guile/doc/ref/api-debug.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2010, 2011, 2012, 2013, 2014, 2018
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Debugging
@section Debugging Infrastructure

@cindex Debugging
In order to understand Guile's debugging facilities, you first need to
understand a little about how Guile represent the Scheme control stack.
With that in place we explain the low level trap calls that the virtual
machine can be configured to make, and the trap and breakpoint
infrastructure that builds on top of those calls.

@menu
* Evaluation Model::            Evaluation and the Scheme stack.
* Source Properties::           From expressions to source locations.
* Programmatic Error Handling::  Debugging when an error occurs.
* Traps::                       Breakpoints, tracepoints, oh my!
* GDB Support::                 C-level debugging with GDB.
@end menu

@node Evaluation Model
@subsection Evaluation and the Scheme Stack

The idea of the Scheme stack is central to a lot of debugging.  The
Scheme stack is a reified representation of the pending function returns
in an expression's continuation.  As Guile implements function calls
using a stack, this reification takes the form of a number of nested
stack frames, each of which corresponds to the application of a
procedure to a set of arguments.

A Scheme stack always exists implicitly, and can be summoned into
concrete existence as a first-class Scheme value by the
@code{make-stack} call, so that an introspective Scheme program -- such
as a debugger -- can present it in some way and allow the user to query
its details. The first thing to understand, therefore, is how Guile's
function call convention creates the stack.

Broadly speaking, Guile represents all control flow on a stack. Calling
a function involves pushing an empty frame on the stack, then evaluating
the procedure and its arguments, then fixing up the new frame so that it
points to the old one. Frames on the stack are thus linked together. A
tail call is the same, except it reuses the existing frame instead of
pushing on a new one.

In this way, the only frames that are on the stack are ``active''
frames, frames which need to do some work before the computation is
complete. On the other hand, a function that has tail-called another
function will not be on the stack, as it has no work left to do.

Therefore, when an error occurs in a running program, or the program
hits a breakpoint, or in fact at any point that the programmer chooses,
its state at that point can be represented by a @dfn{stack} of all the
procedure applications that are logically in progress at that time, each
of which is known as a @dfn{frame}.  The programmer can learn more about
the program's state at that point by inspecting the stack and its
frames.

@menu
* Stack Capture::               Reifying a continuation.
* Stacks::                      Accessors for the stack data type.
* Frames::                      Likewise, accessors for stack frames.
@end menu

@node Stack Capture
@subsubsection Stack Capture

A Scheme program can use the @code{make-stack} primitive anywhere in its
code, with first arg @code{#t}, to construct a Scheme value that
describes the Scheme stack at that point.

@lisp
(make-stack #t)
@result{}
#<stack 25205a0>
@end lisp

Use @code{start-stack} to limit the stack extent captured by future
@code{make-stack} calls.

@deffn {Scheme Procedure} make-stack obj arg @dots{}
@deffnx {C Function} scm_make_stack (obj, args)
Create a new stack. If @var{obj} is @code{#t}, the current
evaluation stack is used for creating the stack frames,
otherwise the frames are taken from @var{obj} (which must be
a continuation or a frame object).

@var{arg} @dots{} can be any combination of integer, procedure, address
range, and prompt tag values.

These values specify various ways of cutting away uninteresting stack
frames from the top and bottom of the stack that @code{make-stack}
returns.  They come in pairs like this:  @code{(@var{inner_cut_1}
@var{outer_cut_1} @var{inner_cut_2} @var{outer_cut_2} @dots{})}.

Each @var{inner_cut_i} can be an integer, a procedure, an address range,
or a prompt tag.  An integer means to cut away exactly that number of
frames.  A procedure means to cut away all frames up to but excluding
the frame whose procedure matches the specified one.  An address range
is a pair of integers indicating the low and high addresses of a
procedure's code, and is the same as cutting away to a procedure (though
with less work).  Anything else is interpreted as a prompt tag which
cuts away all frames that are inside a prompt with the given tag.

Each @var{outer_cut_i} can likewise be an integer, a procedure, an
address range, or a prompt tag.  An integer means to cut away that
number of frames.  A procedure means to cut away frames down to but
excluding the frame whose procedure matches the specified one.  An
address range is the same, but with the procedure's code specified as an
address range.  Anything else is taken to be a prompt tag, which cuts
away all frames that are outside a prompt with the given tag.


