File Handling

PxPlus has the ability to read and write native OS serial files (sometimes referred to as flat files) for easy transfer of information from other applications. It also has the ability to access any data file regardless of the file format.

The special OPEN file option ISZ= allows a file to be accessed as if it was an Indexed file, specifying a logical indexed record size. This means that the file's contents can be read or written directly. Once the file has been opened (with ISZ= set), read and write statements can be issued against it. Optionally, the user can specify the record index via the IND= option to access specific areas of the file directly. The size specified via the ISZ= option is considered the record size for each record in the file. A record index specified on a file read or write directive with an IND= option determines where in the file (at what record index) the I/O function is to be performed.

Use of ISZ=-1 on an OPEN provides direct access to a file on a byte-by-byte basis. Whatever is specified on the IND= clause identifies the actual byte within the file. The SIZ= clause can be used on the READ or WRITE to define the amount of data to transfer.

Example:

0010 OPEN (1,ISZ=100) "CUSTDB"
0020 FOR I = 0 TO 49
0030 READ RECORD (1,IND=I,ERR=0060) R$
0040 PRINT HTA(R$)
0050 NEXT I
0060 END

In the preceding example, the file CUSTDB is opened with an ISZ=100; i.e. the disk space occupied by the file is to be considered as a series of 100 byte records. The FOR..NEXT loop then reads the first 50 records (bytes 0 through 5000) and prints them in Hex via the HTA( ) function. Record index zero (0) in the preceding example would consist of bytes 1 through 100. Index one (1) would be bytes 101 through 200, and so on.

One of the main reasons for accessing files via the ISZ= option is to read and/or update databases or other files maintained by other applications, such as word processors, spreadsheets, etc. Another reason may be to provide direct access to disk contents to correct an error that may have occurred on a file.

Warning!
Use the ISZ= feature carefully, as there is no attempt by PxPlus to verify the data you are writing is correct for the type of file being updated. It is possible to accidentally corrupt data if you update the file with the wrong data.

For other options for storing and retrieving external data, see Data Integration.