Timer and Threads in linux

How to write timer and thread in c for colibri T20?

For threads programming, have a look at the following tutorial. With timers, I guess you would need to be a little more specific about what exactly it is you are trying to achieve (e.g. periodic timer vs. alarm clock type event timer possibly utilising the RTC). You may find countless examples e.g. here. Please also note that none of this is any hardware specific at all.