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Here we describe fully the function used to invoke the debugger.
This function enters the debugger. It switches buffers to a buffer named ‘*Backtrace*’ (or ‘*Backtrace*<2>’ if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode.
The Debugger mode c and r commands exit the recursive edit;
then debug
switches back to the previous buffer and returns to
whatever called debug
. This is the only way the function
debug
can return to its caller.
If the first of the debugger-args passed to debug
is
nil
(or if it is not one of the special values in the table
below), then debug
displays the rest of its arguments at the
top of the ‘*Backtrace*’ buffer. This mechanism is used to display
a message to the user.
However, if the first argument passed to debug
is one of the
following special values, then it has special significance. Normally,
these values are passed to debug
only by the internals of XEmacs
and the debugger, and not by programmers calling debug
.
The special values are:
lambda
A first argument of lambda
means debug
was called because
of entry to a function when debug-on-next-call
was
non-nil
. The debugger displays ‘Entering:’ as a line of
text at the top of the buffer.
debug
debug
as first argument indicates a call to debug
because
of entry to a function that was set to debug on entry. The debugger
displays ‘Entering:’, just as in the lambda
case. It also
marks the stack frame for that function so that it will invoke the
debugger when exited.
t
When the first argument is t
, this indicates a call to
debug
due to evaluation of a list form when
debug-on-next-call
is non-nil
. The debugger displays the
following as the top line in the buffer:
Beginning evaluation of function call form:
exit
When the first argument is exit
, it indicates the exit of a
stack frame previously marked to invoke the debugger on exit. The
second argument given to debug
in this case is the value being
returned from the frame. The debugger displays ‘Return value:’ on
the top line of the buffer, followed by the value being returned.
error
When the first argument is error
, the debugger indicates that
it is being entered because an error or quit
was signaled and not
handled, by displaying ‘Signaling:’ followed by the error signaled
and any arguments to signal
. For example,
(let ((debug-on-error t)) (/ 1 0))
------ Buffer: *Backtrace* ------ Signaling: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------
If an error was signaled, presumably the variable
debug-on-error
is non-nil
. If quit
was signaled,
then presumably the variable debug-on-quit
is non-nil
.
nil
Use nil
as the first of the debugger-args when you want
to enter the debugger explicitly. The rest of the debugger-args
are printed on the top line of the buffer. You can use this feature to
display messages—for example, to remind yourself of the conditions
under which debug
is called.
Next: Internals of Debugger, Previous: Debugger Commands, Up: Debugger [Contents][Index]