@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1994, 1998--1999, 2001--2022 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Debugging
@chapter Debugging Lisp Programs
@cindex debugging lisp programs

  There are several ways to find and investigate problems in an Emacs
Lisp program.

@itemize @bullet
@item
If a problem occurs when you run the program, you can use the built-in
Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or
alter its internal state.

@item
You can use Edebug, a source-level debugger for Emacs Lisp.

@item
@cindex tracing Lisp programs
You can trace the execution of functions involved in the problem using
the tracing facilities provided by the @file{trace.el} package.  This
package provides the functions @code{trace-function-foreground} and
@code{trace-function-background} for tracing function calls, and
@code{trace-values} for adding values of select variables to the
trace.  For the details, see the documentation of these facilities in
@file{trace.el}.

@item
If a syntactic problem is preventing Lisp from even reading the
program, you can locate it using Lisp editing commands.

@item
You can look at the error and warning messages produced by the byte
compiler when it compiles the program.  @xref{Compiler Errors}.

@item
You can use the Testcover package to perform coverage testing on the
program.

@item
You can use the ERT package to write regression tests for the program.
@xref{Top,the ERT manual,, ert, ERT: Emacs Lisp Regression Testing}.

@item
You can profile the program to get hints about how to make it more efficient.
@end itemize

  Other useful tools for debugging input and output problems are the
dribble file (@pxref{Terminal Input}) and the @code{open-termscript}
function (@pxref{Terminal Output}).

@menu
* Debugger::            A debugger for the Emacs Lisp evaluator.
* Edebug::              A source-level Emacs Lisp debugger.
* Syntax Errors::       How to find syntax errors.
* Test Coverage::       Ensuring you have tested all branches in your code.
* Profiling::           Measuring the resources that your code uses.
@end menu

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

  The ordinary @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.
* Variable Debugging::    Entering it when a variable is modified.
* Explicit Debug::        Entering it at a certain point in the program.
* Using Debugger::        What the debugger does.
* Backtraces::            What you see while in the debugger.
* 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 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 @code{debug-on-error} to non-@code{nil}.
(The command @code{toggle-debug-on-error} provides an easy way to do
this.)

@defopt debug-on-error
This variable determines whether the debugger is called when an error
is signaled and not handled.  If @code{debug-on-error} is @code{t},
all kinds of errors call the debugger, except those listed in
@code{debug-ignored-errors} (see below).  If it is @code{nil}, none
call the debugger.

The value can also be a list of error conditions (@pxref{Signaling
Errors}).  Then the debugger is called only for error conditions in
this list (except those also listed in @code{debug-ignored-errors}).
For example, if you set @code{debug-on-error} to the list
@code{(void-variable)}, the debugger is only called for errors about a
variable that has no value.

Note that @code{eval-expression-debug-on-error} overrides this
variable in some cases; see below.

When this variable is non-@code{nil}, Emacs does not create an error
handler around process filter functions and sentinels.  Therefore,
errors in these functions also invoke the debugger.  @xref{Processes}.
@end defopt

@defopt debug-ignored-errors
This variable specifies errors which should not enter the debugger,
regardless of the value of @code{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 @code{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 @code{debug-ignored-errors} to @code{nil}.
@end defopt

@defopt eval-expression-debug-on-error
If this variable has a non-@code{nil} value (the default), running the
command @code{eval-expression} causes @code{debug-on-error} to be
temporarily bound to @code{t}.  @xref{Lisp Eval,, Evaluating
Emacs Lisp Expressions, emacs, The GNU Emacs Manual}.

If @code{eval-expression-debug-on-error} is @code{nil}, then the value
of @code{debug-on-error} is not changed during @code{eval-expression}.
@end defopt

@defopt debug-on-signal
Normally, errors caught by @code{condition-case} never invoke the
debugger.  The @code{condition-case} gets a chance to handle the error
before the debugger gets a chance.

If you change @code{debug-on-signal} to a non-@code{nil} value, the
debugger gets the first chance at every error, regardless of the
presence of @code{condition-case}.  (To invoke the debugger, the error
must still fulfill the criteria specified by @code{debug-on-error} and
@code{debug-ignored-errors}.)

