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(elisp)Debugger Commands

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Debugger Commands
-----------------

   Inside the debugger (in Debugger mode), these special commands are
available in addition to the usual cursor motion commands.  (Keep in
mind that all the usual facilities of Emacs, such as switching windows
or buffers, are still available.)

   The most important use of debugger commands is for stepping through
code, so that you can see how control flows.  The debugger can step
through the control structures of an interpreted function, but cannot do
so in a byte-compiled function.  If you would like to step through a
byte-compiled function, replace it with an interpreted definition of the
same function.  (To do this, visit the source for the function and type
`C-M-x' on its definition.)

   Here is a list of Debugger mode commands:

`c'
     Exit the debugger and continue execution.  When continuing is
     possible, it resumes execution of the program as if the debugger
     had never been entered (aside from any side-effects that you
     caused by changing variable values or data structures while inside
     the debugger).

     Continuing is possible after entry to the debugger due to function
     entry or exit, explicit invocation, or quitting.  You cannot
     continue if the debugger was entered because of an error.

`d'
     Continue execution, but enter the debugger the next time any Lisp
     function is called.  This allows you to step through the
     subexpressions of an expression, seeing what values the
     subexpressions compute, and what else they do.

     The stack frame made for the function call which enters the
     debugger in this way will be flagged automatically so that the
     debugger will be called again when the frame is exited.  You can
     use the `u' command to cancel this flag.

`b'
     Flag the current frame so that the debugger will be entered when
     the frame is exited.  Frames flagged in this way are marked with
     stars in the backtrace buffer.

`u'
     Don't enter the debugger when the current frame is exited.  This
     cancels a `b' command on that frame.  The visible effect is to
     remove the star from the line in the backtrace buffer.

`e'
     Read a Lisp expression in the minibuffer, evaluate it, and print
     the value in the echo area.  The debugger alters certain important
     variables, and the current buffer, as part of its operation; `e'
     temporarily restores their values from outside the debugger, so
     you can examine and change them.  This makes the debugger more
     transparent.  By contrast, `M-:' does nothing special in the
     debugger; it shows you the variable values within the debugger.

`R'
     Like `e', but also save the result of evaluation in the buffer
     `*Debugger-record*'.

`q'
     Terminate the program being debugged; return to top-level Emacs
     command execution.

     If the debugger was entered due to a `C-g' but you really want to
     quit, and not debug, use the `q' command.

`r'
     Return a value from the debugger.  The value is computed by
     reading an expression with the minibuffer and evaluating it.

     The `r' command is useful when the debugger was invoked due to exit
     from a Lisp call frame (as requested with `b' or by entering the
     frame with `d'); then the value specified in the `r' command is
     used as the value of that frame.  It is also useful if you call
     `debug' and use its return value.  Otherwise, `r' has the same
     effect as `c', and the specified return value does not matter.

     You can't use `r' when the debugger was entered due to an error.