The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.
However, entry to the debugger is not a normal consequence of an
error. Many commands frequently get Lisp errors when invoked in
inappropriate contexts (such as C-f at the end of the buffer) and
during ordinary editing it would be very unpleasant to enter the
debugger each time this happens. If you want errors to enter the
debugger, set the variable
debug-on-error to non-
This variable determines whether the debugger is called when an error is
signaled and not handled. If
errors call the debugger. If it is
nil, none call the debugger.
The value can also be a list of error conditions that should call the
debugger. For example, if you set it to the list
(void-variable), then only errors about a variable that has no
value invoke the debugger.
When this variable is non-
nil, SXEmacs does not catch errors that
happen in process filter functions and sentinels. Therefore, these
errors also can invoke the debugger. See Processes.
This variable is similar to
debug-on-error but breaks
whenever an error is signalled, regardless of whether it would be
This variable specifies certain kinds of errors that should not enter
the debugger. Its value is a list of error condition symbols and/or
regular expressions. If the error has any of those condition symbols,
or if the error message matches any of the regular expressions, then
that error does not enter the debugger, regardless of the value of
The normal value of this variable lists several errors that happen often during editing but rarely result from bugs in Lisp programs.
To debug an error that happens during loading of the .emacs
file, use the option ‘-debug-init’, which binds
t while .emacs is loaded and
inhibits use of
condition-case to catch init file errors.
If your .emacs file sets
debug-on-error, the effect may
not last past the end of loading .emacs. (This is an undesirable
byproduct of the code that implements the ‘-debug-init’ command
line option.) The best way to make .emacs set
debug-on-error permanently is with
(add-hook 'after-init-hook '(lambda () (setq debug-on-error t)))