If the @var{outer_cut_i} of the last pair is missing, it is taken as 0.
@end deffn

@deffn {Scheme Syntax} start-stack id exp
Evaluate @var{exp} on a new calling stack with identity @var{id}.  If
@var{exp} is interrupted during evaluation, backtraces will not display
frames farther back than @var{exp}'s top-level form.  This macro is a
way of artificially limiting backtraces and stack procedures, largely as
a convenience to the user.
@end deffn


@node Stacks
@subsubsection Stacks

@deffn {Scheme Procedure} stack? obj
@deffnx {C Function} scm_stack_p (obj)
Return @code{#t} if @var{obj} is a calling stack.
@end deffn

@deffn {Scheme Procedure} stack-id stack
@deffnx {C Function} scm_stack_id (stack)
Return the identifier given to @var{stack} by @code{start-stack}.
@end deffn

@deffn {Scheme Procedure} stack-length stack
@deffnx {C Function} scm_stack_length (stack)
Return the length of @var{stack}.
@end deffn

@deffn {Scheme Procedure} stack-ref stack index
@deffnx {C Function} scm_stack_ref (stack, index)
Return the @var{index}'th frame from @var{stack}.
@end deffn

@deffn {Scheme Procedure} display-backtrace stack port [first [depth [highlights]]]
@deffnx {C Function} scm_display_backtrace_with_highlights (stack, port, first, depth, highlights)
@deffnx {C Function} scm_display_backtrace (stack, port, first, depth)
Display a backtrace to the output port @var{port}.  @var{stack}
is the stack to take the backtrace from, @var{first} specifies
where in the stack to start and @var{depth} how many frames
to display.  @var{first} and @var{depth} can be @code{#f},
which means that default values will be used.
If @var{highlights} is given it should be a list; the elements
of this list will be highlighted wherever they appear in the
backtrace.
@end deffn


@node Frames
@subsubsection Frames

@deffn {Scheme Procedure} frame? obj
@deffnx {C Function} scm_frame_p (obj)
Return @code{#t} if @var{obj} is a stack frame.
@end deffn

@deffn {Scheme Procedure} frame-previous frame
@deffnx {C Function} scm_frame_previous (frame)
Return the previous frame of @var{frame}, or @code{#f} if
@var{frame} is the first frame in its stack.
@end deffn

@deffn {Scheme Procedure} frame-procedure-name frame
@deffnx {C Function} scm_frame_procedure_name (frame)
Return the name of the procedure being applied in @var{frame}, as a
symbol, or @code{#f} if the procedure has no name.
@end deffn

@deffn {Scheme Procedure} frame-arguments frame
@deffnx {C Function} scm_frame_arguments (frame)
Return the arguments of @var{frame}.
@end deffn

@deffn {Scheme Procedure} frame-address frame
@deffnx {Scheme Procedure} frame-instruction-pointer frame
@deffnx {Scheme Procedure} frame-stack-pointer frame
Accessors for the three VM registers associated with this frame: the
frame pointer (fp), instruction pointer (ip), and stack pointer (sp),
respectively. @xref{VM Concepts}, for more information.
@end deffn

@deffn {Scheme Procedure} frame-dynamic-link frame
@deffnx {Scheme Procedure} frame-return-address frame
@deffnx {Scheme Procedure} frame-mv-return-address frame
Accessors for the three saved VM registers in a frame: the previous
frame pointer, the single-value return address, and the multiple-value
return address.  @xref{Stack Layout}, for more information.
@end deffn

@deffn {Scheme Procedure} frame-bindings frame
Return a list of binding records indicating the local variables that are
live in a frame.
@end deffn

@deffn {Scheme Procedure} frame-lookup-binding frame var
Fetch the bindings in @var{frame}, and return the first one whose name
is @var{var}, or @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} binding-index binding
@deffnx {Scheme Procedure} binding-name binding
@deffnx {Scheme Procedure} binding-slot binding
@deffnx {Scheme Procedure} binding-representation binding
Accessors for the various fields in a binding.  The implicit ``callee''
argument is index 0, the first argument is index 1, and so on to the end
of the arguments.  After that are temporary variables.  Note that if a
variable is dead, it might not be available.
@end deffn

@deffn {Scheme Procedure} binding-ref binding
@deffnx {Scheme Procedure} binding-set! binding val
Accessors for the values of local variables in a frame.
@end deffn

@deffn {Scheme Procedure} display-application frame [port [indent]]
@deffnx {C Function} scm_display_application (frame, port, indent)
Display a procedure application @var{frame} to the output port
@var{port}. @var{indent} specifies the indentation of the
output.
@end deffn

Additionally, the @code{(system vm frame)} module defines a number of
higher-level introspective procedures, for example to retrieve the names
of local variables, and the source location to correspond to a
frame. See its source code for more details.


