17.2: Files

A computer file is a computer resource for recording data in a computer storage device. Just as words can be written to paper, so can data be written to a computer file. Files can be edited and transferred through the Internet on that particular computer system.

Different types of computer files are designed for different purposes. A file may be designed to store a picture, a written message, a video, a computer program, or a wide variety of other kinds of data. Certain files can store multiple data types at once.

By using computer programs, a person can open, read, change, save, and close a computer file. Computer files may be reopened, modified, and copied an arbitrary number of times.

Files are typically organized in a file system, which tracks file locations on the disk and enables user access.

File contents

On most modern operating systems, files are organized into one-dimensional arrays of bytes. The format of a file is defined by its content since a file is solely a container for data, although on some platforms the format is usually indicated by its filename extension, specifying the rules for how the bytes must be organized and interpreted meaningfully. For example, the bytes of a plain text file (.txt in Windows) are associated with either ASCII or UTF-8 characters, while the bytes of image, video, and audio files are interpreted otherwise. Most file types also allocate a few bytes for metadata, which allows a file to carry some basic information about itself.

Some file systems can store arbitrary (not interpreted by the file system) file-specific data outside of the file format, but linked to the file, for example extended attributes or forks. On other file systems this can be done via sidecar files or software-specific databases. All those methods, however, are more susceptible to loss of metadata than are container and archive file formats.

File Size

At any instant in time, a file might have a size, normally expressed as number of bytes, that indicates how much storage is associated with the file. In most modern operating systems the size can be any non-negative whole number of bytes up to a system limit. Many older operating systems kept track only of the number of blocks or tracks occupied by a file on a physical storage device. In such systems, software employed other methods to track the exact byte count.

Operations

The most basic operations that programs can perform on a file are:

• Create a new file
• Change the access permissions and attributes of a file
• Open a file, which makes the file contents available to the program
• Read data from a file
• Write data to a file
• Delete a file
• Close a file, terminating the association between it and the program
• Truncate a file, shortening it to a specified size within the file system without rewriting any content

Files on a computer can be created, moved, modified, grown, shrunk (truncated), and deleted. In most cases, computer programs that are executed on the computer handle these operations, but the user of a computer can also manipulate files if necessary. For instance, Microsoft Word files are normally created and modified by the Microsoft Word program in response to user commands, but the user can also move, rename, or delete these files directly by using a file manager program such as Windows Explorer (on Windows computers) or by command lines (CLI).

In Unix-like systems, user space programs do not operate directly, at a low level, on a file. Only the kernel deals with files, and it handles all user-space interaction with files in a manner that is transparent to the user-space programs. The operating system provides a level of abstraction, which means that interaction with a file from user-space is simply through its filename (instead of its inode). For example, rm filename will not delete the file itself, but only a link to the file. There can be many links to a file, but when they are all removed, the kernel considers that file's memory space free to be reallocated. This free space is commonly considered a security risk (due to the existence of file recovery software). Any secure-deletion program uses kernel-space (system) functions to wipe the file's data.

File moves within a file system complete almost immediately because the data content does not need to be rewrittten. Only the paths need to be changed.

File Organization

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

• External fragmentation will occur, making it difficult to find contiguous blocks of space of sufficient length. Compaction algorithm will be necessary to free up additional space on disk.
• Also, with pre-allocation, it is necessary to declare the size of the file at the time of creation.

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

• Internal fragmentation exists in last disk block of file.
• There is an overhead of maintaining the pointer in every disk block.
• If the pointer of any disk block is lost, the file will be truncated.
• It supports only the sequential access of files.

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.

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.

Bit Tables

This method uses a vector containing one bit for each block on the disk. Each entry for a 0 corresponds to a free block and each 1 corresponds to a block in use.
For example: 00011010111100110001

In this vector every bit correspond to a particular block and 0 implies that, that particular block is free and 1 implies that the block is already occupied. A bit table has the advantage that it is relatively easy to find one or a contiguous group of free blocks. Thus, a bit table works well with any of the file allocation methods. Another advantage is that it is as small as possible.

Free Block List

In this method, each block is assigned a number sequentially and the list of the numbers of all free blocks is maintained in a reserved block of the disk.

Adaptred from:
"Computer file" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"File Systems in Operating System" by Aakansha yadav, Geeks for Geeks is licensed under CC BY-SA 4.0