@cindex emacsclient, getting a backtrace
@cindex backtrace from emacsclient's @option{--eval}
For example, setting this variable is useful to get a backtrace from
code evaluated by emacsclient's @option{--eval} option.  If Lisp code
evaluated by emacsclient signals an error while this variable is
non-@code{nil}, the backtrace will popup in the running Emacs.

@strong{Warning:} Setting this variable to non-@code{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 @code{condition-case},
consider using @code{condition-case-unless-debug} (@pxref{Handling
Errors}).
@end defopt

@defopt debug-on-event
If you set @code{debug-on-event} to a special event (@pxref{Special
Events}), Emacs will try to enter the debugger as soon as it receives
this event, bypassing @code{special-event-map}.  At present, the only
supported values correspond to the signals @code{SIGUSR1} and
@code{SIGUSR2} (this is the default).  This can be helpful when
@code{inhibit-quit} is set and Emacs is not otherwise responding.
@end defopt

@cindex message, finding what causes a particular message
@defvar debug-on-message
If you set @code{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.
@end defvar

@defvar debug-allow-recursive-debug
You can evaluate forms in the current stack frame in the
@samp{*Backtrace*} buffer with the @key{e} command, and while
edebugging you can use the @key{e} and @key{C-x C-e} commands to do
something similar.  By default, the debugger is inhibited by these
commands (because (re-)entering the debugger at this point will
usually take you out of the debugging context you're in).  Set
@code{debug-allow-recursive-debug} to a non-@code{nil} value to allow
these commands to enter the debugger recursively.
@end defvar

  To debug an error that happens during loading of the init
file, use the option @samp{--debug-init}.  This binds
@code{debug-on-error} to @code{t} while loading the init file, and
bypasses the @code{condition-case} which normally catches errors in the
init file.

@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 a @dfn{quit}.  @xref{Quitting}.

  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}.  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 may get enough information to solve the problem.

  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}.
Likewise, @code{debug-on-quit} has no effect on errors.

@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} (the
default), then the debugger is not called when you quit.
@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.

Any function or macro 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 also set debug-on-entry for primitive functions
(i.e., those written in C) this way, but it only takes effect when the
primitive is called from Lisp code.  Debug-on-entry is not allowed for
special forms.

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

Here's an example to illustrate use of this function:

@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)
@end group

@group
------ Buffer: *Backtrace* ------
Debugger entered--entering a function:
* fact(3)
  eval((fact 3))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)
------ Buffer: *Backtrace* ------
@end group

@end example
@end deffn

@deffn Command cancel-debug-on-entry &optional 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 @var{function-name} is
omitted or @code{nil}, it cancels break-on-entry for all functions.
Calling @code{cancel-debug-on-entry} does nothing to a function which is
not currently set up to break on entry.
@end deffn

@node Variable Debugging
@subsection Entering the debugger when a variable is modified
@cindex variable write debugging
@cindex debugging changes to variables

Sometimes a problem with a function is due to a wrong setting of a
variable.  Setting up the debugger to trigger whenever the variable is
changed is a quick way to find the origin of the setting.

@deffn Command debug-on-variable-change variable
This function arranges for the debugger to be called whenever
@var{variable} is modified.

It is implemented using the watchpoint mechanism, so it inherits the
same characteristics and limitations: all aliases of @var{variable}
will be watched together, only dynamic variables can be watched, and
changes to the objects referenced by variables are not detected.  For
details, see @ref{Watching Variables}.
@end deffn

@deffn Command cancel-debug-on-variable-change &optional variable
This function undoes the effect of @code{debug-on-variable-change} on
@var{variable}.  When called interactively, it prompts for
@var{variable} in the minibuffer.  If @var{variable} is omitted or
@code{nil}, it cancels break-on-change for all variables.  Calling
@code{cancel-debug-on-variable-change} does nothing to a variable
which is not currently set up to break on change.
@end deffn


@node Explicit Debug
@subsection Explicit Entry to the Debugger
@cindex debugger, explicit entry
@cindex force entry to debugger

@cindex @code{eval-defun}, and explicit entry to 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} (@code{eval-defun}, a Lisp mode key
binding).  @strong{Warning:} if you do this for temporary debugging
purposes, 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
of @code{(debug)} 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}).

  If you don't know exactly where in the source code you want to put
the debug statement, but you want to display a backtrace when a
certain message is displayed, you can set @code{debug-on-message} to a
regular expression matching the desired message.

