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Parameter States
A parameter has a number of states. Initially, a parameter has no
value, and is said to be in the ground state. When the
parameter has been given a value, the parameter moves to the active
state. A parameter acquires a value in the first instance by
looking for one supplied on the command line. Failing that, when the program
requests a parameter value, the parameter system will attempt to get a value
from one of a number of sources specified by
the VPATH Field
in the Interface Module (see SUN/115).
This may cause the user to be prompted and the prompt may contain a
suggested value which can be accepted by just hitting the
RETURN key.
If a program requests a value for a parameter already in the active state,
the existing value is returned.
To obtain a new value the program must first cancel the
parameter, moving it to the cancelled state.
DAT_CANCL does this as well as annulling the
associated locator.
When the application gets a value for a cancelled parameter
the VPATH Field
is ignored and the user prompted. When a value is obtained, the parameter
returns to the active state.
The parameter may also go into the null state. This occurs if the
parameter is given the null value, for example by the user entering !
in response to a prompt. When the parameter is in the null state,
any attempt to get its value will return status PAR__NULL - it
must be cancelled before another value can be obtained.
Next: Creating Objects
Up: HDSPAR - HDS Parameter Routines
Previous: Associating with an Object
HDSPAR - HDS Parameter Routines
Starlink User Note 224
A J Chipperfield
10 February 2000
E-mail:ussc@star.rl.ac.uk
Copyright © 2000 Council for the Central Laboratory of the Research Councils