0.8.9.48:

[sbcl.git] / doc / manual / debugger.texinfo

@node  The Debugger
@comment  node-name,  next,  previous,  up
@chapter The Debugger
@cindex Debugger

The SBCL debugger (as the CMUCL debugger it was derived from) has very
good support for source-level debugging of compiled code.  Although
some other debuggers allow access of variables by name, this seems to
be the first Lisp debugger that:

@itemize

@item
Tells you when a variable doesn't have a value because it hasn't been
initialized yet or has already been deallocated, or

@item
Can display the precise source location corresponding to a code
location in the debugged program.

@end itemize

These features allow the debugging of compiled code to be made almost
indistinguishable from interpreted code debugging.


@menu
* Starting the Debugger::       
* The Debugger Command Loop::   
* Controlling Printing in the Debugger::  
* Stack Frames::                
* Variable Access::             
* Source Location Printing::    
* Debugger Policy Control::     
* Exiting Commands::            
* Information Commands::        
* Function Tracing::            
@end menu

@node  Starting the Debugger
@comment  node-name,  next,  previous,  up
@section Starting the Debugger

The debugger is an interactive command loop that allows a user to examine
the function call stack.  The debugger is invoked when:

@itemize

@item
A @code{serious-condition} is signaled, and it is not handled, or

@item
@code{error} is called, and the condition it signals is not handled,
or

@item
the debugger is explicitly entered with the Lisp @code{break} or
@code{debug} functions.

@end itemize

When you enter the TTY debugger, it looks something like this:

@example
debugger invoked on a TYPE-ERROR in thread 11184:
  The value 3 is not of type LIST.
restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT   ] Reduce debugger level (leaving debugger, returning to toplevel).
  1: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop.
(CAR 1 3)[:EXTERNAL]
0]
@end example

The first group of lines describe what the error was that put us in
the debugger.  In this case @code{car} was called on @code{3}.  After
@samp{restarts} is a list of all the ways that we can restart
execution after this error.  In this case, both options return to
top-level.  After printing its banner, the debugger prints the current
frame and the debugger prompt.


@node  The Debugger Command Loop
@comment  node-name,  next,  previous,  up
@section The Debugger Command Loop
@cindex Evaluation, in the debugger

The debugger is an interactive read-eval-print loop much like the normal
top-level, but some symbols are interpreted as debugger commands instead
of being evaluated.  A debugger command starts with the symbol name of
the command, possibly followed by some arguments on the same line.  Some
commands prompt for additional input.  Debugger commands can be
abbreviated by any unambiguous prefix: @command{help} can be typed as
@samp{h}, @samp{he}, etc.  For convenience, some commands have
ambiguous one-letter abbreviations: @samp{f} for @command{frame}.
@comment FIXME: what does that last bit mean?

The package is not significant in debugger commands; any symbol with the
name of a debugger command will work.  If you want to show the value of
a variable that happens also to be the name of a debugger command, you
can use the @command{list-locals} command or the @code{sb-debug:var}
function, or you can wrap the variable in a @code{progn} to hide it from
the command loop.

The debugger prompt is ``@code{@var{frame}]}'', where @var{frame} is
the number of the current frame.  Frames are numbered starting from
zero at the top (most recent call), increasing down to the bottom.
The current frame is the frame that commands refer to.  The current
frame also provides the lexical environment for evaluation of
non-command forms.

 The debugger evaluates forms in the lexical environment of the
functions being debugged.  The debugger can only access variables.
You can't @code{go} or @code{return-from} into a function, and you
can't call local functions.  Special variable references are evaluated
with their current value (the innermost binding around the debugger
invocation) -- you don't get the value that the special had in the
current frame.  For more information on debugger variable access, see
@ref{Variable Access}.


@node Controlling Printing in the Debugger
@comment  node-name,  next,  previous,  up
@section Controlling Printing in the Debugger

In the debugger, it is possible to override the printing behaviour of
the REPL.

@defvr {Variable} *debug-print-variable-alist*

An association list describing new bindings for special variables
(typically *PRINT-FOO* variables) to be used within the debugger, e.g.
@lisp
((*PRINT-LENGTH* . 10) (*PRINT-LEVEL* . 6) (*PRINT-PRETTY* . NIL))
@end lisp
The variables in the @code{car} position are bound to the values in
the @code{cdr} during the execution of some debug commands.  When
evaluating arbitrary expressions in the debugger, the normal values of
the printer control variables are in effect. @c FIXME: is this correct?
@code{*debug-print-variable-alist*} does not contain any bindings
initially.