@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 @file{*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).

@vindex debugger-bury-or-kill
  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 buries the backtrace
buffer.  (You can customize what the @kbd{q} command does with the
backtrace buffer by setting the variable @code{debugger-bury-or-kill}.
For example, set it to @code{kill} if you prefer to kill the buffer
rather than bury it.  Consult the variable's documentation for more
possibilities.)

  When the debugger has been entered, the @code{debug-on-error}
variable is temporarily set according to
@code{eval-expression-debug-on-error}.  If the latter variable is
non-@code{nil}, @code{debug-on-error} will temporarily be set to
@code{t}.  However, further errors that occur while debugging won't
(by default) trigger another debugger, because @code{inhibit-debugger}
will also be bound to non-@code{nil}.

  The debugger itself must be run byte-compiled, since it makes
assumptions about the state of the Lisp interpreter.  These
assumptions are false if the debugger is running interpreted.

@node Backtraces
@subsection Backtraces
@cindex backtrace buffer

Debugger mode is derived from Backtrace mode, which is also used to
show backtraces by Edebug and ERT@.  (@pxref{Edebug}, and @ref{Top,the
ERT manual,, ert, ERT: Emacs Lisp Regression Testing}.)

@cindex stack frame
The backtrace buffer shows you the functions that are executing and
their argument values.  When a backtrace buffer is created, it shows
each stack frame on one, possibly very long, line.  (A stack frame is
the place where the Lisp interpreter records information about a
particular invocation of a function.)  The most recently called
function will be at the top.

@cindex current stack frame
In a backtrace you can specify a stack frame by moving point to a line
describing that frame.  The frame whose line point is on is considered
the @dfn{current frame}.

If a function name is underlined, that means Emacs knows where its
source code is located.  You can click with the mouse on that name, or
move to it and type @key{RET}, to visit the source code.  You can also
type @key{RET} while point is on any name of a function or variable
which is not underlined, to see help information for that symbol in a
help buffer, if any exists.  The @code{xref-find-definitions} command,
bound to @kbd{M-.}, can also be used on any identifier in a backtrace
(@pxref{Looking Up Identifiers,,,emacs, The GNU Emacs Manual}).

In backtraces, the tails of long lists and the ends of long strings,
vectors or structures, as well as objects which are deeply nested,
will be printed as underlined ``...''.  You can click with the mouse
on a ``...'', or type @key{RET} while point is on it, to show the part
of the object that was hidden.  To control how much abbreviation is
done, customize @code{backtrace-line-length}.

Here is a list of commands for navigating and viewing backtraces:

@table @kbd
@item v
Toggle the display of local variables of the current stack frame.

@item p
Move to the beginning of the frame, or to the beginning
of the previous frame.

@item n
Move to the beginning of the next frame.

@item +
Add line breaks and indentation to the top-level Lisp form at point to
make it more readable.

@item -
Collapse the top-level Lisp form at point back to a single line.

@item #
Toggle @code{print-circle} for the frame at point.

@item :
Toggle @code{print-gensym} for the frame at point.

@item .
Expand all the forms abbreviated with ``...'' in the frame at point.

@end table

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

  The debugger buffer (in Debugger mode) provides special commands in
addition to the usual Emacs commands and to the Backtrace mode commands
described in the previous section.  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 for the function and
type @kbd{C-M-x} on its definition.)  You cannot use the Lisp debugger
to step through a primitive function.

Some of the debugger commands operate on the current frame.  If a
frame 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.

@c FIXME: Add @findex for the following commands?  --xfq
  Here is a list of Debugger mode commands:

@table @kbd
@item c
Exit the debugger and continue execution.  This resumes execution of
the program as if the debugger had never been entered (aside from any
side-effects that you caused by changing variable values or data
structures while inside the debugger).

@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.  The visible effect is to
remove the star from the line in the backtrace buffer.

@item j
Flag the current frame like @kbd{b}.  Then continue execution like
@kbd{c}, but temporarily disable break-on-entry for all functions that
are set up to do so by @code{debug-on-entry}.

