(command-line): If no error data, print "", not nil.

[gnu-emacs] / lispref / debugging.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/debugging
@node Debugging, Streams, Byte Compilation, Top
@chapter Debugging Lisp Programs

  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.

@itemize @bullet
@item
If the problem occurs when you run the program, you can use a Lisp
debugger (either the default debugger or Edebug) to investigate what is
happening during execution.

@item
If the problem is syntactic, so that Lisp cannot even read the program,
you can use the Emacs facilities for editing Lisp to localize it.

@item
If the problem occurs when trying to compile the program with the byte
compiler, you need to know how to examine the compiler's input buffer.
@end itemize

@menu
* Debugger::            How the Emacs Lisp debugger is implemented.
* Syntax Errors::       How to find syntax errors.
* Compilation Errors::  How to find errors that show up in byte compilation.
* Edebug::              A source-level Emacs Lisp debugger.
@end menu

  Another useful debugging tool is the dribble file.  When a dribble
file is open, Emacs 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
@code{open-termscript} function can be useful.  @xref{Terminal Output}.

@node Debugger
@section The Lisp Debugger
@cindex debugger
@cindex Lisp debugger
@cindex break

  The @dfn{Lisp debugger} provides the ability to suspend evaluation of
a form.  While evaluation is suspended (a state that is commonly known
as a @dfn{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 Emacs are
available; you can even run programs that will enter the debugger
recursively.  @xref{Recursive Editing}.

@menu
* Error Debugging::       Entering the debugger when an error happens.
* Infinite Loops::        Stopping and debugging a program that doesn't exit.
* Function Debugging::    Entering it when a certain function is called.
* Explicit Debug::        Entering it at a certain point in the program.
* Using Debugger::        What the debugger does; what you see while in it.
* Debugger Commands::     Commands used while in the debugger.
* Invoking the Debugger:: How to call the function @code{debug}.
* Internals of Debugger:: Subroutines of the debugger, and global variables.
@end menu

@node Error Debugging
@subsection Entering the Debugger on an Error
@cindex error debugging
@cindex debugging errors

  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 @kbd{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 @code{debug-on-error} to non-@code{nil}.

@defopt debug-on-error
This variable determines whether the debugger is called when a error is
signaled and not handled.  If @code{debug-on-error} is @code{t}, all
errors call the debugger.  If it is @code{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
@code{(void-variable)}, then only errors about a variable that has no
value invoke the debugger.
@end defopt

  To debug an error that happens during loading of the @file{.emacs}
file, use the option @samp{-debug-init}, which binds
@code{debug-on-error} to @code{t} while @file{.emacs} is loaded.

  If your @file{.emacs} file sets @code{debug-on-error}, the effect
lasts only until the end of loading @file{.emacs}.  (This is an
undesirable by-product of the @samp{-debug-init} feature.)  If you want
@file{.emacs} to set @code{debug-on-error} permanently, use
@code{after-init-hook}, like this:

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

@node Infinite Loops
@subsection Debugging Infinite Loops
@cindex infinite loops
@cindex loops, infinite
@cindex quitting from infinite loop
@cindex stopping an infinite loop

  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 @kbd{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
@code{debug-on-quit} to non-@code{nil}.  Quitting with @kbd{C-g} is not
considered an error, and @code{debug-on-error} has no effect on the
handling of @kbd{C-g}.  Contrariwise, @code{debug-on-quit} has no effect
on errors.@refill

  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.

@defopt debug-on-quit
This variable determines whether the debugger is called when @code{quit}
is signaled and not handled.  If @code{debug-on-quit} is non-@code{nil},
then the debugger is called whenever you quit (that is, type @kbd{C-g}).
If @code{debug-on-quit} is @code{nil}, then the debugger is not called
when you quit.  @xref{Quitting}.
@end defopt

@node Function Debugging
@subsection Entering the Debugger on a Function Call
@cindex function call debugging
@cindex debugging specific functions

  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.