@node Source Properties
@subsection Source Properties

@cindex source properties
As Guile reads in Scheme code from file or from standard input, it
remembers the file name, line number and column number where each
expression begins. These pieces of information are known as the
@dfn{source properties} of the expression. Syntax expanders and the
compiler propagate these source properties to compiled procedures, so
that, if an error occurs when evaluating the transformed expression,
Guile's debugger can point back to the file and location where the
expression originated.

The way that source properties are stored means that Guile cannot
associate source properties with individual symbols, keywords,
characters, booleans, or small integers.  This can be seen by typing
@code{(xxx)} and @code{xxx} at the Guile prompt (where the variable
@code{xxx} has not been defined):

@example
scheme@@(guile-user)> (xxx)
<unnamed port>:4:1: In procedure module-lookup:
<unnamed port>:4:1: Unbound variable: xxx

scheme@@(guile-user)> xxx
ERROR: In procedure module-lookup:
ERROR: Unbound variable: xxx
@end example

@noindent
In the latter case, no source properties were stored, so the error
doesn't have any source information.

@deffn {Scheme Procedure} supports-source-properties? obj
@deffnx {C Function} scm_supports_source_properties_p (obj)
Return #t if source properties can be associated with @var{obj},
otherwise return #f.
@end deffn

The recording of source properties is controlled by the read option
named ``positions'' (@pxref{Scheme Read}).  This option is switched
@emph{on} by default.

The following procedures can be used to access and set the source
properties of read expressions.

@deffn {Scheme Procedure} set-source-properties! obj alist
@deffnx {C Function} scm_set_source_properties_x (obj, alist)
Install the association list @var{alist} as the source property
list for @var{obj}.
@end deffn

@deffn {Scheme Procedure} set-source-property! obj key datum
@deffnx {C Function} scm_set_source_property_x (obj, key, datum)
Set the source property of object @var{obj}, which is specified by
@var{key} to @var{datum}.  Normally, the key will be a symbol.
@end deffn

@deffn {Scheme Procedure} source-properties obj
@deffnx {C Function} scm_source_properties (obj)
Return the source property association list of @var{obj}.
@end deffn

@deffn {Scheme Procedure} source-property obj key
@deffnx {C Function} scm_source_property (obj, key)
Return the property specified by @var{key} from @var{obj}'s source
properties.
@end deffn

If the @code{positions} reader option is enabled, supported expressions
will have values set for the @code{filename}, @code{line} and
@code{column} properties.

Source properties are also associated with syntax objects.  Procedural
macros can get at the source location of their input using the
@code{syntax-source} accessor.  @xref{Syntax Transformer Helpers}, for
more.

Guile also defines a couple of convenience macros built on
@code{syntax-source}:

@deffn {Scheme Syntax} current-source-location
Expands to the source properties corresponding to the location of the
@code{(current-source-location)} form.
@end deffn

@deffn {Scheme Syntax} current-filename
Expands to the current filename: the filename that the
@code{(current-filename)} form appears in.  Expands to @code{#f} if this
information is unavailable.
@end deffn

If you're stuck with defmacros (@pxref{Defmacros}), and want to preserve
source information, the following helper function might be useful to
you:

@deffn {Scheme Procedure} cons-source xorig x y
@deffnx {C Function} scm_cons_source (xorig, x, y)
Create and return a new pair whose car and cdr are @var{x} and @var{y}.
Any source properties associated with @var{xorig} are also associated
with the new pair.
@end deffn


@node Programmatic Error Handling
@subsection Programmatic Error Handling

For better or for worse, all programs have bugs, and dealing with bugs
is part of programming. This section deals with that class of bugs that
causes an exception to be raised -- from your own code, from within a
library, or from Guile itself.

