When the debugger is entered, it displays the previously selected buffer in one window and a buffer named *Backtrace* in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error).
The backtrace buffer is read-only and uses a special major mode,
Debugger mode, in which letters are defined as debugger commands. The
usual Emacs editing commands are available; thus, you can switch windows
to examine the buffer that was being edited at the time of the error,
switch buffers, visit files, or do any other sort of editing. However,
the debugger is a recursive editing level (see Recursive Editing)
and it is wise to go back to the backtrace buffer and exit the debugger
(with the q command) when you are finished with it. Exiting
the debugger gets out of the recursive edit and buries the backtrace
buffer. (You can customize what the q command does with the
backtrace buffer by setting the variable debugger-bury-or-kill.
For example, set it to
kill if you prefer to kill the buffer
rather than bury it. Consult the variable’s documentation for more
When the debugger has been entered, the debug-on-error
variable is temporarily set according to
eval-expression-debug-on-error. If the latter variable is
nil, debug-on-error will temporarily be set to
t. This means that any further errors that occur while doing a
debugging session will (by default) trigger another backtrace. If
this is not what you want, you can either set
nil, or set
The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the current frame. Some of the debugger commands operate on the current frame. If a line starts with a star, that means that exiting that frame will call the debugger again. This is useful for examining the return value of a function.
If a function name is underlined, that means the debugger knows where its source code is located. You can click with the mouse on that name, or move to it and type RET, to visit the source code.
The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.