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Restrictions on Mapped Access   

Certain restrictions are imposed on mapped access to NDF components, the most important being that each array component may only be mapped for access once through any NDF identifier. Thus, two successive calls to NDF_MAP (or NDF_MAPZ) specifying the same identifier and array component will result in an error. Of course, the component may later be re-mapped if it has first been unmapped, either by an intervening call to NDF_UNMAP, or implicitly.

One way of avoiding this restriction is to clone a duplicate identifier using NDF_CLONE so that a component can be mapped twice, once through each identifier. However, a more fundamental restriction exists if a possible access conflict may occur. Update and write access to NDF components is always exclusive, so the NDF_ system will reject any attempt to access a component twice if one of the attempts involves update or write access, although the same component may be accessed any number of times (through different identifiers) for read access alone.



next up previous
Next: BAD PIXELS
Up: ACCESSING ARRAY COMPONENTS
Previous: Mapping the Variance Component as Standard Deviations


Starlink User Note 33
R.F. Warren-Smith
11th January 2000
E-mail:rfws@star.rl.ac.uk

Copyright © 2000 Council for the Central Laboratory of the Research Councils