What is file in os?

The advantages of using a file system include the:

Disadvantages of using a file system include:

A file is a collection of related information that is recorded on secondary storage. Or file is a collection of logically related entities. From the user’s perspective, a file is the smallest allotment of logical secondary storage.

The name  of the file is divided into two parts as shown below:

FILE DIRECTORIES: Collection of files is a file directory. The directory contains information about the files, including attributes, location and ownership. Much of this information, especially that is concerned with storage, is managed by the operating system. The directory is itself a file, accessible by various file management routines.

Information contained in a device directory are:

Operation performed on directory are:

Advantages of maintaining directories are:

SINGLE-LEVEL DIRECTORY In this a single directory is maintained for all the users.

TWO-LEVEL DIRECTORY In this separate directories for each user is maintained.

TREE-STRUCTURED DIRECTORY : Directory is maintained in the form of a tree. Searching is efficient and also there is grouping capability. We have absolute or relative path name for a file.

FILE ALLOCATION METHODS :

1. Continuous Allocation –A single continuous set of blocks is allocated to a file at the time of file creation. Thus, this is a pre-allocation strategy, using variable size portions. The file allocation table needs just a single entry for each file, showing the starting block and the length of the file. This method is best from the point of view of the individual sequential file. Multiple blocks can be read in at a time to improve I/O performance for sequential processing. It is also easy to retrieve a single block. For example, if a file starts at block b, and the ith block of the file is wanted, its location on secondary storage is simply b+i-1.

Disadvantage –

2. Linked Allocation(Non-contiguous allocation) –Allocation is on an individual block basis. Each block contains a pointer to the next block in the chain. Again the file table needs just a single entry for each file, showing the starting block and the length of the file. Although pre-allocation is possible, it is more common simply to allocate blocks as needed. Any free block can be added to the chain. The blocks need not be continuous. Increase in file size is always possible if free disk block is available. There is no external fragmentation because only one block at a time is needed but there can be internal fragmentation but it exists only in the last disk block of file.

Disadvantage –

3. Indexed Allocation –It addresses many of the problems of contiguous and chained allocation. In this case, the file allocation table contains a separate one-level index for each file: The index has one entry for each block allocated to the file. Allocation may be on the basis of fixed-size blocks or variable-sized blocks. Allocation by blocks eliminates external fragmentation, whereas allocation by variable-size blocks improves locality. This allocation technique supports both sequential and direct access to the file and thus is the most popular form of file allocation.

Disk Free Space Management :

Just as the space that is allocated to files must be managed ,so the space that is not currently allocated to any file must be managed. To perform any of the file allocation techniques,it is necessary to know what blocks on the disk are available. Thus we need a disk allocation table in addition to a file allocation table.The following are the approaches used for free space management.

This article is contributed by Aakansha yadav

To a user, the file is the smallest unit of storage on a computer system. The user performs file operations such as open, close, read, write and modify. They can create or delete files from the system. In this article, you will learn the basics of file and file system.

Computer store information in storage media such as disk, tape drives, and optical disks. The operating system provides a logical view of the information stored in the disk. This logical storage unit is a file.

The information stored in files are non-volatile, means they are not lost during power failures. A file is named collection of related information that is stored on physical storage.

Data cannot be written to a computer unless it is written to a file. A file, in general, is a sequence of bits, bytes, lines, or records defined by its owner or creator. The file has a structure defined by its owner or creator and depends on the file type.

A file has a single e editable name given by its creator. The name never changes unless a user has necessary permission to change the file name. The names are given because it is humanly understandable.

The properties of a file can be different on many systems, however, some of them are common:

The information about the file is kept in a directory structure which also resides on the secondary storage.

The operating system performs the following file operations using various system calls:

Create Files – User must have necessary disk space on the file system to create a file. A directory entry is required where the file is created.

Read Files – The system call requires file name and next block in memory to be read. The system needs a read pointer to read the file from a specific location in the file and this pointer is updated for the next read from the file.

