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There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.
1.1 The Lisp Debugger | How the Emacs Lisp debugger is implemented. | |
1.2 Debugging Invalid Lisp Syntax | How to find syntax errors. | |
1.3 Debugging Problems in Compilation | How to find errors that show up in byte compilation. | |
• Edebug | A source-level Emacs Lisp debugger. |
Another useful debugging tool is the dribble file. When a dribble file is open, XEmacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. @xref{Terminal Input}.
For debugging problems in terminal descriptions, the
open-termscript
function can be useful. @xref{Terminal Output}.
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The Lisp debugger provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of XEmacs are available; you can even run programs that will enter the debugger recursively. @xref{Recursive Editing}.
1.1.1 Entering the Debugger on an Error | Entering the debugger when an error happens. | |
1.1.2 Debugging Infinite Loops | Stopping and debugging a program that doesn’t exit. | |
1.1.3 Entering the Debugger on a Function Call | Entering it when a certain function is called. | |
1.1.4 Explicit Entry to the Debugger | Entering it at a certain point in the program. | |
1.1.5 Using the Debugger | What the debugger does; what you see while in it. | |
1.1.6 Debugger Commands | Commands used while in the debugger. | |
1.1.7 Invoking the Debugger | How to call the function debug .
| |
1.1.8 Internals of the Debugger | Subroutines of the debugger, and global variables. |
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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-nil
.
This variable determines whether the debugger is called when an error is
signaled and not handled. If debug-on-error
is t
, all
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
, Emacs does not catch errors that
happen in process filter functions and sentinels. Therefore, these
errors also can invoke the debugger. @xref{Processes}.
To debug an error that happens during loading of the ‘.emacs’
file, use the option ‘-debug-init’, which binds
debug-on-error
to 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 after-init-hook
, like
this:
(add-hook 'after-init-hook '(lambda () (setq debug-on-error t)))
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When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes quit.
Ordinary quitting gives no information about why the program was
looping. To get more information, you can set the variable
debug-on-quit
to non-nil
. Quitting with C-g is not
considered an error, and debug-on-error
has no effect on the
handling of C-g. Likewise, debug-on-quit
has no effect on
errors.
Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you will probably get enough information to solve the problem.
This variable determines whether the debugger is called when quit
is signaled and not handled. If debug-on-quit
is non-nil
,
then the debugger is called whenever you quit (that is, type C-g).
If debug-on-quit
is nil
, then the debugger is not called
when you quit. @xref{Quitting}.
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To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.
This function requests function-name to invoke the debugger each time
it is called. It works by inserting the form (debug 'debug)
into
the function definition as the first form.
Any function defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can’t debug primitive functions (i.e., those written in C) this way.
When debug-on-entry
is called interactively, it prompts
for function-name in the minibuffer.
If the function is already set up to invoke the debugger on entry,
debug-on-entry
does nothing.
Note: if you redefine a function after using
debug-on-entry
on it, the code to enter the debugger is lost.
debug-on-entry
returns function-name.
(defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) ⇒ fact
(debug-on-entry 'fact) ⇒ fact
(fact 3)
------ Buffer: *Backtrace* ------ Entering: * fact(3) eval-region(4870 4878 t) byte-code("...") eval-last-sexp(nil) (let ...) eval-insert-last-sexp(nil) * call-interactively(eval-insert-last-sexp) ------ Buffer: *Backtrace* ------
(symbol-function 'fact) ⇒ (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n)))))
This function undoes the effect of debug-on-entry
on
function-name. When called interactively, it prompts for
function-name in the minibuffer. If function-name is
nil
or the empty string, it cancels debugging for all functions.
If cancel-debug-on-entry
is called more than once on the same
function, the second call does nothing. cancel-debug-on-entry
returns function-name.
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You can cause the debugger to be called at a certain point in your
program by writing the expression (debug)
at that point. To do
this, visit the source file, insert the text ‘(debug)’ at the
proper place, and type C-M-x. Be sure to undo this insertion
before you save the file!
The place where you insert ‘(debug)’ must be a place where an
additional form can be evaluated and its value ignored. (If the value
of (debug)
isn’t ignored, it will alter the execution of the
program!) The most common suitable places are inside a progn
or
an implicit progn
(@pxref{Sequencing}).
