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K.2 Debugger Mode

• https://www.emacswiki.org/emacs/DebuggerMode

Debugger mode is the mode used in the BacktraceBuffer (buffer ‘*Backtrace*’). It shows the “frames” of your execution stack. A line starts with ‘*’ if exiting that frame will call the debugger.

Debugger Keys

‘C-h m’ gives you information on the current mode.

b

break point – place an asterisk (‘*’) on a stack frame

u

remove break point

c

continue

d

single step

j

keep existing frames until you reach the current frame

e

eval another sexp that you input

q

quit

Entering debugger mode

• You can use M-x debug-on-entry to put breakpoints at the start of functions, and M-x cancel-debug-on-entry to remove them.
• You can put (debug) in EmacsLisp code at arbitrary points (and then eval that code) to put breakpoints at those places.
• You can set any of the following variables:

  debug-on-quit

  Debugger is entered when the user hits ‘C-g’. This is useful in debugging infinite loops and long delays. You can toggle this variable using M-x toggle-debug-on-quit.

  debug-on-error

  Debugger is entered when an uncaught error happens. This is foiled by condition-case forms that catch errors. You can toggle this variable using ‘M-x toggle-debug-on-error’. Note that some errors are ignored — you may have to temporarily set debug-ignored-errors to ‘nil’ in order to debug such errors.

  stack-trace-on-error

  An alternative to debug-on-error. It shows trace buffers instead of entering the debugger. Example: (setq stack-trace-on-error '(buffer-read-only))