@end defvr

@node  Stack Frames
@comment  node-name,  next,  previous,  up
@section Stack Frames
@cindex Stack frames

A @dfn{stack frame} is the run-time representation of a call to a
function; the frame stores the state that a function needs to remember
what it is doing.  Frames have:

@itemize

@item
@dfn{variables} (@pxref{Variable Access}), which are the values being operated
on;

@item
@dfn{arguments} to the call (which are really just particularly
interesting variables), and

@item
a current location (@pxref{Source Location Printing}), which is the place in
the program where the function was running when it stopped to call
another function, or because of an interrupt or error.

@end itemize


@menu
* Stack Motion::                
* How Arguments are Printed::   
* Function Names::              
* Funny Frames::                
* Debug Tail Recursion::        
* Unknown Locations and Interrupts::  
@end menu

@node  Stack Motion
@comment  node-name,  next,  previous,  up
@subsection Stack Motion

These commands move to a new stack frame and print the name of the
function and the values of its arguments in the style of a Lisp
function call:

@deffn {Debugger Command} up
Move up to the next higher frame.  More recent function calls are
considered to be higher on the stack.
@end deffn

@deffn {Debugger Command} down
Move down to the next lower frame.
@end deffn

@deffn {Debugger Command} top
Move to the highest frame, that is, the frame where the debugger was
entered.
@end deffn

@deffn {Debugger Command} bottom
Move to the lowest frame.
@end deffn

@deffn {Debugger Command} frame [@var{n}]
Move to the frame with the specified number.  Prompts for the number if not
supplied.  The frame with number 0 is the frame where the debugger
was entered.
@end deffn


@node  How Arguments are Printed
@comment  node-name,  next,  previous,  up
@subsection How Arguments are Printed

A frame is printed to look like a function call, but with the actual
argument values in the argument positions.  So the frame for this call
in the source:

@lisp
(myfun (+ 3 4) 'a)
@end lisp

would look like this:

@example
(MYFUN 7 A)
@end example

All keyword and optional arguments are displayed with their actual
values; if the corresponding argument was not supplied, the value will
be the default.  So this call:

@lisp
(subseq "foo" 1)
@end lisp

would look like this:

@example
(SUBSEQ "foo" 1 3)
@end example

And this call:

@lisp
(string-upcase "test case")
@end lisp

would look like this:

@example
(STRING-UPCASE "test case" :START 0 :END NIL)
@end example

The arguments to a function call are displayed by accessing the
argument variables.  Although those variables are initialized to the
actual argument values, they can be set inside the function; in this
case the new value will be displayed.

@code{&rest} arguments are handled somewhat differently.  The value of
the rest argument variable is displayed as the spread-out arguments to
the call, so:

@lisp
(format t "~A is a ~A." "This" 'test)
@end lisp

would look like this:

@example
(FORMAT T "~A is a ~A." "This" 'TEST)
@end example

Rest arguments cause an exception to the normal display of keyword
arguments in functions that have both @code{&rest} and @code{&key}
arguments.  In this case, the keyword argument variables are not
displayed at all; the rest arg is displayed instead.  So for these
functions, only the keywords actually supplied will be shown, and the
values displayed will be the argument values, not values of the
(possibly modified) variables.

If the variable for an argument is never referenced by the function,
it will be deleted.  The variable value is then unavailable, so the
debugger prints @samp{#<unused-arg>} instead of the value.  Similarly,
if for any of a number of reasons the value of the variable is
unavailable or not known to be available (@pxref{Variable Access}),
then @samp{#<unavailable-arg>} will be printed instead of the argument
value.

@vindex *debug-print-variable-alist*
Printing of argument values is controlled by
@code{*debug-print-variable-alist*}.  @xref{Controlling Printing in
the Debugger}.


@node  Function Names
@comment  node-name,  next,  previous,  up
@subsection Function Names

If a function is defined by @code{defun}, @code{labels}, or
@code{flet}, then the debugger will print the actual function name
after the open parenthesis, like:

@example
(STRING-UPCASE "test case" :START 0 :END NIL)
((SETF AREF) #\a "for" 1)
@end example

Otherwise, the function name is a string, and will be printed in
quotes:

@example
("DEFUN MYFUN" BAR)
("DEFMACRO DO" (DO ((I 0 (1+ I))) ((= I 13))) NIL)
("SETQ *GC-NOTIFY-BEFORE*")
@end example

This string name is derived from the @code{def@var{mumble}} form
that encloses or expanded into the lambda, or the outermost enclosing
form if there is no @code{def@var{mumble}}.


