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GNU Emacs Lisp Reference Manual

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18.1.1 Entering the Debugger on an Error

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 cause Lisp errors when invoked inappropriately (such as C-f at the end of the buffer), 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-nil. (The command toggle-debug-on-error provides an easy way to do this.)

User Option: debug-on-error
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). 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, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. See section 37. Processes.

User Option: debug-ignored-errors
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 debug-on-error.

The normal value of this variable lists 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 will need to change this list in order to debug the error. The easiest way is usually to set debug-ignored-errors to nil.

User Option: debug-on-signal
Normally, errors that are caught by condition-case never run the debugger, even if debug-on-error is non-nil. In other words, condition-case gets a chance to handle the error before the debugger gets a chance.

If you set debug-on-signal to a non-nil value, then the debugger gets the first chance at every error; an error will invoke the debugger regardless of any condition-case, if it fits the criteria specified by the values of debug-on-error and debug-ignored-errors.

Warning: This variable is strong medicine! Various parts of Emacs handle errors in the normal course of affairs, and you may not even realize that errors happen there. If you set debug-on-signal to a non-nil value, those errors will enter the debugger.

Warning: debug-on-signal has no effect when debug-on-error is nil.

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 bypasses the condition-case which normally catches errors in the init file.

If your init file sets debug-on-error, the effect may not last past the end of loading the init file. (This is an undesirable byproduct of the code that implements the `--debug-init' command line option.) The best way to make the init file set debug-on-error permanently is with after-init-hook, like this:

(add-hook 'after-init-hook
          (lambda () (setq debug-on-error t)))

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