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Avoiding Unwanted Recursion  

When writing format conversion utilities, it is often convenient to use the NDF library to access the native NDF format version of the data (see §[*]). However, you should bear in mind that the NDF library's ability to invoke format conversion commands will still be active unless you take action to switch it off. This means that unwanted recursion is possible if a conversion utility accesses a foreign dataset that in turn causes a further conversion utility to be invoked, and so on...

In practice, this is unlikely to be a problem if care is taken to ensure that NDF datasets are never stored in objects whose names might be mistaken for foreign format data files. If it does prove necessary to suppress unwanted format conversion, however, this can be achieved by setting the NDF_ library's DOCVT tuning parameter to zero. This will have the effect of disabling recognition of foreign data files by the conversion utility.

One way of doing this is by setting the environment variable NDF_DOCVT to 0 as part of the format conversion command, immediately before the conversion utility itself is invoked. Alternatively, the conversion utility may call the NDF_TUNE routine itself in order to control recognition of foreign data formats. The latter approach allows individual control over each dataset accessed by the utility if necessary.



next up previous
Next: Debugging Conversion Commands
Up: ADDITIONAL FACILITIES
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Starlink System Note 20
R.F.Warren-Smith & D.S.Berry
17th July 2000
E-mail:ussc@star.rl.ac.uk

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