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3.4.1 Variable Value Availability

The value of a variable may be unavailable to the debugger in portions of the program where Common Lisp says that the variable is defined. If a variable value is not available, the debugger will not let you read or write that variable. With one exception, the debugger will never display an incorrect value for a variable. Rather than displaying incorrect values, the debugger tells you the value is unavailable.

The one exception is this: if you interrupt (e.g., with ^C) or if there is an unexpected hardware error such as “bus error” (which should only happen in unsafe code), then the values displayed for arguments to the interrupted frame might be incorrect.4 This exception applies only to the interrupted frame: any frame farther down the stack will be fine.

The value of a variable may be unavailable for these reasons:

Since it is especially useful to be able to get the arguments to a function, argument variables are treated specially when the speed optimization quality is less than 3 and the debug quality is at least 1. With this compilation policy, the values of argument variables are almost always available everywhere in the function, even at unknown locations. For non-argument variables, debug must be at least 2 for values to be available, and even then, values are only available at known locations.


Footnotes

(4)

Since the location of an interrupt or hardware error will always be an unknown location (see unknown-locations), non-argument variable values will never be available in the interrupted frame.


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