by Robert MacLachlan
| 3.1 |
Debugger
Introduction |
|
The cmucl debugger is unique in its level
of support for source-level debugging of compiled code. Although
some other debuggers allow access of variables by name, this seems
to be the first Common Lisp debugger that:
- Tells you when a variable doesn't have a value because it
hasn't been initialized yet or has already been deallocated, or
- Can display the precise source location corresponding to a code
location in the debugged program.
These features allow the debugging of compiled code to be made
almost indistinguishable from interpreted code debugging.
The debugger is an interactive command loop that allows a user to
examine the function call stack. The debugger is invoked when:
- A serious-condition is signaled, and it is not handled,
or
- error is called,
and the condition it signals is not handled, or
- The debugger is explicitly invoked with the Common Lisp
break or debug functions.
Note: there are two debugger interfaces in cmucl: the TTY debugger (described below) and the
Motif debugger. Since the difference is only in the user interface,
much of this chapter also applies to the Motif version. See
section 2.8.1 for a very
brief discussion of the graphical interface.
When you enter the TTY debugger, it looks something like this:
Error in function CAR.
Wrong type argument, 3, should have been of type LIST.
Restarts:
0: Return to Top-Level.
Debug (type H for help)
(CAR 3)
0]
The first group of lines describe what the error was that put us in
the debugger. In this case car was called on
3. After Restarts: is a
list of all the ways that we can restart execution after this
error. In this case, the only option is to return to top-level.
After printing its banner, the debugger prints the current frame
and the debugger prompt.
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:
help can be typed as h,
he, etc. For convenience, some commands have
ambiguous one-letter abbreviations: f for
frame.
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 list-locals
command or the debug:var function, or you can
wrap the variable in a progn to hide it from
the command loop.
The debugger prompt is ``frame]'', where 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 go or
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. See section 3.4 for
more information on debugger variable access.
A 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:
- Variables (see section 3.4),
which are the values being operated on, and
- Arguments to the call (which are really just particularly
interesting variables), and
- A current location (see section 3.5), 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.
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:
- up
- Move up to the next higher frame. More recent function calls
are considered to be higher on the stack.
- down
- Move down to the next lower frame.
- top
- Move to the highest frame.
- bottom
- Move to the lowest frame.
- frame [n
- ] Move to the frame with the specified number. Prompts for the
number if not supplied.
| 3.3.2 |
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:
(myfun (+ 3 4) 'a)
would look like this:
(MYFUN 7 A)
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:
(subseq "foo" 1)
would look like this:
(SUBSEQ "foo" 1 3)
And this call:
(string-upcase "test case")
would look like this:
(STRING-UPCASE "test case" :START 0 :END NIL)
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.
&rest arguments are
handled somewhat differently. The value of the rest argument
variable is displayed as the spread-out arguments to the call, so:
(format t "~A is a ~A." "This" 'test)
would look like this:
(FORMAT T "~A is a ~A." "This" 'TEST)
Rest arguments cause an exception to the normal display of keyword
arguments in functions that have both &rest and &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 #unused-arg instead of the value. Similarly, if for any
of a number of reasons (described in more detail in section
3.4) the value of the variable is
unavailable or not known to be available, then #unavailable-arg will be printed instead of the
argument value.
Printing of argument values is controlled by *debug-print-level* and *debug-print-length*.
If a function is defined by defun, labels, or flet, then the
debugger will print the actual function name after the open
parenthesis, like:
(STRING-UPCASE "test case" :START 0 :END NIL)
((SETF AREF) #\a "for" 1)
Otherwise, the function name is a string, and will be printed in
quotes:
("DEFUN MYFUN" BAR)
("DEFMACRO DO" (DO ((I 0 (1+ I))) ((= I 13))) NIL)
("SETQ *GC-NOTIFY-BEFORE*")
This string name is derived from the defmumble form that encloses
or expanded into the lambda, or the outermost enclosing form if
there is no defmumble.
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 ``[keyword]'' after the parentheses. For example, this call:
(car 'a 'b)
will look like this:
(CAR 2 A) [:EXTERNAL]
And this call:
(string-upcase "test case" :end)
would look like this:
("DEFUN STRING-UPCASE" "test case" 335544424 1) [:OPTIONAL]
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 (see section 3.5.)
With recursive or block compiled functions (see
section 5.7), an :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 :CLEANUP frames during the
execution of unwind-protect cleanup code.