Write Files – The system call uses same file pointers of the process to write to a file. This saves space and reduces complexity.

Repositioning within a file – The current-file-position pointer is repositioned to a given value. It does not involve an I/O, and known as file seek operation.

Deleting a file – We look into the directory for the file name, if a file is found, release the space occupied by the file and remove directory entries for the deleted file.

Truncating a file – Sometimes the user does not want to delete a file, but remove some information from it. This will change file length attribute, however, other attributes remain unchanged.

There are other file operations such asappending a file,renaming a file, create a duplicate copy of the file.

The file operations require searching a directory every time. To avoid frequent searches, the OS allows a system call – open() and keeps an.open file table When the file operation is requested, the system refers to the file via an index value. The file is not used actively, the process closes the file and the OS remove its entry from the open file table.

The open() system call can take mode information such as read-only, write, append, create, and so on. The mode is checked against the file permission and it allowed file is open for the process.

The open call returns a pointer to the entry in the open-file table and the pointer, not the file is used for all I/O operations.

Several different files may open the same file, therefore, the system maintains two file-tables – per-process table and system-wide table. The per-process table contains information about files opened by the file.For example, current file-pointer for each file, access rights, and accounting information for the files.

Each entry in the process file-table points to a system-wide table, which contains process independent information such as the location of the file on the disk, access dates, and file size. When a process opens a file, an entry is added to the processor open file table of the process and points to the entry in the system-wide table.

The open file table keeps an open count indicating the number of processes that have opened the file. The close operation decreases the count and when it reaches 0 the file entry is removed from the open-file table.

The following information is associated with an open file:

File pointer – It helps in read() and write() operation. It is unique to each process.File Open count – When multiple processes open the same file, therefore, file open-count keep track of open files and closed files and when the count reaches 0 and process can remove the entry from the open-table.Disk location of the file – To modify data the path to the file is kept in memory. The system does not have to read it for each operation.Access rights – The process opens the file in access mode and information regarding the access rights is in the per-process table, based on which OS can reject or accept the I/O request.

Apart from the above, some OS provides ways to lock files. This is true in the case of shared files where only one process can write to the file while other processes read the information on the file.

A file is a named collection of related information that is recorded on secondary storage such as magnetic disks, magnetic tapes and optical disks. In general, a file is a sequence of bits, bytes, lines or records whose meaning is defined by the files creator and user.

A file is a named collection of related information that is recorded on secondary storage such as magnetic disks, magnetic tapes and optical disks. In general, a file is a sequence of bits, bytes, lines or records whose meaning is defined by the files creator and user.

A File Structure should be according to a required format that the operating system can understand.

File type refers to the ability of the operating system to distinguish different types of file such as text files source files and binary files etc. Many operating systems support many types of files. Operating system like MS-DOS and UNIX have the following types of files −

These files are of two types −

File access mechanism refers to the manner in which the records of a file may be accessed. There are several ways to access files −

A sequential access is that in which the records are accessed in some sequence, i.e., the information in the file is processed in order, one record after the other. This access method is the most primitive one. Example: Compilers usually access files in this fashion.

Files are allocated disk spaces by operating system. Operating systems deploy following three main ways to allocate disk space to files.

What is the file?

The file can be explained as the smallest unit of storage on a computer system. The user can perform file operations like open, close, read, write, and modify.

File concept

The operating system can provide a logical view of the information stored in the disks, this logical unit is known as a file. The information stored in files is not lost during power failures.

A file helps to write data on the computer. It is a sequence of bits, bytes, or records, the structure of which is defined by the owner and depends on the type of the file.

Different types of files are:

In an executable file, the binary code that is loaded in the memory for execution is stored. It is stored in an exe type file.

The source file has subroutines and functions that are compiled later.

An object file is a sequence of bytes used by the linker.

A text file is a sequence of characters.

An image file is a sequence of visual information, for example, vector art.

File Attributes

Some of the common file attributes are:

File Concept in OS Reference