Protection, directory implementation
- 1. Protection, Directory Implementation By C.Rajeswari M.Sc (info.tech), Nadar saraswathi college of arts and science, theni.
- 2. Protection When information is stored in a computer system, we want to keep it safe from physical damage(reliability) and improper access(protection). Protection can be provided in many ways. For a small single-user system, we might provide protection by physically removing the floppy disks and locking them in a desk drawer or file cabinet.
- 3. Continue… I. Types of access: The need to protect files is a direct result of the ability to access files. Several different types of operations may be controlled: Read : Read from the file. Write : Write or rewrite the file. Execute : Load the file into memory and execute it Append : Write new information at the end of the file. Delete : Delete the file and free its space for possible reuse. List : List the name and attributes of the file.
- 4. Continue… II. Access control: The most common approach to the protection problem is to make access dependent on the identity of the user. To condense the length of the access-control list, many systems recognize three classifications of users in connection with each file: Owner : The user who created the file is the owner. Group : A set of users who are sharing the file and need similar access is a group, or work group. Universe : All other users in the system constitute the universe.
- 5. Continue… III. Other protection approaches: Another approach to the protection problem is to associate a password with each file. Just as access to the computer system is often controlled by a password, access to each file can be controlled in the same way. The IBMVM/CMS operating system allows three passwords for a minidisk-one each for read, write, and multiwrite access. Some single-user operating systems--such as MS-DOS and earlier versions of file protection. Protection mechanisms must be retrofitted into them.
- 6. Directory Implementation The selection of directory-allocation and directory- management algorithms significantly affects the efficiency, performance, and reliability of the file system. In trade-offs involved in choosing one of these algorithms.
- 7. Continue… I. Linear List: The simplest method of implementing a directory is to use a linear list of file names with pointers to the data blocks. This method is simple to program but time-consuming to execute. Linear search time Could keep ordered alphabetically via linked list or use B+ tree
- 8. Continue… Advantage: It also be used to decrease the time required to delete file We can mark blank space and attached to free list Disadvantage: Finding a file requires a linear search Access is very slow
- 9. Continue… II. Hash Table: Another data structure used for a file directory is a hash table. A linear list stores the directory entries, but a hash data structure is also used. Insertion and deletion are also fairly straightforward. although some provision must be made for collisions-situations in which two file names hash to the same location. Drawback Only good if entries are fixed size, or use chained-overflow method
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