Note that inline expansion and open-coding affect what frames are
present in the debugger, see sections 3.6 and 4.8.
| 3.3.5 |
Debug Tail
Recursion |
|
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 at the time of the
call, rather than after the call returns. Consider this
backtrace:
(BAR ...)
(FOO ...)
Because of tail recursion, it is not necessarily the case that
FOO directly called BAR. It may be that FOO called
some other function FOO2 which then called
BAR tail-recursively, as in this example:
(defun foo ()
...
(foo2 ...)
...)
(defun foo2 (...)
...
(bar ...))
(defun bar (...)
...)
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 (section 3.5.)
The elimination of tail-recursive frames can be prevented by
disabling tail-recursion optimization, which happens when the
debug optimization quality is greater than
2 (see section 3.6.)
For a more thorough discussion of tail recursion, see
section 5.5.
| 3.3.6 |
Unknown Locations
and Interrupts |
|
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 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:
- There is inadequate debug information due to the value of the
debug optimization quality. See
section 3.6.
- The debugger was entered because of an interrupt such as
C.
- A hardware error such as ``bus error''
occurred in code that was compiled unsafely due to the value of the
safety optimization quality. See
section 4.7.1.
In the last two cases, the values of argument variables are
accessible, but may be incorrect. See section 3.4.1 for more details on when variable
values are accessible.
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.
When running interpreted code, all locations should be known.
However, an interrupt might catch some subfunction of the
interpreter at an unknown location. In this case, you should be
able to go up the stack a frame or two and reach an interpreted
frame which can be debugged.
There are three ways to access the current frame's local variables
in the debugger. The simplest is to type the variable's name into
the debugger's read-eval-print loop. The debugger will evaluate the
variable reference as though it had appeared inside that frame.
The debugger doesn't really understand lexical scoping; it has just
one namespace for all the variables in a function. 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 debug:var
function and the list-locals command use this
identifier to distinguish between ambiguous variables:
- list-locals {prefix}
- This command prints the name and value of all variables in the
current frame whose name has the specified prefix. prefix may be a
string or a symbol. If no prefix is
given, then all available variables are printed. If a variable has
a potentially ambiguous name, then the name is printed with a
``#identifier''
suffix, where identifier is the small
integer used to make the name unique.
[Function]
debug:var name &optional identifier
This function returns the value of the variable in the current
frame with the specified name. If
supplied, identifier determines which
value to return when there are ambiguous variables.
When name is a symbol, it is interpreted
as the symbol name of the variable, i.e. the package is
significant. If name is an uninterned
symbol (gensym), then return the value of the uninterned variable
with the same name. If name is a string,
debug:var interprets it as the prefix of a
variable name, and must unambiguously complete to the name of a
valid variable.
This function is useful mainly for accessing the value of
uninterned or ambiguous variables, since most variables can be
evaluated directly.
| 3.4.1 |
Variable Value
Availability |
|
The value of a variable may be unavailable to the debugger in
portions of the program where Common 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 C) or if there is an unexpected hardware error
such as ``bus error'' (which should only
happen in unsafe code), then the values displayed for arguments to
the interrupted frame might be incorrect.1 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:
- The value of the 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 debug is at least 2.
- 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 debug optimization quality is 3.
- The variable's name is an uninterned symbol (gensym). To save
space, the compiler only dumps debug information about uninterned
variables when the debug optimization quality
is 3.
- The frame's location is unknown (see section 3.3.6) 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 above.
- The variable was optimized out of existence. Variables with no
reads are always optimized away, even in the interpreter. The
degree to which the compiler deletes variables will depend on the
value of the compile-speed optimization
quality, but most source-level optimizations are done under all
compilation policies.
Since it is especially useful to be able to get the arguments to a
function, argument variables are treated specially when the
speed optimization quality is less than
3 and the debug quality
is at least 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, debug must be at
least 2 for values to be available, and even
then, values are only available at known locations.
| 3.4.2 |
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.2 You can close over them, but such closures
can't be used as upward funargs.
You can also set local variables using 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.
| 3.5 |
Source Location
Printing |
|
One of cmucl's unique capabilities is
source level debugging of compiled code. These commands display the
source location for the current frame:
- source {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 context
is specified, then it is an integer specifying the number of
enclosing levels of list structure to print.
- vsource {context}
- This command is identical to source,
except that it uses the global values of *print-level* and *print-length*
instead of the debugger printing control variables *debug-print-level* and *debug-print-length*.
