The set of existing files can vary while the program executes. Files that are known to the operating system do not necessarily exist for an executable program at a given time. O statements can be applied only to files that exist. Files that have names are called named files . Names are simply character strings.
Every data file has a position.
The point immediately before the first record. The point immediately after the last record. The record containing the point where the file is positioned. The record immediately before the current record. The preceding record is undefined if the file is positioned in the first record or at the initial point.