HP 3000 Manuals

Accessing Files Programmer's Guide

Record Pointers 

The file system uses record pointers to find specific records for your
use.  Physical record pointers (also referred to as block pointers) are
used to locate specific blocks on disk; logical record pointers blocks
and deblocks the logical records in a physical record and indicate
specific logical records within a file.  A file opened with the inhibit 
buffering option parameter set to BUF (the default) is accessed with a
logical record pointer.  A file opened with the inhibit buffering option 
parameter set to NOBUF is accessed with a physical record (or block)
pointer.

Figure 7-1  shows how the physical and logical record pointers operate
together to locate any record in a file.  For any record, the physical
record pointer indicates the correct block, and the logical record
pointer locates the logical record within the block.

The file system uses both the physical and the logical record pointers to
locate records.  Future references to "record pointer" in this manual
will imply this combination.

When you open your file the HPFOPEN/FOPEN intrinsic sets the record
pointer to record 0 (the first record in your file) for all operations.
If you have opened the file with APPEND access, though (using the access 
type option parameter available in HPFOPEN/FOPEN), MPE/iX moves the
record pointer to the end of the file prior to a write operation; this
ensures that any data that you write to the file is added to the end of
the file rather than written over existing data.


          Figure 7-1.  Record Pointers 

Following initialization, the record pointer may remain in position at
the head of your file, or it may be moved by the intrinsics used in
record selection.

MPE/iX 5.0 Documentation