rpi_hal from https://github.com/rudiratlos/rpi-hal is an incredible rich and uptodate unit for Freepascal to use the I/O of the Raspberry.

From Freepascal programs all works as expected when run as root. Otherwise accessing /dev/mem is causing real problems.

The unit can be used from Lazarus also (avoid the test procedures with writeln’ 😉 ) but also require root access and setting cthreads in the program heading.

For X apps  running as root is not possible with just sudo.
Seems I found a solution here: https://wiki.archlinux.org/index.php/Running_GUI_applications_as_root

I applied method 2:

sudo nano /etc/profile

and added this line as last line

export XAUTHORITY=/home/pi/.Xauthority

Now i can do

$ sudo ./testinit (my simple test program with only the init line of rpi_hal)

and this worked without error, init succeeded.

If you want debugging the app in Lazarus, start Lazarus as root (and ignore all the warnings)

xhost +localhost
sudo /usr/bin/startlazarus

Original text from https://wiki.archlinux.org/index.php/Running_GUI_applications_as_root

Method 2: Globally in /etc/profile
Add the following line to /etc/profile:

export XAUTHORITY=/home/username/.Xauthority
This will permanently allow root to connect to a non-root user’s X server.

Or, merely specify a particular app:

XAUTHORITY=/home/username/.Xauthority appname
where appname is the name of the particular app. (e.g. kwrite