@deffn Command debug-on-entry function-name
  This function requests @var{function-name} to invoke the debugger each time
it is called.  It works by inserting the form @code{(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 @code{debug-on-entry} is called interactively, it prompts
for @var{function-name} in the minibuffer.

  If the function is already set up to invoke the debugger on entry,
@code{debug-on-entry} does nothing.

  Caveat: if you redefine a function after using @code{debug-on-entry}
on it, the code to enter the debugger is lost.

  @code{debug-on-entry} returns @var{function-name}.

@example
@group
(defun fact (n)
  (if (zerop n) 1
      (* n (fact (1- n)))))
     @result{} fact
@end group
@group
(debug-on-entry 'fact)
     @result{} fact
@end group
@group
(fact 3)
     @result{} 6
@end group

@group
------ 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* ------
@end group

@group
(symbol-function 'fact)
     @result{} (lambda (n)
          (debug (quote debug))
          (if (zerop n) 1 (* n (fact (1- n)))))
@end group
@end example
@end deffn

@deffn Command cancel-debug-on-entry function-name
This function undoes the effect of @code{debug-on-entry} on
@var{function-name}.  When called interactively, it prompts for
@var{function-name} in the minibuffer.

If @code{cancel-debug-on-entry} is called more than once on the same
function, the second call does nothing.  @code{cancel-debug-on-entry}
returns @var{function-name}.
@end deffn

@node Explicit Debug
@subsection Explicit Entry to the Debugger

  You can cause the debugger to be called at a certain point in your
program by writing the expression @code{(debug)} at that point.  To do
this, visit the source file, insert the text @samp{(debug)} at the
proper place, and type @kbd{C-M-x}.  Be sure to undo this insertion
before you save the file!

  The place where you insert @samp{(debug)} must be a place where an
additional form can be evaluated and its value ignored.  (If the value
isn't ignored, it will alter the execution of the program!)  The most
common suitable places are inside a @code{progn} or an implicit
@code{progn} (@pxref{Sequencing}).

@node Using Debugger
@subsection Using the Debugger

  When the debugger is entered, it displays the previously selected
buffer in one window and a buffer named @samp{*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 (@pxref{Recursive Editing})
and it is wise to go back to the backtrace buffer and exit the debugger
(with the @kbd{q} command) when you are finished with it.  Exiting
the debugger gets out of the recursive edit and kills the backtrace
buffer.

@cindex current stack frame
  The contents of the backtrace buffer show 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 @dfn{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.

@need 3000

@node Debugger Commands
@subsection Debugger Commands
@cindex debugger command list

  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 Emacs, 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 @kbd{C-M-x} on its definition.)

  Here is a list of Debugger mode commands:

@table @kbd
@item c
Exit the debugger and continue execution.  When continuing is possible,
it 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 is possible after entry to the debugger due to function entry
or exit, explicit invocation, or quitting.  You cannot continue if the
debugger was entered because of an error.

@item d
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 @kbd{u} command
to cancel this flag.

@item b
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.

@item u
Don't enter the debugger when the current frame is exited.  This
cancels a @kbd{b} command on that frame.

@item e
Read a Lisp expression in the minibuffer, evaluate it, and print the
value in the echo area.  The debugger alters certain important variables
as part of its operation; @kbd{e} temporarily restores their
outside-the-debugger values so you can examine them.  This makes the
debugger more transparent.  By contrast, @kbd{M-@key{ESC}} does nothing
special in the debugger; it shows you the variable values within the
debugger.

@item q
Terminate the program being debugged; return to top-level Emacs
command execution.

If the debugger was entered due to a @kbd{C-g} but you really want
to quit, and not debug, use the @kbd{q} command.

@item r
Return a value from the debugger.  The value is computed by reading an
expression with the minibuffer and evaluating it.

The @kbd{r} command makes a difference when the debugger was invoked due
to exit from a Lisp call frame (as requested with @kbd{b}); then the
value specified in the @kbd{r} command is used as the value of that
frame.

You can't use @kbd{r} when the debugger was entered due to an error.
@end table

@node Invoking the Debugger
@subsection Invoking the Debugger

  Here we describe fully the function used to invoke the debugger.

