NoteRecord access refers to how records will be read from or written to a file, regardless of the file's organization. Record access is specified each time you open a file; it can be different each time. The type of record access permitted is determined by the combination of file organization and record type.
For instance, you can:
Add records to a sequential file with ORGANIZATION= ' SEQUENTIAL ' and POSITION= ' APPEND ' (or use ACCESS= ' APPEND ' ).
Add records sequentially by using multiple WRITE statements, close the file, and then open it again with ORGANIZATION= ' SEQUENTIAL ' and ACCESS= ' SEQUENTIAL ' (or ACCESS= ' DIRECT ' if the sequential file has fixed-length records).
Sequential access transfers records sequentially to or from files or I/O devices such as terminals. You can use sequential I/O with any type of supported file organization and record type.
If you select sequential access mode for files with sequential or relative organization, records are written to or read from the file starting at the beginning of the file and continuing through it, one record after another. A particular record can be retrieved only after all of the records preceding it have been read; new records can be written only at the end of the file.
Direct access transfers records selected by record number to and from either sequential files stored on disk with a fixed-length record type or relative organization files.
If you select direct access mode, you can determine the order in which records are read or written. Each READ or WRITE statement must include the relative record number, indicating the record to be read or written.
You can directly access a sequential disk file only if it contains fixed-length records. Because direct access uses cell numbers to find records, you can enter successive READ or WRITE statements requesting records that either precede or follow previously requested records. For example, the first of the following statements reads record 24; the second reads record 10:
READ (12,REC=24) I
READ (12,REC=10) J
You can use both access modes on sequential and relative files. However, direct access to a sequential organization file can only be done if the file resides on disk and contains fixed-length records.
The table below summarizes the types of access permitted for the various combinations of file organizations and record types.
|
Record Type |
Sequential Access? |
Direct Access? |
|
Sequential file organization |
||
|
Fixed |
Yes |
Yes |
|
Variable |
Yes |
No |
|
Segmented |
Yes |
No |
|
Stream |
Yes |
No |
|
Stream_CR |
Yes |
No |
|
Stream_LF |
Yes |
No |
|
Relative file organization |
||
|
Fixed |
Yes |
Yes |
Note
Direct access and relative files require that the file resides on a disk device.