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XMM-Newton Science Analysis System


dal (dal-1.182.1) [xmmsas_20080701_1801-8.0.0]

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Access Modes

DataSets are accessed with one of the following modes:

• READ Read an existing dataset with the given name. An error is raised if the dataset is not found, or cannot be opened.
• CREATE Create a new dataset with the given name. If an dataset already exisits with the given name, the behaviour is dependent on the setting of the environment variable SAS_FORMAT. Any changes made to the dataset will be discarded upon closure,
• MODIFY Open an existing dataset with the given name. All changes made to the dataset will be written saved upon closure.
• TEMP Open a new dataset. The dataset is discarded upon closure.

When accessing tables, arrays and columns the mode is understood to be only a hint, and gives the DAL an opportunity to be more memory efficient. It should be noted that it is not intended to safeguard the developer from making logical programming mistakes. The underlying reason for this is that the DAL hands out data pointers, and has no way of preventing modifications or even knowing if the data has been modified.

This simple scenario has been adopted because the full bookkeeping on what data has to be written and what data has to be extracted from the original file would have been too expensive in terms of performance and coding.

By default an object inherits the access mode from its parent.