CIS 305 Unit 4 Assignment: File Access Methods

Information in files could be accessed in many ways. It is usually dependent on an application. Sequential Access: A simple access method, information in a file is accessed sequentially one record after another. To process the with record all the 1-1 records previous to 1 must be accessed There are several ways that the information in the file can be accessed. Some systems provide only one access method for files. On other systems, many different access methods are supported, and choosing the right one for a particular application is a major design problem. Sequential Access Information in the file is processed in order, one record after the other. This is by far the most common mode of access of files. For example, computer editors usually access files in this fashion. A read operation reads the next portion of the file and automatically advances the file pointer. Similarly, a write appends to the end of the file and the file pointer. Similarly, a write appends to the end of the file and the file pointer. Similarly, a write appends to the end of the end of the file and advances to the end of the newly written material (the new end of file). Such a file can be reset to the beginning, and, on some systems, a program may be able to skip forward or backward n records, for some integer n. This scheme is known as sequential access to a file. Sequential access is based on a tape model of a file. Direct Access Direct access is based on a disk model of a file. For direct access, the file is viewed as a numbered sequence of block or records. A direct-access file allows arbitrary blocks to be read or written. Thus, after block 18 has been read, block 57 could be next, and then block 3. There are no restrictions on the order of reading and writing for a direct access file. Direct access files are of great use for intermediate access to large amounts of information. The file operations must be modified to include the block number as a parameter. Thus, we have "read n", where n is the block number, rather than "read next", and "write n", rather that "write next". An alternative approach is to retain "read next" and "write next" and to add an operation; "position file to n" where n is the block number. Then, to effect a "read n", we would issue the commands "position to n" and then "read next". Not all O...

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