example: CONFIG_PRINTK=y

src/main.c

#include <zephyr/kernel.h>

int main(void)
{
    printk("Hello World! %s\n", CONFIG_BOARD);
    return 0;
}

Creating Your Own Application

Let’s create a custom application from scratch.

Step 1: Create Directory Structure

mkdir -p ~/zephyrproject/my-apps/my-first-app/src
cd ~/zephyrproject/my-apps/my-first-app

Step 2: Create CMakeLists.txt

cmake_minimum_required(VERSION 3.20.0)
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
project(my_first_app)

target_sources(app PRIVATE src/main.c)

Step 3: Create prj.conf

# Enable printk for debug output
CONFIG_PRINTK=y

Step 4: Create src/main.c

#include <zephyr/kernel.h>

int main(void)
{
    int count = 0;

    printk("My First Zephyr App!\n");
    printk("Board: %s\n", CONFIG_BOARD);

    while (1) {
        printk("Loop count: %d\n", count++);
        k_sleep(K_SECONDS(1));
    }

    return 0;
}

Step 5: Build and Run

cd ~/zephyrproject

# Build your app
west build -b qemu_cortex_m3 my-apps/my-first-app

# Run it
west build -t run

Expected Output

*** Booting Zephyr OS build v3.6.0 ***
My First Zephyr App!
Board: qemu_cortex_m3
Loop count: 0
Loop count: 1
Loop count: 2
...

Application Build Flow

flowchart TD
    A[west build -b board app/] --> B[CMake Configure]
    B --> C[Process CMakeLists.txt]
    C --> D[Process prj.conf]
    D --> E[Process Device Tree]
    E --> F[Generate Headers]
    F --> G[Compile Sources]
    G --> H[Link Application]
    H --> I[Generate zephyr.elf]
    I --> J[west build -t run]
    J --> K[QEMU Execution]

Building for Hardware

If you have a development board:

# Nordic nRF52840 DK
west build -b nrf52840dk_nrf52840 my-apps/my-first-app
west flash

# STM32F4 Discovery
west build -b stm32f4_disco my-apps/my-first-app
west flash

Common Issues

“ZEPHYR_BASE not set”

source ~/zephyrproject/zephyr/zephyr-env.sh

“Board not found”

# List available boards
west boards | grep cortex

Build errors

# Clean rebuild
west build -p -b qemu_cortex_m3 my-apps/my-first-app

QEMU won’t start

# Check if QEMU is installed
which qemu-system-arm

# Install if needed (Ubuntu)
sudo apt install qemu-system-arm

Example Code

Complete, tested examples are available:

• Hello World Example - Basic application
• Blinky Example - LED blinking with GPIO

Exercises

1. Modify the message: Change the print statement and rebuild
2. Change sleep interval: Make the loop faster or slower
3. Add a counter limit: Stop the loop after 10 iterations
4. Try different boards: Build for qemu_x86 or native_posix

What’s Next?

Congratulations! You’ve successfully:

• Built a Zephyr application
• Run it in QEMU
• Created your own application from scratch

In Part 2, we’ll dive deeper into the build system, learning about west, CMake, Kconfig, and device tree.