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 *some-random-special*, then the innermost enclosing
evaluated form will be printed. Here are some possible enclosing
forms:
(let ((a *some-random-special*))
...)
(+ *some-random-special* ...)
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 /usr/me/mystuff.lisp
looked like this:
(defmacro mymac ()
'(myfun))
(defun foo ()
(mymac)
...)
If foo has called myfun, and is waiting for it to return, then the
source command would print:
; File: /usr/me/mystuff.lisp
(MYMAC)
Note that the macro use was printed, not the actual function call
form, (myfun).
If enclosing source is printed by giving an argument to source or vsource, then the
actual source form is marked by wrapping it in a list whose first
element is #:***HERE***. In the previous
example, source 1 would print:
; File: /usr/me/mystuff.lisp
(DEFUN FOO ()
(#:***HERE***
(MYMAC))
...)
| 3.5.1 |
How the Source is
Found |
|
If the code was defined from Common Lisp by compile or eval, then the source
can always be reliably located. If the code was defined from a
fasl file created by 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 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:
; File has been modified since compilation:
; filename
; Using form offset instead of character position.
where 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:
- No top-level forms before the top-level form containing the
source have been added or deleted, and
- 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.)
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 eq list twice. If you don't define
read macros and don't use ## in perverted
ways, you don't need to worry about this.
| 3.5.2 |
Source Location
Availability |
|
Source location information is only
available when the debug optimization quality
is at least 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 (see section 3.3.6),
then the command will print:
Unknown location: using block start.
and then proceed to print the source location for the start of the
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:
- The block start location may be the same as the true
location.
- The block start location will never be later in the the
program's flow of control than the true location.
- No conditional control structures (such as if, cond, 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 do not end
basic blocks.
- The head of a loop will be the start of a block.
- The programming language concept of ``block structure'' and the
Common Lisp block special form are totally
unrelated to the compiler's basic block.
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.)
| 3.6 |
Compiler Policy
Control |
|
The compilation policy specified by optimize
declarations affects the behavior seen in the debugger. The
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 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 debug
quality:
- 0
- Only the function name and enough information to allow the
stack to be parsed.
- > 0
- Any level greater than 0 gives level
0 plus all argument variables. Values will
only be accessible if the argument variable is never set and
speed is not 3.
cmucl allows any real value for
optimization qualities. It may be useful to specify 0.5 to get backtrace argument display without argument
documentation.
- 1
- Level 1 provides argument documentation
(printed arglists) and derived argument/result type information.
This makes describe
more informative, and allows the compiler to do compile-time
argument count and type checking for any calls compiled at
run-time.
- 2
- Level 1 plus all interned local
variables, source location information, and lifetime information
that tells the debugger when arguments are available (even when
speed is 3 or the
argument is set.) This is the default.
- > 2
- Any level greater than 2 gives level
2 and in addition disables tail-call
optimization, so that the backtrace will contain frames for all
invoked functions, even those in tail positions.
- 3
- Level 2 plus all uninterned variables. In
addition, lifetime analysis is disabled (even when speed is 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.
As you can see, if the speed quality is
3, debugger performance is degraded. This
effect comes from the elimination of argument variable
special-casing (see section 3.4.1.) Some degree of
speed/debuggability tradeoff is unavoidable, but the effect is not
too drastic when debug is at least 2.
In addition to
inline and notinline
declarations, the relative values of the speed and space qualities also
change whether functions are inline expanded (see
section 5.8.) 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.
These commands get you out of the debugger.
- quit
- Throw to top level.
- restart {n}
- Invokes the nth restart case as
displayed by the error command. If n is not specified, the available restart cases are
reported.
- go
- Calls continue on the condition given to
debug. If there is no restart case named
continue, then an error is signaled.
- abort
- Calls abort on the condition given to
debug. This is useful for popping debug
command loop levels or aborting to top level, as the case may
be.
Most of these commands print information about the current frame or
function, but a few show general information.
- help, ?
- Displays a synopsis of debugger commands.
- describe
- Calls describe on the current function,
displays number of local variables, and indicates whether the
function is compiled or interpreted.
- print
- Displays the current function call as it would be displayed by
moving to this frame.
- vprint (or pp)
{verbosity}
- Displays the current function call using *print-level* and *print-length*
instead of *debug-print-level* and *debug-print-length*. verbosity is a small integer (default 2) that
controls other dimensions of verbosity.
- error
- Prints the condition given to invoke-debugger and the active proceed cases.
