In a computer, a file is an integral part of the computer and refers to some associated element in the file name, the element is usually a record, and a record is a set of meaningful data items.combine pdfs for free windows In modern operating systems, the file system is an important part of organizing and managing the large amount of programs and data stored in the computer.

In a file system, data can be categorized into three levels:data items, records, and files. Data items are the lowest form of data organization,convert scanned pdf to word online free large files
can be divided into basic data items and combined data items. Basic data items are used to describe the object of a certain attribute of the character set, is the organization of data can be named the smallest logical data unit. Combined data items consist of several data items. In addition to the data name, the basic data item should also have a data type. Because the basic item describes only one attribute of an object, different data types are needed to describe it according to different attributes.

A record is a collection of related data items that describe properties of an object in a particular aspect, depending on which aspect of the object is to be described.pdf editor online In many records, in order to uniquely identify a record, one or several data items must be identified within the record, and their collection is called a keyword. This keyword is the data item that uniquely identifies the record.

A file, which is a collection of related elements with a file name defined by the creator, can be categorized as structured or unstructured. A structured file consists of a number of related records, while an unstructured file is viewed as a stream of characters. A file is the largest unit of data in a file system and describes a set of objects. Files also have their own attributes, which include file type, file length, physical location of the file, and file creation time.

File types are categorized according to their use as system files, user files, and library files. According to the form of data in the file, it can be categorized as source file, target file and executable file. Classified according to access control attributes, they can be divided into execution-only files, read-only files, and read-write files. Classified according to the form of organization and processing, ordinary files, directory files and special files.

The file system model can be divided into three levels, the bottom level is the object and its attributes, the middle level is the collection of software that operates and manages the object, and the top level is the interface that the file system provides to the user. The objects managed by the file management system are: files, which are the direct objects of file management; directories, which must be configured in the file system in order to facilitate user access and retrieval of files, and each directory entry, which must contain the file name and the physical address where the file is located. Disk storage space, files and directories must occupy storage space, the effective management of this part of the space.

In daily work, the user through the file system provides the system call to implement the operation of the file. The basic operations are: creating files, deleting files, reading files, writing files, truncating files and setting the read/write location of files. In order to facilitate the user to use the file, but also provides more operations, such as open and close a file and change the file name and other operations.

In practice, the operation of a file is roughly divided into two steps: the first step is to retrieve the file directory to find the attributes of the specified file and its location on the external memory; the second step is to implement the corresponding operation on the file. In order to speed up the retrieval of the directory, most of the OS has introduced the file system call open. When the user requests an operation on a file for the first time, open means that the system copies the attributes of the named file from the external memory to one of the table entries of the open file table in the memory and returns the index pointer of the table entry to the user. Later, the system can use the index pointer to look up the file directly. When the file operation is complete, the file can be closed with the close system call, and the OS will delete the file from the table of open files.