@defun debug &rest debugger-args
This function enters the debugger.  It switches buffers to a buffer
named @samp{*Backtrace*} (or @samp{*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 @kbd{c} and @kbd{r} commands exit the recursive edit;
then @code{debug} switches back to the previous buffer and returns to
whatever called @code{debug}.  This is the only way the function
@code{debug} can return to its caller.

If the first of the @var{debugger-args} passed to @code{debug} is
@code{nil} (or if it is not one of the special values in the table
below), then @code{debeg} displays the rest of its arguments at the the
top of the @samp{*Backtrace*} buffer.  This mechanism is used to display
a message to the user.

However, if the first argument passed to @code{debug} is one of the
following special values, then it has special significance.  Normally,
these values are passed to @code{debug} only by the internals of Emacs
and the debugger, and not by programmers calling @code{debug}.

The special values are:

@table @code
@item lambda
@cindex @code{lambda} in debug
A first argument of @code{lambda} means @code{debug} was called because
of entry to a function when @code{debug-on-next-call} was
non-@code{nil}.  The debugger displays @samp{Entering:} as a line of
text at the top of the buffer.

@item debug
@code{debug} as first argument indicates a call to @code{debug} because
of entry to a function that was set to debug on entry.  The debugger
displays @samp{Entering:}, just as in the @code{lambda} case.  It also
marks the stack frame for that function so that it will invoke the
debugger when exited.

@item t
When the first argument is @code{t}, this indicates a call to
@code{debug} due to evaluation of a list form when
@code{debug-on-next-call} is non-@code{nil}.  The debugger displays the
following as the top line in the buffer:

@smallexample
Beginning evaluation of function call form:
@end smallexample

@item exit
When the first argument is @code{exit}, it indicates the exit of a
stack frame previously marked to invoke the debugger on exit.  The
second argument given to @code{debug} in this case is the value being
returned from the frame.  The debugger displays @samp{Return value:} on
the top line of the buffer, followed by the value being returned.

@item error
@cindex @code{error} in debug
When the first argument is @code{error}, the debugger indicates that
it is being entered because an error or @code{quit} was signaled and not
handled, by displaying @samp{Signaling:} followed by the error signaled
and any arguments to @code{signal}.  For example,

@example
@group
(let ((debug-on-error t))
  (/ 1 0))
@end group

@group
------ Buffer: *Backtrace* ------
Signaling: (arith-error)
  /(1 0)
...
------ Buffer: *Backtrace* ------
@end group
@end example

If an error was signaled, presumably the variable
@code{debug-on-error} is non-@code{nil}.  If @code{quit} was signaled,
then presumably the variable @code{debug-on-quit} is non-@code{nil}.

@item nil
Use @code{nil} as the first of the @var{debugger-args} when you want
to enter the debugger explicitly.  The rest of the @var{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 @code{debug} is called.
@end table
@end defun

@need 5000

@node Internals of Debugger
@subsection Internals of the Debugger

  This section describes functions and variables used internally by the
debugger.

@defvar 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
@code{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 @code{debug}.
@end defvar

@deffn Command backtrace
@cindex run time stack
@cindex call stack
This function prints a trace of Lisp function calls currently active.
This is the function used by @code{debug} to fill up the
@samp{*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 @code{nil}.

In the following example, a Lisp expression calls @code{backtrace}
explicitly.  This prints the backtrace to the stream
@code{standard-output}: in this case, to the buffer
@samp{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.

@smallexample
@group
(with-output-to-temp-buffer "backtrace-output"
  (let ((var 1))
    (save-excursion
      (setq var (eval '(progn
                         (1+ var)
                         (list 'testing (backtrace))))))))

     @result{} nil
@end group

@group
----------- 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 ------------
@end group
@end smallexample

The character @samp{*} indicates a frame whose debug-on-exit flag is
set.
@end deffn

@ignore @c Not worth mentioning
@defopt stack-trace-on-error
@cindex stack trace
This variable controls whether Lisp automatically displays a
backtrace buffer after every error that is not handled.  A quit signal
counts as an error for this variable.  If it is non-@code{nil} then a
backtrace is shown in a pop-up buffer named @samp{*Backtrace*} on every
error.  If it is @code{nil}, then a backtrace is not shown.