@vindex debug-allow-recursive-debug
@item e
Read a Lisp expression in the minibuffer, evaluate it (with the
relevant lexical environment, if applicable), and print the
value in the echo area.  The debugger alters certain important
variables, and the current buffer, as part of its operation; @kbd{e}
temporarily restores their values from outside the debugger, so you can
examine and change them.  This makes the debugger more transparent.  By
contrast, @kbd{M-:} does nothing special in the debugger; it shows you
the variable values within the debugger.  By default, this command
suppresses the debugger during evaluation, so that an error in the
evaluated expression won't add a new error on top of the existing one.
Set the @code{debug-allow-recursive-debug} user option to a
non-@code{nil} value to override this.

@item R
Like @kbd{e}, but also save the result of evaluation in the
buffer @file{*Debugger-record*}.

@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 is useful when the debugger was invoked due to exit
from a Lisp call frame (as requested with @kbd{b} or by entering the
frame with @kbd{d}); then the value specified in the @kbd{r} command is
used as the value of that frame.  It is also useful if you call
@code{debug} and use its return value.  Otherwise, @kbd{r} has the same
effect as @kbd{c}, and the specified return value does not matter.

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

@item l
Display a list of functions that will invoke the debugger when called.
This is a list of functions that are set to break on entry by means of
@code{debug-on-entry}.

@end table

@node Invoking the Debugger
@subsection Invoking the Debugger
@cindex invoking lisp debugger

  Here we describe in full detail the function @code{debug} that is used
to invoke the debugger.

@deffn Command debug &rest debugger-args
This function enters the debugger.  It switches buffers to a buffer
named @file{*Backtrace*} (or @file{*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}, @kbd{d}, @kbd{j}, 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.

The use of the @var{debugger-args} is that @code{debug} displays the
rest of its arguments at the top of the @file{*Backtrace*} buffer, so
that the user can see them.  Except as described below, this is the
@emph{only} way these arguments are used.

However, certain values for first argument to @code{debug} have a
special significance.  (Normally, these values are used only by the
internals of Emacs, and not by programmers calling @code{debug}.)  Here
is a table of these special values:

@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{Debugger
entered--entering a function:} as a line of text at the top of the
buffer.

@item debug
@code{debug} as first argument means @code{debug} was called because
of entry to a function that was set to debug on entry.  The debugger
displays the string @samp{Debugger entered--entering a function:},
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 function call form when
@code{debug-on-next-call} is non-@code{nil}.  The debugger displays
@samp{Debugger entered--beginning evaluation of function call form:}
as the top line in the buffer.

@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{Debugger
entered--returning value:} in 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{Debugger entered--Lisp error:}
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* ------
Debugger entered--Lisp error: (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 deffn

@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.  This function should invoke
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} (@pxref{Invoking the Debugger}).
@end defvar

@defun backtrace
@cindex run time stack
@cindex call stack
This function prints a trace of Lisp function calls currently active.
The trace is identical to the one that @code{debug} would show in the
@file{*Backtrace*} buffer.  The return value is always nil.

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

Each line of the backtrace represents one function call.  The line
shows the function followed by a list of the values of the function's
arguments if they are all known; if they are still being computed, the
line consists of a list containing the function and its unevaluated
arguments.  Long lists or deeply nested structures may be 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{} (testing nil)
@end group

@group
----------- Buffer: backtrace-output ------------
  backtrace()
  (list 'testing (backtrace))
@end group
  (progn ...)
  eval((progn (1+ var) (list 'testing (backtrace))))
  (setq ...)
  (save-excursion ...)
  (let ...)
  (with-output-to-temp-buffer ...)
  eval((with-output-to-temp-buffer ...))
  eval-last-sexp-1(nil)
@group
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)
----------- Buffer: backtrace-output ------------
@end group
@end smallexample
@end defun

@defopt debugger-stack-frame-as-list
If this variable is non-@code{nil}, every stack frame of the backtrace
is displayed as a list.  This aims at improving the backtrace
readability at the cost of special forms no longer being visually
different from regular function calls.

With @code{debugger-stack-frame-as-list} non-@code{nil}, the above
example would look as follows:

@smallexample
@group
----------- Buffer: backtrace-output ------------
  (backtrace)
  (list 'testing (backtrace))
@end group
  (progn ...)
  (eval (progn (1+ var) (list 'testing (backtrace))))
  (setq ...)
  (save-excursion ...)
  (let ...)
  (with-output-to-temp-buffer ...)
  (eval (with-output-to-temp-buffer ...))
  (eval-last-sexp-1 nil)
@group
  (eval-last-sexp nil)
  (call-interactively eval-last-sexp)
----------- Buffer: backtrace-output ------------
@end group
@end smallexample
@end defopt

@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 down the stack, 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.