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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 XEmacs 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 (@pxref{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 kills the backtrace buffer.
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.
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.
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Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of XEmacs, such as switching windows or buffers, are still available.)
The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source file for the function and type C-M-x on its definition.)
Here is a list of Debugger mode commands:
Exit the debugger and continue execution. This resumes execution of the program as if the debugger had never been entered (aside from the effect of any variables or data structures you may have changed while inside the debugger).
Continuing when an error or quit was signalled will cause the normal
action of the signalling to take place. If you do not want this to
happen, but instead want the program execution to continue as if
the call to signal
did not occur, use the r command.
Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do.
The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag.
Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer.
Don’t enter the debugger when the current frame is exited. This cancels a b command on that frame.
Read a Lisp expression in the minibuffer, evaluate it, and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; e temporarily restores their outside-the-debugger values so you can examine them. This makes the debugger more transparent. By contrast, M-: does nothing special in the debugger; it shows you the variable values within the debugger.
Terminate the program being debugged; return to top-level XEmacs command execution.
If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command.
Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it.
The r command is useful when the debugger was invoked due to exit
from a Lisp call frame (as requested with b); then the value
specified in the r command is used as the value of that frame. It
is also useful if you call debug
and use its return value.
If the debugger was entered at the beginning of a function call, r has the same effect as c, and the specified return value does not matter.
If the debugger was entered through a call to signal
(i.e. as a
result of an error or quit), then returning a value will cause the
call to signal
itself to return, rather than throwing to
top-level or invoking a handler, as is normal. This allows you to
correct an error (e.g. the type of an argument was wrong) or continue
from a debug-on-quit
as if it never happened.
Note that some errors (e.g. any error signalled using the error
function, and many errors signalled from a primitive function) are not
continuable. If you return a value from them and continue execution,
then the error will immediately be signalled again. Other errors
(e.g. wrong-type-argument errors) will be continually resignalled
until the problem is corrected.
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Here we describe fully the function used to invoke the debugger.
This function enters the debugger. It switches buffers to a buffer named ‘*Backtrace*’ (or ‘*Backtrace*<2>’ if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode.
The Debugger mode c and r commands exit the recursive edit;
then debug
switches back to the previous buffer and returns to
whatever called debug
. This is the only way the function
debug
can return to its caller.
If the first of the debugger-args passed to debug
is
nil
(or if it is not one of the special values in the table
below), then debug
displays the rest of its arguments at the
top of the ‘*Backtrace*’ buffer. This mechanism is used to display
a message to the user.
However, if the first argument passed to debug
is one of the
following special values, then it has special significance. Normally,
these values are passed to debug
only by the internals of XEmacs
and the debugger, and not by programmers calling debug
.
The special values are:
lambda
A first argument of lambda
means debug
was called because
of entry to a function when debug-on-next-call
was
non-nil
. The debugger displays ‘Entering:’ as a line of
text at the top of the buffer.
debug
debug
as first argument indicates a call to debug
because
of entry to a function that was set to debug on entry. The debugger
displays ‘Entering:’, just as in the lambda
case. It also
marks the stack frame for that function so that it will invoke the
debugger when exited.
t
When the first argument is t
, this indicates a call to
debug
due to evaluation of a list form when
debug-on-next-call
is non-nil
. The debugger displays the
following as the top line in the buffer:
Beginning evaluation of function call form:
exit
When the first argument is exit
, it indicates the exit of a
stack frame previously marked to invoke the debugger on exit. The
second argument given to debug
in this case is the value being
returned from the frame. The debugger displays ‘Return value:’ on
the top line of the buffer, followed by the value being returned.
error
When the first argument is error
, the debugger indicates that
it is being entered because an error or quit
was signaled and not
handled, by displaying ‘Signaling:’ followed by the error signaled
and any arguments to signal
. For example,
(let ((debug-on-error t)) (/ 1 0))
------ Buffer: *Backtrace* ------ Signaling: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------
If an error was signaled, presumably the variable
debug-on-error
is non-nil
. If quit
was signaled,
then presumably the variable debug-on-quit
is non-nil
.
nil
Use nil
as the first of the debugger-args when you want
to enter the debugger explicitly. The rest of the debugger-args
are printed on the top line of the buffer. You can use this feature to
display messages—for example, to remind yourself of the conditions
under which debug
is called.
