High level languages such as COBOL, FORTRAN and C, is commonly known as procedure oriented programming(POP). In the procedure oriented programming, program is divided into sub programs or modules and then assembled to form a complete program. These modules are called functions.
In a multi-function program, many important data items are placed as global so that they may be accessed by all functions. Each function may have its own local data. If a function made any changes to global data, these changes will reflect in other functions. Global data are more unsafe to an accidental change by a function. In a large program it is very difficult to identify what data is used by which function.