This function is used only by the debugger.
@end defun

@defvar command-debug-status
This variable records the debugging status of the 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
invocation.

The advantage of using this variable rather than an ordinary global
variable is that the data will never carry over to a subsequent command
invocation.

This variable is obsolete and will be removed in future versions.
@end defvar

@defun backtrace-frame frame-number &optional base
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 return value is @code{(t @var{function}
@var{arg-values}@dots{})}.

In the return value, @var{function} is whatever was supplied as the
@sc{car} of the 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 @var{base} is specified, @var{frame-number} counts relative to
the topmost frame whose @var{function} is @var{base}.

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

@defun mapbacktrace function &optional base
The function @code{mapbacktrace} calls @var{function} once for each
frame in the backtrace, starting at the first frame whose function is
@var{base} (or from the top if @var{base} is omitted or @code{nil}).

@var{function} is called with four arguments: @var{evald}, @var{func},
@var{args}, and @var{flags}.

If a frame has not evaluated its arguments yet or is a special form,
@var{evald} is @code{nil} and @var{args} is a list of forms.

If a frame has evaluated its arguments and called its function
already, @var{evald} is @code{t} and @var{args} is a list of values.
@var{flags} is a plist of properties of the current frame: currently,
the only supported property is @code{:debug-on-exit}, which is
@code{t} if the stack frame's @code{debug-on-exit} flag is set.
@end defun

@include edebug.texi

@node Syntax Errors
@section Debugging Invalid Lisp Syntax
@cindex debugging invalid Lisp syntax

  The Lisp reader reports invalid syntax, but cannot say where the real
problem is.  For example, the error @samp{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, @samp{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} (@code{end-of-defun}, @pxref{Moving by
Defuns,,,emacs, The GNU Emacs Manual}) 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.

@cindex unbalanced parentheses
@cindex parenthesis mismatch, debugging
  However, unmatched parentheses are the most common syntax errors in
Lisp, and we can give further advice for those cases.  (In addition,
just moving point through the code with Show Paren mode enabled might
find the mismatch.)

@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
@cindex 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 go to the end of
the file and type @kbd{C-u C-M-u} (@code{backward-up-list},
@pxref{Moving by Parens,,,emacs, The GNU Emacs Manual}).  This will
move you to the beginning of the first defun that is unbalanced.

  The next step is to determine precisely what is wrong.  There is no
way to be sure of this except by studying 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}
(@code{indent-pp-sexp}, @pxref{Multi-line Indent,,,emacs, The GNU
Emacs Manual}) and see what moves.  @strong{But don't do this yet!}
Keep reading, first.

  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} (@code{end-of-defun}) 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} with @kbd{C-_} (@code{undo}),
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
@cindex excess close parentheses

  To deal with an excess close parenthesis, first go to the beginning
of the file, then type @kbd{C-u -1 C-M-u} (@code{backward-up-list}
with an argument of @minus{}1) to find the end of the first unbalanced
defun.

  Then find the actual matching close parenthesis by typing @kbd{C-M-f}
(@code{forward-sexp}, @pxref{Expressions,,,emacs, The GNU Emacs Manual})
at the beginning of that 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} (@code{indent-pp-sexp}) 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}
with @kbd{C-_} (@code{undo}), 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 fits the intended nesting of parentheses,
and you have put back those parentheses, @kbd{C-M-q} should not change
anything.

@node Test Coverage
@section Test Coverage
@cindex coverage testing

@findex testcover-start
@findex testcover-mark-all
@findex testcover-next-mark
  You can do coverage testing for a file of Lisp code by loading the
@code{testcover} library and using the command @kbd{M-x
testcover-start @key{RET} @var{file} @key{RET}} to instrument the
code.  Then test your code by calling it one or more times.  Then use
the command @kbd{M-x testcover-mark-all} to display colored highlights
on the code to show where coverage is insufficient.  The command
@kbd{M-x testcover-next-mark} will move point forward to the next
highlighted spot.

