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18.1.7 Invoking the Debugger

Here we describe in full detail the function debug that is used to invoke the debugger.

— Command: debug &rest debugger-args

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, d, j, 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.

The use of the debugger-args is that debug displays the rest of its arguments at the top of the *Backtrace* buffer, so that the user can see them. Except as described below, this is the only way these arguments are used.

However, certain values for first argument to debug have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling debug.) Here is a table of these special values:

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 ‘Debugger entered--entering a function:’ as a line of text at the top of the buffer.
debug
debug as first argument means debug was called because of entry to a function that was set to debug on entry. The debugger displays the string ‘Debugger entered--entering a function:’, 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 function call form when debug-on-next-call is non-nil. The debugger displays ‘Debugger entered--beginning evaluation of function call form:’ as the top line in the buffer.
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 ‘Debugger entered--returning value:’ in 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 ‘Debugger entered--Lisp error:’ followed by the error signaled and any arguments to signal. For example,
               (let ((debug-on-error t))
                 (/ 1 0))
               
               ------ Buffer: *Backtrace* ------
               Debugger entered--Lisp error: (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.