@node  Funny Frames
@comment  node-name,  next,  previous,  up
@subsection Funny Frames
@cindex External entry points
@cindex Entry points, external
@cindex Block compilation, debugger implications
@cindex External, stack frame kind
@cindex Optional, stack frame kind
@cindex Cleanup, stack frame kind

Sometimes the evaluator introduces new functions that are used to
implement a user function, but are not directly specified in the
source.  The main place this is done is for checking argument type and
syntax.  Usually these functions do their thing and then go away, and
thus are not seen on the stack in the debugger.  But when you get some
sort of error during lambda-list processing, you end up in the
debugger on one of these funny frames.

These funny frames are flagged by printing
``@code{[@var{keyword}]}'' after the parentheses.  For example,
this call:

@lisp
(car 'a 'b)
@end lisp

will look like this:

@example
(CAR 2 A)[:EXTERNAL]
@end example

And this call:

@lisp
(string-upcase "test case" :end)
@end lisp

would look like this:

@example
("SB!INT:&MORE processing" "test case" 1053984 1)[:OPTIONAL]
@end example

As you can see, these frames have only a vague resemblance to the
original call.  Fortunately, the error message displayed when you
enter the debugger will usually tell you what problem is (in these
cases, too many arguments and odd keyword arguments.)  Also, if you go
down the stack to the frame for the calling function, you can display
the original source.  @xref{Source Location Printing}.

@c FIXME: is the block-compilation part correct for SBCL?

With recursive or block compiled functions, an @code{:EXTERNAL} frame
may appear before the frame representing the first call to the
recursive function or entry to the compiled block. This is a
consequence of the way the compiler does block compilation: there is
nothing odd with your program. You will also see @code{:CLEANUP}
frames during the execution of @code{unwind-protect} cleanup
code. Note that inline expansion and open-coding affect what frames
are present in the debugger, see @ref{Debugger Policy Control}.
@comment FIXME: link here to section about open coding once it exists.
@c @ref{open-coding}


@node  Debug Tail Recursion
@comment  node-name,  next,  previous,  up
@subsection Debug Tail Recursion
@cindex Tail recursion
@cindex Recursion, tail

Both the compiler and the interpreter are ``properly tail recursive.''
If a function call is in a tail-recursive position, the stack frame
will be deallocated @emph{at the time of the call}, rather than after
the call returns.  Consider this backtrace:

@example
(BAR ...) 
(FOO ...)
@end example

Because of tail recursion, it is not necessarily the case that
@code{FOO} directly called @code{BAR}.  It may be that @code{FOO}
called some other function @code{FOO2} which then called @code{BAR}
tail-recursively, as in this example:

@lisp
(defun foo ()
  ...
  (foo2 ...)
  ...)

(defun foo2 (...)
  ...
  (bar ...))

(defun bar (...)
  ...)
@end lisp

Usually the elimination of tail-recursive frames makes debugging more
pleasant, since theses frames are mostly uninformative.  If there is
any doubt about how one function called another, it can usually be
eliminated by finding the source location in the calling frame.
@xref{Source Location Printing}.

The elimination of tail-recursive frames can be prevented by disabling
tail-recursion optimization, which happens when the @code{debug}
optimization quality is greater than @code{2}.  
@xref{Debugger Policy Control}.

@comment FIXME: reinstate this link once the chapter is in the manual.
@c For a more thorough discussion of tail recursion, @ref{tail-recursion}.


@node Unknown Locations and Interrupts
@comment  node-name,  next,  previous,  up
@subsection Unknown Locations and Interrupts
@cindex Unknown code locations
@cindex Locations, unknown
@cindex Interrupts
@cindex Errors, run-time

The debugger operates using special debugging information attached to
the compiled code.  This debug information tells the debugger what it
needs to know about the locations in the code where the debugger can
be invoked.  If the debugger somehow encounters a location not
described in the debug information, then it is said to be
@dfn{unknown}.  If the code location for a frame is unknown, then some
variables may be inaccessible, and the source location cannot be
precisely displayed.

There are three reasons why a code location could be unknown:

@itemize

@item
There is inadequate debug information due to the value of the @code{debug}
optimization quality.  @xref{Debugger Policy Control}.

@item
The debugger was entered because of an interrupt such as @key{C-c}.

