For default history recording to occur, the NDF's history component must be in a defined state (as explained above). Control over this is normally in the hands of the user of the application, who can run a separate utility to modify the history component of specified NDFs and hence nominate which of them are to accumulate history information.
In addition, the NDF must have been opened either for `WRITE' or `UPDATE' access, so that the current application will have modified (or created) it. Some further conditions are also necessary, as discussed later, but no explicit action is required on the part of the programmer when writing the application. Thus, default history recording is available to even the most rudimentary of NDF applications.
Default history information is normally written to the history
component of an NDF when it is released from the NDF_ system, either
by a call to NDF_ANNUL which annuls the last valid identifier for the
NDF, or implicitly via the cleaning-up action of the NDF_END routine
(see §). In any event, the information written will be the
same, consisting of the following items:
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- | Name of the application which wrote the information |
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- | Date and time the information was written |
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- | User ID for the person who ran the application |
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- | Name of the machine on which the application was running |
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- | Reference name, which fully identifies the NDF dataset |
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- | Free-format text containing any additional information |
The first five of these items contain ``fixed'' information which is
always automatically associated with any new history record and
which may be read back at a later date and acted upon.
The final TEXT item is ``free format'' and there is no restriction on
what this may contain. It is available for applications to write their
own history information, either to augment or to replace that written
by default. When written by default, this history text will
typically record the command line used to invoke the application (or,
depending on the programming environment in use, the values of its
parameters) together with the name of the file which was executed.
The precise content and format of the default history text may depend
on a number of factors, so you
should not write software which depends upon such details.