Why use fflush(stdin) in c programming language?

It is a a function which helps us  you to clean the input buffer.
you can  use this function if you are thinking that there might be some problem creating data  during  user inputs from stdin. 

Solution: the categorization of functional and non-functional requirement are as follows: some of the more typical functional requirements include: • business rules• transaction corrections, adjustments and cancellations• administrative functions• authentication• authorization levels• audit tracking• external interfaces• certification requirements• reporting requirements• historical data• legal or regulatory requirementswhile,some typical non-functional requirements are: • performance – for example response time, throughput, utilization, static volumetric• scalability• capacity• availability• reliability• recoverability• maintainability• serviceability• security• regulatory• manageability• environmental• data integrity• usability• interoperability

Aconsequence when using open source software is c. no support