聊城大学计算机学院计算机操作系统（1）英文课件 Chapter12.ppt

File Management Chapter 12 File Management File management system consists of system utility programs that run as privileged applications Input to applications is by means of a file Output is saved in a file for long-term storage File System Properties Long-term existence Sharable between processes Structure File Operations Create Delete Open Close Read Write Terms Used with Files Field Basic element of data Contains a single value Characterized by its length and data type Record Collection of related fields Treated as a unit Example: employee record Terms Used with Files File Collection of similar records Treated as a single entity Have file names May restrict access Database Collection of related data Relationships exist among elements Typical Operations Retrieve_All Retrieve_One Retrieve_Next Retrieve_Previous Insert_One Delete_One Update_One Retrieve_Few File Management Systems The way a user of application may access files Programmer does not need to develop file management software Objectives for aFile Management System Meet the data management needs and requirements of the user Guarantee that the data in the file are valid Optimize performance Provide I/O support for a variety of storage device types Objectives for aFile Management System Minimize or eliminate the potential for lost or destroyed data Provide a standardized set of I/O interface routines Provide I/O support for multiple users Minimal Set of Requirements Each user should be able to create, delete, read, write and modify files Each user may have controlled access to other users’ files Each user may control what type of accesses are allowed to the users’ files Each user should be able to restructure the user’s files in a form appropriate to the problem Minimal Set of Requirements Each user should be able to move data between files Each user should be able to back up and recover the user’s files in case of damage Each user should be able to access the user’s files by using symbolic names Devic