- backtrace {n}
Displays all the frames from the current to the bottom. Only shows
n frames if specified. The printing is
controlled by *debug-print-level* and
*debug-print-length*.
cmucl supports setting of breakpoints
inside compiled functions and stepping of compiled code.
Breakpoints can only be set at at known locations (see
section 3.3.6), so these
commands are largely useless unless the debug
optimize quality is at least 2 (see
section 3.6). These commands
manipulate breakpoints:
- breakpoint location {option value}*
- Set a breakpoint in some function. location may be an integer code location number (as
displayed by list-locations) or a keyword.
The keyword can be used to indicate setting a breakpoint at the
function start (:start, :s) or function end (:end,
:e). The breakpoint
command has :condition, :break, :print and :function options which work similarly to the
trace options.
- list-locations (or ll) {function}
- List all the code locations in the current frame's function, or
in function if it is supplied. The
display format is the code location number, a colon and then the
source form for that location:
3: (1- N)
If consecutive locations have the same source, then a numeric range
like 3-5: will be printed. For example, a
default function call has a known location both immediately before
and after the call, which would result in two code locations with
the same source. The listed function becomes the new default
function for breakpoint setting (via the breakpoint) command.
- list-breakpoints (or lb)
- List all currently active breakpoints with their breakpoint
number.
- delete-breakpoint (or db) {number}
- Delete a breakpoint specified by its breakpoint number. If no
number is specified, delete all breakpoints.
- step
- Step to the next possible breakpoint location in the current
function. This always steps over function calls, instead of
stepping into them
Consider this definition of the factorial function:
(defun ! (n)
(if (zerop n)
1
(* n (! (1- n)))))
This debugger session demonstrates the use of breakpoints:
common-lisp-user> (break) ; Invoke debugger
Break
Restarts:
0: [CONTINUE] Return from BREAK.
1: [ABORT ] Return to Top-Level.
Debug (type H for help)
(INTERACTIVE-EVAL (BREAK))
0] ll #'!
0: #'(LAMBDA (N) (BLOCK ! (IF # 1 #)))
1: (ZEROP N)
2: (* N (! (1- N)))
3: (1- N)
4: (! (1- N))
5: (* N (! (1- N)))
6: #'(LAMBDA (N) (BLOCK ! (IF # 1 #)))
0] br 2
(* N (! (1- N)))
1: 2 in !
Added.
0] q
common-lisp-user> (! 10) ; Call the function
*Breakpoint hit*
Restarts:
0: [CONTINUE] Return from BREAK.
1: [ABORT ] Return to Top-Level.
Debug (type H for help)
(! 10) ; We are now in first call (arg 10) before the multiply
Source: (* N (! (1- N)))
3] st
*Step*
(! 10) ; We have finished evaluation of (1- n)
Source: (1- N)
3] st
*Breakpoint hit*
Restarts:
0: [CONTINUE] Return from BREAK.
1: [ABORT ] Return to Top-Level.
Debug (type H for help)
(! 9) ; We hit the breakpoint in the recursive call
Source: (* N (! (1- N)))
3]
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.
[Macro]
trace {option
global-value}* {name {option value}*}*
trace is a debugging tool that prints
information when specified functions are called. In its simplest
form:
(trace name-1 name-2 ...)
trace causes a printout on *trace-output* each time that
one of the named functions is entered or returns (the names are not evaluated.) Trace output is indented
according to the number of pending traced calls, and this trace
depth is printed at the beginning of each line of output. Printing
verbosity of arguments and return values is controlled by *debug-print-level* and
*debug-print-length*.
If no names or options are are given, trace
returns the list of all currently traced functions, *traced-function-list*.
Trace options can cause the normal printout to be suppressed, or
cause extra information to be printed. Each option is a pair of an
option keyword and a value form. Options may be interspersed with
function names. Options only affect tracing of the function whose
name they appear immediately after. Global options are specified
before the first name, and affect all functions traced by a given
use of trace. If an already traced function
is traced again, any new options replace the old options. The
following options are defined:
- :condition form, :condition-after
form, :condition-all form
- If :condition is specified, then
trace does nothing unless form evaluates to true at the time of the call.
:condition-after is similar, but suppresses
the initial printout, and is tested when the function returns.
:condition-all tries both before and
after.
- :wherein names
- If specified, names is a function
name or list of names. trace does nothing
unless a call to one of those functions encloses the call to this
function (i.e. it would appear in a backtrace.) Anonymous functions
have string names like "DEFUN FOO".
