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Primary and Secondary Locators   

Since data objects are stored in container files, HDS has to decide when to open and close these files (it would be very inefficient if a file had to be opened every time an object within it was referenced). To allow control over this, HDS locators are divided into two classes termed primary and secondary. It is primary locators that are responsible for holding container files open.

To be more specific, an HDS container file will remain open, so that data objects within it are accessible, for as long as there is at least one valid primary locator associated with it (that is, with one of the data objects within the file). Not surprisingly, those routines intended for opening container files (HDS_OPEN, HDS_NEW and HDS_WILD) will return primary locators - so that the file subsequently remains open. However, all other routines return secondary locators (with the exception of DAT_PRMRY, which may be used to manipulate this attribute - see §[*]).



next up previous
Next: Container File Reference Counts
Up: USING LOCATORS
Previous: Cloning Locators

HDS Hierarchical Data System
Starlink User Note 92
R.F. Warren-Smith & M.D. Lawden
23rd February 1999
E-mail:rfws@star.rl.ac.uk

Copyright (C) 1999 Central Laboratory of the Research Councils