@item
A hardware error such as ``@samp{bus error}'' occurred in code that was
compiled unsafely due to the value of the @code{safety} optimization
quality.
@comment FIXME: reinstate link when section on optimize qualities exists.
@c  @xref{optimize-declaration}.

@end itemize

In the last two cases, the values of argument variables are
accessible, but may be incorrect.  For more details on when variable
values are accessible, @ref{Variable Value Availability}.

It is possible for an interrupt to happen when a function call or
return is in progress.  The debugger may then flame out with some
obscure error or insist that the bottom of the stack has been reached,
when the real problem is that the current stack frame can't be
located.  If this happens, return from the interrupt and try again.


@node Variable Access
@comment  node-name,  next,  previous,  up
@section Variable Access
@cindex Debug variables
@cindex Variables, debugger access

There are two ways to access the current frame's local variables in
the debugger: @command{list-locals} and @code{sb-debug:var}.

The debugger doesn't really understand lexical scoping; it has just
one namespace for all the variables in the current stack frame.  If a
symbol is the name of multiple variables in the same function, then
the reference appears ambiguous, even though lexical scoping specifies
which value is visible at any given source location.  If the scopes of
the two variables are not nested, then the debugger can resolve the
ambiguity by observing that only one variable is accessible.

When there are ambiguous variables, the evaluator assigns each one a
small integer identifier.  The @code{sb-debug:var} function uses this
identifier to distinguish between ambiguous variables.  The
@command{list-locals} command prints the identifier.  In the
following example, there are two variables named @code{X}.  The first
one has identifier 0 (which is not printed), the second one has
identifier 1.

@example
X  =  1
X#1  =  2
@end example

@deffn {Debugger Command} list-locals [@var{prefix}]
This command prints the name and value of all variables in the current
frame whose name has the specified @var{prefix}.  @var{prefix} may be
a string or a symbol.  If no @var{prefix} is given, then all available
variables are printed.  If a variable has a potentially ambiguous
name, then the name is printed with a ``@code{#@var{identifier}}''
suffix, where @var{identifier} is the small integer used to make the
name unique.
@end deffn

@defun sb-debug:var @var{name} &optional @var{identifier}
This function returns the value of the variable in the current frame
with the specified @var{name}.  If supplied, @var{identifier}
determines which value to return when there are ambiguous variables.
  
When @var{name} is a symbol, it is interpreted as the symbol name of
the variable, i.e. the package is significant.  If @var{name} is an
uninterned symbol (gensym), then return the value of the uninterned
variable with the same name.  If @var{name} is a string,
@code{sb-debug:var} interprets it as the prefix of a variable name
that must unambiguously complete to the name of a valid variable.

@var{identifier} is used to disambiguate the variable name; use
@command{list-locals} to find out the identifiers.
@end defun


@menu
* Variable Value Availability::  
* Note On Lexical Variable Access::  
@end menu

@node Variable Value Availability
@comment  node-name,  next,  previous,  up
@subsection Variable Value Availability
@cindex Availability of debug variables
@cindex Validity of debug variables
@cindex Debug optimization quality

The value of a variable may be unavailable to the debugger in portions
of the program where Lisp says that the variable is defined.  If a
variable value is not available, the debugger will not let you read or
write that variable.  With one exception, the debugger will never
display an incorrect value for a variable.  Rather than displaying
incorrect values, the debugger tells you the value is unavailable.

The one exception is this: if you interrupt (e.g., with @key{C-c}) or
if there is an unexpected hardware error such as ``@samp{bus error}''
(which should only happen in unsafe code), then the values displayed
for arguments to the interrupted frame might be
incorrect.@footnote{Since the location of an interrupt or hardware
error will always be an unknown location, non-argument variable values
will never be available in the interrupted frame.  @xref{Unknown
Locations and Interrupts}.}  This exception applies only to the
interrupted frame: any frame farther down the stack will be fine.

The value of a variable may be unavailable for these reasons:

@itemize

@item
The value of the @code{debug} optimization quality may have omitted debug
information needed to determine whether the variable is available.
Unless a variable is an argument, its value will only be available when
@code{debug} is at least @code{2}.

@item
The compiler did lifetime analysis and determined that the value was no longer
needed, even though its scope had not been exited.  Lifetime analysis is
inhibited when the @code{debug} optimization quality is @code{3}.

@item
The variable's name is an uninterned symbol (gensym).  To save space, the
compiler only dumps debug information about uninterned variables when the
@code{debug} optimization quality is @code{3}.