- :break form,
:break-after form,
:break-all form
- If specified, and form evaluates to
true, then the debugger is invoked at the start of the function, at
the end of the function, or both, according to the respective
option.
- :print form,
:print-after form,
:print-all form
- In addition to the usual printout, the result of evaluating
form is printed at the start of the
function, at the end of the function, or both, according to the
respective option. Multiple print options cause multiple values to
be printed.
- :function function-form
- This is a not really an option, but rather another way of
specifying what function to trace. The function-form is evaluated immediately, and the
resulting function is traced.
- :encapsulate {:default | t |
nil}
- In cmucl, tracing can be done either
by temporarily redefining the function name (encapsulation), or
using breakpoints. When breakpoints are used, the function object
itself is destructively modified to cause the tracing action. The
advantage of using breakpoints is that tracing works even when the
function is anonymously called via funcall.
When :encapsulate is true, tracing is done
via encapsulation. :default is the default,
and means to use encapsulation for interpreted functions and
funcallable instances, breakpoints otherwise. When encapsulation is
used, forms are not evaluated in the function's lexical
environment, but debug:arg can still be
used.
Note that if you trace using :encapsulate,
you will only get a trace or breakpoint at the outermost call to
the traced function, not on recursive calls.
In the case of functions where the known return convention is used
to optimize, encapsulation may be necessary in order to make
tracing work at all. The symptom of this occurring is an error
stating
Error in function foo: :FUNCTION-END breakpoints are
currently unsupported for the known return convention.
in such cases we recommend using (trace foo :encapsulate t)
:condition, :break and
:print forms are evaluated in the lexical
environment of the called function; debug:var
and debug:arg can be used. The -after and -all forms are
evaluated in the null environment.
[Macro]
untrace &rest function-names
This macro turns off tracing for the specified functions, and
removes their names from *traced-function-list*. If no function-names are given, then all currently traced
functions are untraced.
[Variable]
extensions:*traced-function-list*
A list of function names maintained and used by trace, untrace, and untrace-all. This list should contain the names of all
functions currently being traced.
[Variable]
extensions:*max-trace-indentation*
The maximum number of spaces which should be used to indent trace
printout. This variable is initially set to 40.
| 3.10.1 |
Encapsulation
Functions |
|
The encapsulation functions provide a mechanism for intercepting
the arguments and results of a function. encapsulate changes the function definition of a
symbol, and saves it so that it can be restored later. The new
definition normally calls the original definition. The Common Lisp
fdefinition function
always returns the original definition, stripping off any
encapsulation.
The original definition of the symbol can be restored at any time
by the unencapsulate function. encapsulate and unencapsulate
allow a symbol to be multiply encapsulated in such a way that
different encapsulations can be completely transparent to each
other.
Each encapsulation has a type which may be an arbitrary lisp
object. If a symbol has several encapsulations of different types,
then any one of them can be removed without affecting more recent
ones. A symbol may have more than one encapsulation of the same
type, but only the most recent one can be undone.
[Function]
extensions:encapsulate symbol
type body
Saves the current definition of symbol,
and replaces it with a function which returns the result of
evaluating the form, body. Type is an arbitrary lisp object which is the type
of encapsulation.
When the new function is called, the following variables are bound
for the evaluation of body:
- extensions:argument-list
- A list of the arguments to the function.
- extensions:basic-definition
- The unencapsulated definition of the function.
The unencapsulated definition may be called with the original
arguments by including the form
(apply extensions:basic-definition extensions:argument-list)
encapsulate always returns symbol.
[Function]
extensions:unencapsulate symbol
type
Undoes symbol's most recent encapsulation
of type type. Type is compared with eq.
Encapsulations of other types are left in place.
[Function]
extensions:encapsulated-p symbol type
Returns t if symbol
has an encapsulation of type type.
Returns nil otherwise. type is compared with eq.
These are the special variables that control the debugger
action.
[Variable]
debug:*debug-print-level*
[Variable]
debug:*debug-print-length*
*print-level* and *print-length* are bound to these values during the
execution of some debug commands. When evaluating arbitrary
expressions in the debugger, the normal values of *print-level* and *print-length*
are in effect. These variables are initially set to 3 and 5,
respectively.
- 1
- Since the location of an interrupt or hardware error will
always be an unknown location (see section 3.3.6), non-argument variable values will
never be available in the interrupted frame.
- 2
- The variable bindings are actually created using the Common
Lisp symbol-macrolet special form.
© 1995-2003 CMUCL Project