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This section describes functions and variables used internally by the debugger.
The value of this variable is the function to call to invoke the
debugger. Its value must be a function of any number of arguments (or,
more typically, the name of a function). Presumably this function will
enter some kind of debugger. The default value of the variable is
debug
.
The first argument that Lisp hands to the function indicates why it
was called. The convention for arguments is detailed in the description
of debug
.
This function prints a trace of Lisp function calls currently active.
This is the function used by debug
to fill up the
‘*Backtrace*’ buffer. It is written in C, since it must have access
to the stack to determine which function calls are active. The return
value is always nil
.
In the following example, a Lisp expression calls backtrace
explicitly. This prints the backtrace to the stream
standard-output
: in this case, to the buffer
‘backtrace-output’. Each line of the backtrace represents one
function call. The line shows the values of the function’s arguments if
they are all known. If they are still being computed, the line says so.
The arguments of special forms are elided.
(with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) ⇒ nil
----------- Buffer: backtrace-output ------------ backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval-region(1973 2142 #<buffer *scratch*>) byte-code("... for eval-print-last-sexp ...") eval-print-last-sexp(nil) * call-interactively(eval-print-last-sexp) ----------- Buffer: backtrace-output ------------
The character ‘*’ indicates a frame whose debug-on-exit flag is set.
If this variable is non-nil
, it says to call the debugger before
the next eval
, apply
or funcall
. Entering the
debugger sets debug-on-next-call
to nil
.
The d command in the debugger works by setting this variable.
This function sets the debug-on-exit flag of the stack frame level
levels down the stack, giving it the value flag. If flag is
non-nil
, this will cause the debugger to be entered when that
frame later exits. Even a nonlocal exit through that frame will enter
the debugger.
This function is used only by the debugger.
This variable records the debugging status of the current interactive
command. Each time a command is called interactively, this variable is
bound to nil
. The debugger can set this variable to leave
information for future debugger invocations during the same command.
The advantage, for the debugger, of using this variable rather than another global variable is that the data will never carry over to a subsequent command invocation.
The function backtrace-frame
is intended for use in Lisp
debuggers. It returns information about what computation is happening
in the stack frame frame-number levels down.
If that frame has not evaluated the arguments yet (or is a special
form), the value is (nil function arg-forms…)
.
If that frame has evaluated its arguments and called its function
already, the value is (t function
arg-values…)
.
In the return value, function is whatever was supplied as the
CAR of the evaluated list, or a lambda
expression in the
case of a macro call. If the function has a &rest
argument, that
is represented as the tail of the list arg-values.
If frame-number is out of range, backtrace-frame
returns
nil
.
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The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error “End of file during parsing” in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, “Invalid read syntax: ")"” indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?
If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.
However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases.
1.2.1 Excess Open Parentheses | How to find a spurious open paren or missing close. | |
1.2.2 Excess Close Parentheses | How to find a spurious close paren or missing open. |
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The first step is to find the defun that is unbalanced. If there is
an excess open parenthesis, the way to do this is to insert a
close parenthesis at the end of the file and type C-M-b
(backward-sexp
). This will move you to the beginning of the
defun that is unbalanced. (Then type C-<SPC> C-_ C-u
C-<SPC> to set the mark there, undo the insertion of the
close parenthesis, and finally return to the mark.)
The next step is to determine precisely what is wrong. There is no way to be sure of this except to study the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves.
Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don’t use C-M-e to move there, since that too will fail to work until the defun is balanced.
Now you can go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.
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To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file, back up over it, and type C-M-f to find the end of the unbalanced defun. (Then type C-<SPC> C-_ C-u C-<SPC> to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.)
Then find the actual matching close parenthesis by typing C-M-f at the beginning of the defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.
If you don’t see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q with C-_, since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use C-M-q again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, C-M-q should not change anything.
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When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the ‘*Compile-Log*’ buffer, and then stops. The message may state a function name in which the error was found, or it may not. Either way, here is how to find out where in the file the error occurred.
What you should do is switch to the buffer ‘ *Compiler Input*’. (Note that the buffer name starts with a space, so it does not show up in M-x list-buffers.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read.
If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.
If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, this technique can’t localize the error precisely, but can still show you which function to check.
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