Which is the hardest C program You ever done?

This is the hardest program I have written. This was a project I had to do during first semester of College in C (only C as we only had C Programming that semester. C++ was for next semester). The project was to create a simple Inventory Management and billing application for a store (scenario and not an actual store by the way) that can take care of billing, inventory control and price control. Sounds simple enough. We then thought we could have a UI (GUI) for the application and hence we narrowed down the option to GTK, which was one of the best UI toolkits for C. So we started thinking it would be pretty easy. Then when we got to coding the UI all hell broke lose because this was the first programming language I learnt and I was not even introduced to GUI programming in any language other than HTML. read less