At any time, Prolog's execution can be interrupted by typing ^c. Under Windows, interruption may be delayed if Prolog is in certain OS routines, especially when waiting for input; the interrupt will then happen when returning to the OS routine. The following prompt is then displayed:
Prolog interruption (h for help)?
If you then type h, followed by <RET>, you will get a list of the possible responses to this prompt:
Prolog interrupt options:
h help - this list
c continue - do nothing
d debug - debugger will start leaping
t trace - debugger will start creeping
a abort - abort to the current break level
q really abort - abort to the top level
e exit - exit from Prolog
The d option will cause you to enter the debugger the next time control passes to a spypoint. You can then use the g (ancestors) option of the debugger to find out at what level of execution you interrupted the program.
The t option will also cause you to enter the debugger at the next call.
The a option causes an abort to the current break level. In previous releases of Quintus Prolog, the a option ignored break levels, always aborting to the top level (break level 0).
The q option causes an abort to the top level (break level 0). This behavior is identical to the a option in previous Quintus Prolog systems.
In runtime systems, the default behavior on a ^c interrupt is to abort immediately, rather than display the above menu.
For more information on ^c interrupts and signal handlers, see ref-iex-iha.
The predicates that control this are:
halt/0
break/0
abort/0