When a backtrace is shown, that buffer is not selected.  If either
@code{debug-on-quit} or @code{debug-on-error} is also non-@code{nil}, then
a backtrace is shown in one buffer, and the debugger is popped up in
another buffer with its own backtrace.

We consider this feature to be obsolete and superseded by the debugger
itself.
@end defopt
@end ignore

@defvar debug-on-next-call
@cindex @code{eval}, and debugging
@cindex @code{apply}, and debugging
@cindex @code{funcall}, and debugging
If this variable is non-@code{nil}, it says to call the debugger before
the next @code{eval}, @code{apply} or @code{funcall}.  Entering the
debugger sets @code{debug-on-next-call} to @code{nil}.

The @kbd{d} command in the debugger works by setting this variable.
@end defvar

@defun backtrace-debug level flag
This function sets the debug-on-exit flag of the stack frame @var{level}
levels, giving it the value @var{flag}.  If @var{flag} is
non-@code{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.

Normally, this function is only called by the debugger.
@end defun

@defvar command-debug-status
This variable records the debugging status of current interactive
command.  Each time a command is called interactively, this variable is
bound to @code{nil}.  The debugger can set this variable to leave
information for future debugger invocations during the same command.

The advantage of using this variable rather that defining another global
variable is that the data will never carry over to a subsequent command
invocation.
@end defvar

@defun backtrace-frame frame-number
The function @code{backtrace-frame} is intended for use in Lisp
debuggers.  It returns information about what computation is happening
in the stack frame @var{frame-number} levels down.

If that frame has not evaluated the arguments yet (or is a special
form), the value is @code{(nil @var{function} @var{arg-forms}@dots{})}.

If that frame has evaluated its arguments and called its function
already, the value is @code{(t @var{function}
@var{arg-values}@dots{})}.

In the return value, @var{function} is whatever was supplied as @sc{car}
of evaluated list, or a @code{lambda} expression in the case of a macro
call.  If the function has a @code{&rest} argument, that is represented
as the tail of the list @var{arg-values}.

If the argument is out of range, @code{backtrace-frame} returns
@code{nil}.
@end defun

@node Syntax Errors
@section Debugging Invalid Lisp Syntax

  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 @kbd{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.

@menu
* Excess Open::     How to find a spurious open paren or missing close.
* Excess Close::    How to find a spurious close paren or missing open.
@end menu

@node Excess Open
@subsection Excess Open Parentheses

  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 @kbd{C-M-b}
(@code{backward-sexp}).  This will move you to the beginning of the
defun that is unbalanced.  (Then type @kbd{C-@key{SPC} C-_ C-u
C-@key{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 @kbd{C-M-q}
and see what moves.

  Before you do this, make sure the defun has enough close parentheses.
Otherwise, @kbd{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 @kbd{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 @kbd{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 @kbd{C-M-q}, since the old indentation is
probably appropriate to the intended parentheses.

  After you think you have fixed the problem, use @kbd{C-M-q} again.  If
the old indentation actually fit the intended nesting of parentheses,
and you have put back those parentheses, @kbd{C-M-q} should not change
anything.

@node Excess Close
@subsection Excess Close Parentheses

  To deal with an excess close parenthesis, first insert an
open parenthesis at the beginning of the file and type @kbd{C-M-f} to
find the end of the unbalanced defun.  (Then type @kbd{C-@key{SPC} C-_
C-u C-@key{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 @kbd{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 @kbd{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 @kbd{C-M-q}, since the old indentation
is probably appropriate to the intended parentheses.

@node Compilation Errors
@section Debugging Problems in Compilation

  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 @samp{*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 @w{@samp{ *Compiler Input*}}.
(Note that the buffer name starts with a space, so it does not show
up in @kbd{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 @emph{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, it can't localize the error precisely, but can still show you
which function to check.

@include edebug.texi