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 signal Lisp errors when invoked inappropriately,
and during ordinary editing it would be very inconvenient to enter the
debugger each time this happens. So if you want errors to enter the
debugger, set the variable debug-on-error to non-
toggle-debug-on-error provides an easy way to do
This variable determines whether the debugger is called when an error
is signaled and not handled. If debug-on-error is t,
all kinds of errors call the debugger, except those listed in
debug-ignored-errors (see below). If it is
call the debugger.
The value can also be a list of error conditions (see Signaling Errors). Then the debugger is called only for error conditions in
this list (except those also listed in debug-ignored-errors).
For example, if you set debug-on-error to the list
(void-variable), the debugger is only called for errors about a
variable that has no value.
Note that eval-expression-debug-on-error overrides this variable in some cases; see below.
When this variable is non-
nil, Emacs does not create an error
handler around process filter functions and sentinels. Therefore,
errors in these functions also invoke the debugger. See Processes.
This variable specifies errors which should not enter the debugger, regardless of the value of debug-on-error. 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.
The normal value of this variable includes
user-error, as well
as several errors that happen often during editing but rarely result
from bugs in Lisp programs. However, “rarely” is not “never”; if
your program fails with an error that matches this list, you may try
changing this list to debug the error. The easiest way is usually to
set debug-ignored-errors to
If this variable has a non-
nil value (the default), running the
eval-expression causes debug-on-error to be
temporarily bound to to t. See Evaluating
Emacs-Lisp Expressions in The GNU Emacs Manual.
If eval-expression-debug-on-error is
nil, then the value
of debug-on-error is not changed during
Normally, errors caught by
condition-case never invoke the
condition-case gets a chance to handle the error
before the debugger gets a chance.
If you change debug-on-signal to a non-
nil value, the
debugger gets the first chance at every error, regardless of the
condition-case. (To invoke the debugger, the error
must still fulfill the criteria specified by debug-on-error and
Warning: Setting this variable to non-
nil may have
annoying effects. Various parts of Emacs catch errors in the normal
course of affairs, and you may not even realize that errors happen
there. If you need to debug code wrapped in
condition-case-unless-debug (see Handling Errors).
If you set debug-on-event to a special event (see Special Events), Emacs will try to enter the debugger as soon as it receives
this event, bypassing special-event-map. At present, the only
supported values correspond to the signals
SIGUSR2 (this is the default). This can be helpful when
inhibit-quit is set and Emacs is not otherwise responding.
If you set debug-on-message to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message.
To debug an error that happens during loading of the init
file, use the option ‘--debug-init’. This binds
debug-on-error to t while loading the init file, and
condition-case which normally catches errors in the