How to generate random number within x-y range

How to generate random number within x-y range in C where X : [t , t+m] and Y : [r , r+m ]. That is , if x varies from t to t+n and y varies from r to r+n.

For example, if x varies from 0 to 1/3 and y varies from 0 to 1/3 as well.