@item
The frame's location is unknown (@pxref{Unknown Locations and
Interrupts}) because the debugger was entered due to an interrupt or
unexpected hardware error.  Under these conditions the values of
arguments will be available, but might be incorrect.  This is the
exception mentioned above.

@item
The variable (or the code referencing it) was optimized out
of existence.  Variables with no reads are always optimized away.  The
degree to which the compiler deletes variables will depend on the
value of the @code{compilation-speed} optimization quality, but most
source-level optimizations are done under all compilation policies.

@item
The variable is never set and its definition looks like
@lisp
(LET ((var1 var2))
   ...)
@end lisp
In this case, @code{var1} is substituted with @code{var2}.

@item 
The variable is never set and is referenced exactly once.  In this
case, the reference is substituted with the variable initial value.

@end itemize

Since it is especially useful to be able to get the arguments to a
function, argument variables are treated specially when the
@code{speed} optimization quality is less than @code{3} and the
@code{debug} quality is at least @code{1}.  With this compilation
policy, the values of argument variables are almost always available
everywhere in the function, even at unknown locations.  For
non-argument variables, @code{debug} must be at least @code{2} for
values to be available, and even then, values are only available at
known locations.


@node  Note On Lexical Variable Access
@comment  node-name,  next,  previous,  up
@subsection Note On Lexical Variable Access

When the debugger command loop establishes variable bindings for
available variables, these variable bindings have lexical scope and
dynamic extent.@footnote{The variable bindings are actually created
using the Lisp @code{symbol-macrolet} special form.}  You can close
over them, but such closures can't be used as upward funargs.

You can also set local variables using @code{setq}, but if the
variable was closed over in the original source and never set, then
setting the variable in the debugger may not change the value in all
the functions the variable is defined in.  Another risk of setting
variables is that you may assign a value of a type that the compiler
proved the variable could never take on.  This may result in bad
things happening.


@node Source Location Printing
@comment  node-name,  next,  previous,  up
@section Source Location Printing
@cindex Source location printing, debugger

One of the debugger's capabilities is source level debugging of
compiled code.  These commands display the source location for the
current frame:

@deffn {Debugger Command} source [@var{context}]
This command displays the file that the current frame's function was
defined from (if it was defined from a file), and then the source form
responsible for generating the code that the current frame was
executing.  If @var{context} is specified, then it is an integer
specifying the number of enclosing levels of list structure to print.
@end deffn

The source form for a location in the code is the innermost list present
in the original source that encloses the form responsible for generating
that code.  If the actual source form is not a list, then some enclosing
list will be printed.  For example, if the source form was a reference
to the variable @code{*some-random-special*}, then the innermost
enclosing evaluated form will be printed.  Here are some possible
enclosing forms:

@lisp
(let ((a *some-random-special*))
  ...)

(+ *some-random-special* ...)
@end lisp

If the code at a location was generated from the expansion of a macro
or a source-level compiler optimization, then the form in the original
source that expanded into that code will be printed.  Suppose the file
@file{/usr/me/mystuff.lisp} looked like this:

@lisp
(defmacro mymac ()
  '(myfun))

(defun foo ()
  (mymac)
  ...)
@end lisp

If @code{foo} has called @code{myfun}, and is waiting for it to
return, then the @command{source} command would print:

@example
; File: /usr/me/mystuff.lisp

(MYMAC)
@end example

Note that the macro use was printed, not the actual function call form,
@code{(myfun)}.

If enclosing source is printed by giving an argument to
@command{source} or @command{vsource}, then the actual source form is
marked by wrapping it in a list whose first element is
@samp{#:***HERE***}.  In the previous example, @code{source 1} would
print:

@example
; File: /usr/me/mystuff.lisp

(DEFUN FOO ()
  (#:***HERE***
   (MYMAC))
  ...)
@end example


@menu
* How the Source is Found::     
* Source Location Availability::  
@end menu

@node  How the Source is Found
@comment  node-name,  next,  previous,  up
@subsection How the Source is Found

If the code was defined from Lisp by @code{compile} or
@code{eval}, then the source can always be reliably located.  If the
code was defined from a @file{fasl} file created by
@code{compile-file}, then the debugger gets the source forms it
prints by reading them from the original source file.  This is a
potential problem, since the source file might have moved or changed
since the time it was compiled.

The source file is opened using the @code{truename} of the source file
pathname originally given to the compiler.  This is an absolute pathname
with all logical names and symbolic links expanded.  If the file can't
be located using this name, then the debugger gives up and signals an
error.

