Hello, World!

This tutorial guides you through getting a simple “Hello World” program running as a user-level application on seL4. It allows you to test your local set up and make sure all the tools are working before moving onto more detailed tutorials.


  1. Set up your machine.


# For instructions about obtaining the tutorial sources see https://docs.sel4.systems/Tutorials/#get-the-code
# Follow these instructions to initialise the tutorial
# initialising the build directory with a tutorial exercise
./init --tut hello-world
# building the tutorial exercise
cd hello-world_build


By the end of this tutorial, you should:

  • Be familiar with the jargon root task.
  • Be able to build and simulate seL4 projects.
  • Have a basic understanding of the role of the CMakeLists.txt file in applications.

Building your first program

seL4 is a microkernel, not an operating system, and as a result only provides very minimal services. After the kernel boots, an initial thread called the root task is started, which is then responsible for setting up the user-level system. When the root task starts there are no available drivers, however a minimal C library is provided.

The tutorial is already set up to print “Hello, world!”, so at this point all you need to do is build and run the tutorial:

# In build directory

If successful, you should see the final ninja rule passing:

[150/150] objcopy kernel into bootable elf

The final image can be run by:

# In build directory

This will run the result on an instance of the QEMU simulator. If everything has worked, you should see:

Booting all finished, dropped to user space

Hello, World!

Looking at the sources

In your tutorial directory, you will find the following files:

  • CMakeLists.txt - the file that incorporates the root task into the wider seL4 build system.
  • src/main.c - the single source file for the initial task.
  • hello-world.md - A generated README for the tutorial.


Every application and library in an seL4 project requires a CMakeLists.txt file in order to be incorporated into the project build system.


cmake_minimum_required(VERSION 3.7.2)
# declare the hello-world CMake project and the languages it is written in (just C)
project(hello-world C ASM)

# In future tutorials, these setup steps will be replaced with
# sel4_tutorials_setup_roottask_tutorial_environment()
find_package(seL4 REQUIRED)
find_package(elfloader-tool REQUIRED)
find_package(musllibc REQUIRED)
find_package(util_libs REQUIRED)
find_package(seL4_libs REQUIRED)


# This sets up environment build flags and imports musllibc and runtime libraries.

# Name the executable and list source files required to build it
add_executable(hello-world src/main.c)

# List of libraries to link with the application.
    sel4runtime sel4
    muslc utils sel4tutorials
    sel4muslcsys sel4platsupport sel4utils sel4debug)

# Tell the build system that this application is the root task. 


The main C is a very typical C file. For a basic root server application, the only requirement is that a main function is provided.

#include <stdio.h>
    printf("Hello, World!\n");

    return 0;

Making a change

Test making a change to main.c by adding a second printf to output "Second hello\n".

    printf("Hello, World!\n");

    printf("Second hello\n");
    return 0;

Once you have made your change, rerun ninja to rebuild the project:

# In build directory

Then run the simulator again:

# In build directory

On success, you should see the following:

Second hello

Getting help

Stuck? See the resources below.