A file is a group of related data that has a name and is stored on secondary storage like magnetic discs, magnetic tapes, and optical discs. Generally speaking, a file is a collection of bits, bytes, lines, or records that have meanings determined by the user and creator of the file.
A File Structure should be according to a required format that the operating system can understand.
- A file has a certain defined structure according to its type.
- A text file is a sequence of characters organized into lines.
- A source file is a sequence of procedures and functions.
- An object file is a sequence of bytes organized into blocks that are understandable by the machine.
- When the operating system defines different file structures, it also contains the code to support these file structures. Unix, and MS-DOS support a minimum number of file structures.
The ability of the operating system to distinguish between various file types, such as text files, source files, binary files, etc., is referred to as file type. Numerous operating systems can handle a wide variety of files. UNIX and MS-DOS operating systems have the following types of files:
- These are the files that contain user information.
- These may have text, databases or executable program.
- The user can apply various operations on such files like add, modify, delete or even remove the entire file.
Directory files - These files contain list of file names and other information related to these files.
- These files are also known as device files.
- These files represent physical device like disks, terminals, printers, networks, tape drive etc.
These files are of two types −
- Character special files − data is handled character by character as in case of terminals or printers.
- Block special files − data is handled in blocks as in the case of disks and tapes.
File Access Mechanisms
File access mechanism refers to the manner in which the records of a file may be accessed. There are several ways to access files −
- Sequential access
- Direct/Random access
- Indexed sequential access
Sequential access is one in which the records are accessed in some order, meaning that each record’s contents is processed one at a time. The most basic access method is this one. Example: This is how compilers typically access files.
- Random access file organization provides, accessing the records directly.
- Each record has its own address on the file with by the help of which it can be directly accessed for reading or writing.
- The records need not be in any sequence within the file and they need not be in adjacent locations on the storage medium.
Indexed sequential access
- This mechanism is built on the base of sequential access.
- An index is created for each file that contains pointers to various blocks.
- Index is searched sequentially and its pointer is used to access the file directly.
Files are allocated disk spaces by the operating system. Operating systems deploy the following three main ways to allocate disk space to files.
- Contiguous Allocation
- Linked Allocation
- Indexed Allocation
- Each file occupies a contiguous address space on disk.
- Assigned disk address is in linear order.
- Easy to implement.
- External fragmentation is a major issue with this type of allocation technique.
- Each file carries a list of links to disk blocks.
- Directory contains link / pointer to first block of a file.
- No external fragmentation
- Effectively used in sequential access file.
- Inefficient in case of direct access file.
- Provides solutions to problems of contiguous and linked allocation.
- A index block is created having all pointers to files.
- Each file has its own index block which stores the addresses of disk space occupied by the file.
- Directory contains the addresses of index blocks o