If the source file can be found, but has been modified since the time it was
compiled, the debugger prints this warning:

@example
; File has been modified since compilation:
;   @var{filename}
; Using form offset instead of character position.
@end example

where @var{filename} is the name of the source file.  It then proceeds
using a robust but not foolproof heuristic for locating the source.
This heuristic works if:

@itemize

@item
No top-level forms before the top-level form containing the source
have been added or deleted, and

@item
The top-level form containing the source has not been modified much.
(More precisely, none of the list forms beginning before the source
form have been added or deleted.)

@end itemize

If the heuristic doesn't work, the displayed source will be wrong, but will
probably be near the actual source.  If the ``shape'' of the top-level form in
the source file is too different from the original form, then an error will be
signaled.  When the heuristic is used, the the source location commands are
noticeably slowed.

Source location printing can also be confused if (after the source was
compiled) a read-macro you used in the code was redefined to expand
into something different, or if a read-macro ever returns the same
@code{eq} list twice.  If you don't define read macros and don't use
@code{##} in perverted ways, you don't need to worry about this.


@node  Source Location Availability
@comment  node-name,  next,  previous,  up
@subsection Source Location Availability
@cindex Debug optimization quality
@cindex Block, basic
@cindex Block, start location

Source location information is only available when the @code{debug}
optimization quality is at least @code{2}.  If source location
information is unavailable, the source commands will give an error
message.

If source location information is available, but the source location
is unknown because of an interrupt or unexpected hardware error
(@pxref{Unknown Locations and Interrupts}), then the command will
print:

@example
Unknown location: using block start.
@end example

and then proceed to print the source location for the start of the
@emph{basic block} enclosing the code location.  It's a bit
complicated to explain exactly what a basic block is, but here are
some properties of the block start location:

@itemize

@item The block start location may be the same as the true location.

@item The block start location will never be later in the the
program's flow of control than the true location.

@item No conditional control structures (such as @code{if},
@code{cond}, @code{or}) will intervene between the block start and the
true location (but note that some conditionals present in the original
source could be optimized away.)  Function calls @emph{do not} end
basic blocks.

@item The head of a loop will be the start of a block.

@item The programming language concept of ``block structure'' and the
Lisp @code{block} special form are totally unrelated to the compiler's
basic block.

@end itemize

In other words, the true location lies between the printed location and the
next conditional (but watch out because the compiler may have changed the
program on you.)


@node Debugger Policy Control
@comment  node-name,  next,  previous,  up
@section Debugger Policy Control
@cindex Policy, debugger
@cindex Debug optimization quality
@cindex Optimize declaration
@cindex Inline expansion
@cindex Semi-inline expansion

The compilation policy specified by @code{optimize} declarations
affects the behavior seen in the debugger.  The @code{debug} quality
directly affects the debugger by controlling the amount of debugger
information dumped.  Other optimization qualities have indirect but
observable effects due to changes in the way compilation is done.

Unlike the other optimization qualities (which are compared in relative value
to evaluate tradeoffs), the @code{debug} optimization quality is directly
translated to a level of debug information.  This absolute interpretation
allows the user to count on a particular amount of debug information being
available even when the values of the other qualities are changed during
compilation.  These are the levels of debug information that correspond to the
values of the @code{debug} quality:

@table @code

@item 0
Only the function name and enough information to allow the stack to
be parsed.

@item > 0
Any level greater than @code{0} gives level @code{0} plus all argument
variables.  Values will only be accessible if the argument variable is
never set and @code{speed} is not @code{3}.  SBCL allows any real
value for optimization qualities.  It may be useful to specify
@code{0.5} to get backtrace argument display without argument
documentation.

@item 1
Level @code{1} provides argument documentation (printed arglists) and
derived argument/result type information.  This makes @code{describe}
more informative, and allows the compiler to do compile-time argument
count and type checking for any calls compiled at run-time.  This is
the default.

@item 2
Level @code{1} plus all interned local variables, source location
information, and lifetime information that tells the debugger when
arguments are available (even when @code{speed} is @code{3} or the
argument is set).

@item > 2
Any level greater than @code{2} gives level @code{2} and in addition
disables tail-call optimization, so that the backtrace will contain
frames for all invoked functions, even those in tail positions.

@item 3
Level @code{2} plus all uninterned variables.  In addition, lifetime
analysis is disabled (even when @code{speed} is @code{3}), ensuring
that all variable values are available at any known location within
the scope of the binding.  This has a speed penalty in addition to the
obvious space penalty.

@item > (max speed space)
If @code{debug} is greater than both @code{speed} and @code{space},
the command @command{return} can be used to continue execution by
returning a value from the current stack frame.