  Normally, a red highlight indicates the form was never completely
evaluated; a brown highlight means it always evaluated to the same
value (meaning there has been little testing of what is done with the
result).  However, the red highlight is skipped for forms that can't
possibly complete their evaluation, such as @code{error}.  The brown
highlight is skipped for forms that are expected to always evaluate to
the same value, such as @code{(setq x 14)}.

  For difficult cases, you can add do-nothing macros to your code to
give advice to the test coverage tool.

@defmac 1value form
Evaluate @var{form} and return its value, but inform coverage testing
that @var{form}'s value should always be the same.
@end defmac

@defmac noreturn form
Evaluate @var{form}, informing coverage testing that @var{form} should
never return.  If it ever does return, you get a run-time error.
@end defmac

  Edebug also has a coverage testing feature (@pxref{Coverage
Testing}).  These features partly duplicate each other, and it would
be cleaner to combine them.


@node Profiling
@section Profiling
@cindex profiling
@cindex profile
@cindex performance analysis
@cindex measuring resource usage
@cindex memory usage

If your program is working correctly, but not fast enough, and you
want to make it run more quickly or efficiently, the first thing to do
is @dfn{profile} your code so that you know where it spends most of
the execution time.  If you find that one particular function is
responsible for a significant portion of the execution time, you can
start looking for ways to optimize that piece.

@findex profiler-start
@findex profiler-report
@findex profiler-stop
Emacs has built-in support for this.  To begin profiling, type
@w{@kbd{M-x profiler-start}}.  You can choose to sample CPU usage
periodically (@code{cpu}), when memory is allocated (@code{memory}),
or both.  Then run the code you'd like to speed up.  After that, type
@kbd{M-x profiler-report} to display a summary buffer for CPU usage
sampled by each type (cpu and memory) that you chose to profile.  The
names of the report buffers include the times at which the reports
were generated, so you can generate another report later on without
erasing previous results.  When you have finished profiling, type
@kbd{M-x profiler-stop} (there is a small overhead associated with
profiling, so we don't recommend leaving it active except when you are
actually running the code you want to examine).

The profiler report buffer shows, on each line, a function that was
called, preceded by how much CPU resources it used in
absolute and percentage terms since profiling started.  If a given
line has a @samp{+} symbol to the left of the function name, you can
expand that line by typing @kbd{@key{RET}}, in order to see the
function(s) called by the higher-level function.  Use a prefix
argument (@kbd{C-u @key{RET}}) to see the whole call tree below a
function.  Pressing @kbd{@key{RET}} again will collapse back to the
original state.

Press @kbd{j} or @kbd{mouse-2} to jump to the definition of a function
at point.  Press @kbd{d} to view a function's documentation.  You can
save a profile to a file using @kbd{C-x C-w}.  You can compare two
profiles using @kbd{=}.

@c FIXME reversed calltree?

@cindex @file{elp.el}
@cindex timing programs
The @file{elp} library offers an alternative approach, which is useful
when you know in advance which Lisp function(s) you want to profile.
Using that library, you begin by setting @code{elp-function-list} to
the list of function symbols---those are the functions you want to
profile.  Then type @w{@kbd{M-x elp-instrument-list @key{RET} nil
@key{RET}}} to arrange for profiling those functions.  After running
the code you want to profile, invoke @w{@kbd{M-x elp-results}} to
display the current results.  See the file @file{elp.el} for more
detailed instructions.  This approach is limited to profiling
functions written in Lisp, it cannot profile Emacs primitives.

@cindex @file{benchmark.el}
@cindex benchmarking
You can measure the time it takes to evaluate individual Emacs Lisp
forms using the @file{benchmark} library.  See the function
@code{benchmark-call} as well as the macros @code{benchmark-run},
@code{benchmark-run-compiled}, @code{benchmark-progn} and
@code{benchmark-call} in @file{benchmark.el}.  You can also use the
@code{benchmark} command for timing forms interactively.

@c Not worth putting in the printed manual.
@ifnottex
@cindex --enable-profiling option of configure
To profile Emacs at the level of its C code, you can build it using the
@option{--enable-profiling} option of @command{configure}.  When Emacs
exits, it generates a file @file{gmon.out} that you can examine using
the @command{gprof} utility.  This feature is mainly useful for
debugging Emacs.  It actually stops the Lisp-level @kbd{M-x
profiler-@dots{}} commands described above from working.
@end ifnottex