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4-3-5 Programming Programmers who use a le-access method can operate on one of three levels: They can write instructions to manipulate the le on the basis of understanding the logical functions to be executed They can attempt to also understand the underlying physical structure in order to produce programs that are e cient within the perceived system environment They can use attractive features of the le system and attempt to bypass or disable parts of the le-system support which do not suit them, and replace these parts with programs of their own invention Each of these choices has associated costs and bene ts, and the decisions should be made based on documented evaluations of the trade-o s which were considered The cost of the rst alternative may be inadequate performance and the cost of the third alternative may be intolerable in exibility A philosophy in structured programming, information hiding, advocates the imposition of the rst approach by not revealing the underlying structure of system programs This is sometimes unintentionally aided by the lack of clear system documentation The author prefers to work with a good knowledge of the available tools and use them to a maximal extent
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Statements The operations that are basic to the use of vsam and similar methods include the following statements An OPEN statement is to be issued before the le can be processed The OPEN statement is used to specify the le name, the type of operations to be performed, and optionally, a password to be veri ed The OPEN statement also speci es the area or areas where the parameters for further processing requests will be found The catalog, the le listing all les, will be searched to locate the le The device or devices which contain index and data will be accessed and directory information extracted A check to ensure the use of the right versions of the index les and the data le is made by matching the contents of a eld in each le directory which contains the most recent update times Bu er storage for le processing also is obtained and initialized A CLOSE statement is used to indicate that processing of the le is completed The CLOSE statement empties any outstanding bu ers and updates the le directory A GET statement is used to read data from the le Before its execution the search argument and other parameters are placed into a parameter area which speci es what is to be read The parameters to be set for indexed access include the key value and indicate whether an exact or a less-than-or-equal match is desired The position and length of the area into which the record is to be placed have to be speci ed An area for receiving error messages should also be provided A further parameter states whether the process will wait until the reading is completed Otherwise the process can continue computation while the system executes the GET process asynchronously The calling process can check later if the system process is complete The length and address of the record read will be placed into the parameter list upon completion The advantage of using a separate xed area for the parameter list is that only parameters that change have to be respeci ed when the next GET is issued To provide more exibility of operation, it is possible to generate new or to modify existing request parameter areas during program execution by means of GENCB and MODCB statements
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The PUT statement initiates the writing of a record, again using parameters contained in the speci ed parameter area When writing, the length of the record has to be set into the parameter list prior to execution of the statement The parameter area can be separate since its address is speci ed as part of a GET or PUT operation The ERASE statement causes deletion of the indicated record The POINT statement provides a capability to reset the current-record pointer for serial GET and PUT statements The CHECK statement is used to determine whether a previous asynchronous operation has been completed The ENDREQ statement can be used to terminate an outstanding request This will cancel an operation which is no longer desirable, perhaps because of a detected error or because a parallel operation has delivered the desired data already Escape Procedures Optional parameters allow the speci cation of program segments which are to be executed when the le system performs certain operations or senses certain conditions in the le One use of these escape procedures is to compress or expand data records in order to save le space Others can be used when End-of-file conditions occur Considerable modi cation of the basic operations is possible when these escape hatches or exits are used These facilities are also exploited to create multi-indexed les within vsam Upon insertion of a record into the data or primary le an escape procedure will initiate updating of all secondary indexes The programmer who uses escape procedures has access to all the options and the underlying code at the same structural level Now the distinction between the three levels of programming described above is not very explicit, and good intentions can easily come to naught Programs which are invoked as escape procedures may introduce errors which are not visible when the program which invokes the le services is checked out These escape hatches can also easily be misused to violate system security
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