@end table

As you can see, if the @code{speed} quality is @code{3}, debugger performance is
degraded.  This effect comes from the elimination of argument variable
special-casing (@pxref{Variable Value Availability}).  Some degree of
speed/debuggability tradeoff is unavoidable, but the effect is not too drastic
when @code{debug} is at least @code{2}.

In addition to @code{inline} and @code{notinline} declarations, the
relative values of the @code{speed} and @code{space} qualities also
change whether functions are inline expanded.
@comment FIXME: link to section about inline expansion when it exists
@c (\pxlref{inline-expansion}.)
If a function is inline expanded, then
there will be no frame to represent the call, and the arguments will
be treated like any other local variable.  Functions may also be
``semi-inline'', in which case there is a frame to represent the call,
but the call is to an optimized local version of the function, not to
the original function.


@node  Exiting Commands
@comment  node-name,  next,  previous,  up
@section Exiting Commands

These commands get you out of the debugger.

@deffn {Debugger Command} toplevel
Throw to top level.
@end deffn

@deffn {Debugger Command} restart [@var{n}]
Invokes the @var{n}th restart case as displayed by the @code{error}
command.  If @var{n} is not specified, the available restart cases are
reported.
@end deffn

@deffn {Debugger Command} continue
Calls @code{continue} on the condition given to @code{debug}.  If there is no
restart case named @var{continue}, then an error is signaled.
@end deffn

@deffn {Debugger Command} abort
Calls @code{abort} on the condition given to @code{debug}.  This is
useful for popping debug command loop levels or aborting to top level,
as the case may be.
@end deffn

@deffn {Debugger Command} return @var{value}
Returns @var{value} from the current stack frame.  This command is
available when the @code{debug} optimization quality is greater than
both @code{speed} and @code{space}.  Care must be taken that the value
is of the same type as SBCL expects the stack frame to return.
@end deffn


@node  Information Commands
@comment  node-name,  next,  previous,  up
@section Information Commands

Most of these commands print information about the current frame or
function, but a few show general information.

@deffn {Debugger Command} help
@deffnx {Debugger Command} ?
Displays a synopsis of debugger commands.
@end deffn

@deffn {Debugger Command} describe
Calls @code{describe} on the current function and displays the number of
local variables.
@end deffn

@deffn {Debugger Command} print
Displays the current function call as it would be displayed by moving to
this frame.
@end deffn

@deffn {Debugger Command} error
Prints the condition given to @code{invoke-debugger} and the active
proceed cases.
@end deffn

@deffn {Debugger Command} backtrace [@var{n}]
Displays all the frames from the current to the bottom.  Only shows
@var{n} frames if specified.  The printing is controlled by @code{*debug-print-variable-alist*}.
@end deffn

@comment  FIXME (rudi 2004-03-31): sbcl doesn't support breakpoints
@comment  and stepping as of version 0.8.9.  The `list-locations'
@comment  command works, but executing a function leads to an error
@comment  when a breakpoint is hit.  When stepping works, the
@comment  commented-out section below should be reinstated and the
@comment  example output updated to correspont to sbcl's behaviour.

@c @node  Breakpoint Commands, , Information Commands, The Debugger
@c @comment  node-name,  next,  previous,  up
@c @section Breakpoint Commands
@c @cindex Breakpoints

@c SBCL supports setting of breakpoints inside compiled functions and
@c stepping of compiled code.  Breakpoints can only be set at at known
@c locations (@pxref{Unknown Locations and Interrupts}), so these
@c commands are largely useless unless the @code{debug} optimize quality
@c is at least @code{2} (@pxref{Debugger Policy Control}).  These
@c commands manipulate breakpoints:

@c @deffn {Debugger Command} breakpoint @var{location} [@var{option} @var{value}]*
@c Set a breakpoint in some function.  @var{location} may be an integer
@c code location number (as displayed by @command{list-locations}) or a
@c keyword.  The keyword can be used to indicate setting a breakpoint at
@c the function start (@code{:start}, @code{:s}) or function end
@c (@code{:end}, @code{:e}).  The @command{breakpoint} command has
@c @code{:condition}, @code{:break}, @code{:print} and @code{:function}
@c options which work similarly to the @code{trace} options.
@c @end deffn

@c @deffn {Debugger Command} list-locations [@var{function}]
@c @deffnx {Debugger Command} ll  [@var{function}]
@c List all the code locations in the current frame's function, or in
@c @var{function} if it is supplied.  The display format is the code
@c location number, a colon and then the source form for that location:

@c @example
@c 3: (1- N)
@c @end example

@c If consecutive locations have the same source, then a numeric range
@c like @code{3-5:} will be printed.  For example, a default function
@c call has a known location both immediately before and after the call,
@c which would result in two code locations with the same source.  The
@c listed function becomes the new default function for breakpoint
@c setting (via the @command{breakpoint}) command.
@c @end deffn

@c @deffn {Debugger Command} list-breakpoints
@c @deffnx {Debugger Command} lb
@c List all currently active breakpoints with their breakpoint number.
@c @end deffn

@c @deffn {Debugger Command} delete-breakpoint [@var{number}]
@c @deffnx {Debugger Command} db  [@var{number}]
@c Delete a breakpoint specified by its breakpoint number.  If no number
@c is specified, delete all breakpoints.
@c @end deffn

@c @deffn {Debugger Command} step
@c Step to the next possible breakpoint location in the current function.
@c This always steps over function calls, instead of stepping into them.
@c @end deffn


@c @menu
@c * Breakpoint Example::          
@c @end menu

@c @node  Breakpoint Example,  , Breakpoint Commands, Breakpoint Commands
@c @comment  node-name,  next,  previous,  up
@c @subsection Breakpoint Example

@c Consider this definition of the factorial function:

@c @lisp
@c (defun ! (n)
@c   (if (zerop n)
@c       1
@c       (* n (! (1- n)))))
@c @end lisp

@c This debugger session demonstrates the use of breakpoints:

@c @example
@c * (break)  ; invoke debugger

@c debugger invoked on a SIMPLE-CONDITION in thread 11184: break

@c restarts (invokable by number or by possibly-abbreviated name):
@c   0: [CONTINUE] Return from BREAK.
@c   1: [ABORT   ] Reduce debugger level (leaving debugger, returning to toplevel).
@c   2: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop.
@c ("varargs entry for top level local call BREAK" "break")
@c 0] ll #'!

@c 0-1: (SB-INT:NAMED-LAMBDA ! (N) (BLOCK ! (IF (ZEROP N) 1 (* N (! #)))))
@c 2: (BLOCK ! (IF (ZEROP N) 1 (* N (! (1- N)))))
@c 3: (ZEROP N)
@c 4: (* N (! (1- N)))
@c 5: (1- N)
@c 6: (! (1- N))
@c 7-8: (* N (! (1- N)))
@c 9-10: (IF (ZEROP N) 1 (* N (! (1- N))))
@c 0] br 4

@c (* N (! (1- N)))
@c 1: 4 in !
@c added
@c 0] toplevel

@c FIXME: SBCL errored out, and not in the expected way ... Copying the
@c output verbatim from the CMUCL manual for now.

@c common-lisp-user> (! 10) ; Call the function

@c *Breakpoint hit*

@c Restarts:
@c   0: [CONTINUE] Return from BREAK.
@c   1: [ABORT   ] Return to Top-Level.

@c Debug  (type H for help)

@c (! 10) ; We are now in first call (arg 10) before the multiply
@c Source: (* N (! (1- N)))
@c 3] st

@c *Step*

@c (! 10) ; We have finished evaluation of (1- n)
@c Source: (1- N)
@c 3] st

@c *Breakpoint hit*

@c Restarts:
@c   0: [CONTINUE] Return from BREAK.
@c   1: [ABORT   ] Return to Top-Level.

@c Debug  (type H for help)

@c (! 9) ; We hit the breakpoint in the recursive call
@c Source: (* N (! (1- N)))
@c 3] 
@c @end example


@node  Function Tracing
@comment  node-name,  next,  previous,  up
@section Function Tracing
@cindex Tracing
@cindex Function, tracing

The tracer causes selected functions to print their arguments and
their results whenever they are called.  Options allow conditional
printing of the trace information and conditional breakpoints on
function entry or exit.

@include macro-common-lisp-trace.texinfo

@include macro-common-lisp-untrace.texinfo

@include var-sb-debug-star-trace-indentation-step-star.texinfo

@include var-sb-debug-star-max-trace-indentation-star.texinfo

@include var-sb-debug-star-trace-encapsulate-default-star.texinfo

@include var-sb-debug-star-trace-values-star.texinfo

@comment FIXME rudi 2004-03-26: encapsulate is (per TODO file as of
@comment 0.8.9) in a state of flux.  When it's sorted out, revive the
@comment cmucl documentation.