When a user may choose a device for I/O, GT.M routines can take one of two basic programming approaches.
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The user selection directs the program into different code branches, each of which handles a different device type.
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The user selection identifies the device. There is a single code path written with a full complement of deviceparameters to handle all selectable device types.
The latter approach is called device-independent programming. To permit device independent programming, GT.M uses the same deviceparameter for all devices that have an equivalent facility, and ignores deviceparameters applied to a device that does not support that facility.
Example:
OPEN dev:(EXCE=exc:REWIND:VARIABLE:NEWVERSION:WRITEONLY)
This OPENs a device with deviceparameters that affect different devices. Only the EXCEPTION has an effect for all device types. When dev is a terminal or a null device, GT.M ignores the other deviceparameters. When dev is an RMS file on disk, GT.M uses REWIND, VARIABLE, and NEWVERSION. When dev is a mailbox, GT.M only uses WRITEONLY. This command performs a valid OPEN for all the different device types.
![[Note]](images/note.png)
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A file that has been previously created and contains data that should be retained can also be opened with the device parameter APPEND. |