Many times I hear from customers – Linux has no good UI, it’s made for servers but we want to make apps with windows drop boxes, check boxes etc.
Recently we found Simon Long’s C GUI Programming book and made two “windows” examples for OLinuXino.
We recommend you first to download the book and read it then to look at the code we made and uploaded to GitHub.
In the README.TXT you will find step by step instructions how to install and configure GTK for OLinuXino.
Then you can try the code and modify for your need.
GPIO example allow you to read or write GPIO state to any of the GPIO connectors of OLinuXino.
I2C example will allow you to interface to any of our UEXT boards with I2C interface. The example we test with MOD-IO.
olimex 
Aug 04, 2019 @ 10:45:49
Probably, your customers still use something like Slackware 3.0… otherwise is definitely not the case. The fastest way to write professional applications with gtk+ or Qt interfaces is to use the Lazarus + FreePascal. Lazarus is a RAD (Rapid Application Development) application where you visually design your graphical interface. But if you prefer C++, then the Qt Creator is the best.
Anyway, LCL (used in Lazarus and named Lazarus Component Library – which is a port of Borland’s Visual Component Library) that is built on top of gtk+ or Qt is the best GUI in the world.