@menu
* Catching Exceptions::    Handling errors after the stack is unwound.
* Capturing Stacks::       Capturing the stack at the time of error.
* Pre-Unwind Debugging::   Debugging before the exception is thrown.
* Stack Overflow::         Detecting and handling runaway recursion.
* Debug Options::          A historical interface to debugging.
@end menu

@node Catching Exceptions
@subsubsection Catching Exceptions

A common requirement is to be able to show as much useful context as
possible when a Scheme program hits an error.  The most immediate
information about an error is the kind of error that it is -- such as
``division by zero'' -- and any parameters that the code which signalled
the error chose explicitly to provide.  This information originates with
the @code{error} or @code{throw} call (or their C code equivalents, if
the error is detected by C code) that signals the error, and is passed
automatically to the handler procedure of the innermost applicable
@code{catch} or @code{with-throw-handler} expression.

Therefore, to catch errors that occur within a chunk of Scheme code, and
to intercept basic information about those errors, you need to execute
that code inside the dynamic context of a @code{catch} or
@code{with-throw-handler} expression, or the equivalent in C. In Scheme,
this means you need something like this:

@lisp
(catch #t
       (lambda ()
         ;; Execute the code in which
         ;; you want to catch errors here.
         ...)
       (lambda (key . parameters)
         ;; Put the code which you want
         ;; to handle an error here.
         ...))
@end lisp

@noindent
The @code{catch} here can also be @code{with-throw-handler}; see
@ref{Throw Handlers} for information on the when you might want to use
@code{with-throw-handler} instead of @code{catch}.

For example, to print out a message and return #f when an error occurs,
you might use:

@smalllisp
(define (catch-all thunk)
  (catch #t
    thunk
    (lambda (key . parameters)
      (format (current-error-port)
              "Uncaught throw to '~a: ~a\n" key parameters)
      #f)))

(catch-all
 (lambda () (error "Not a vegetable: tomato")))
@print{} Uncaught throw to 'misc-error: (#f ~A (Not a vegetable: tomato) #f)
@result{} #f
@end smalllisp

The @code{#t} means that the catch is applicable to all kinds of error.
If you want to restrict your catch to just one kind of error, you can
put the symbol for that kind of error instead of @code{#t}. The
equivalent to this in C would be something like this:

@lisp
SCM my_body_proc (void *body_data)
@{
  /* Execute the code in which
     you want to catch errors here. */
  ...
@}

SCM my_handler_proc (void *handler_data,
                     SCM key,
                     SCM parameters)
@{
  /* Put the code which you want
     to handle an error here. */
  ...
@}

@{
  ...
  scm_c_catch (SCM_BOOL_T,
               my_body_proc, body_data,
               my_handler_proc, handler_data,
               NULL, NULL);
  ...
@}
@end lisp

@noindent
Again, as with the Scheme version, @code{scm_c_catch} could be replaced
by @code{scm_c_with_throw_handler}, and @code{SCM_BOOL_T} could instead
be the symbol for a particular kind of error.

@node Capturing Stacks
@subsubsection Capturing the full error stack

The other interesting information about an error is the full Scheme
stack at the point where the error occurred; in other words what
innermost expression was being evaluated, what was the expression that
called that one, and so on.  If you want to write your code so that it
captures and can display this information as well, there are a couple
important things to understand.

Firstly, the stack at the point of the error needs to be explicitly
captured by a @code{make-stack} call (or the C equivalent
@code{scm_make_stack}).  The Guile library does not do this
``automatically'' for you, so you will need to write code with a
@code{make-stack} or @code{scm_make_stack} call yourself.  (We emphasise
this point because some people are misled by the fact that the Guile
interactive REPL code @emph{does} capture and display the stack
automatically.  But the Guile interactive REPL is itself a Scheme
program@footnote{In effect, it is the default program which is run when
no commands or script file are specified on the Guile command line.}
running on top of the Guile library, and which uses @code{catch} and
@code{make-stack} in the way we are about to describe to capture the
stack when an error occurs.)

And secondly, in order to capture the stack effectively at the point
where the error occurred, the @code{make-stack} call must be made before
Guile unwinds the stack back to the location of the prevailing catch
expression. This means that the @code{make-stack} call must be made
within the handler of a @code{with-throw-handler} expression, or the
optional "pre-unwind" handler of a @code{catch}. (For the full story of
how these alternatives differ from each other, see @ref{Exceptions}. The
main difference is that @code{catch} terminates the error, whereas
@code{with-throw-handler} only intercepts it temporarily and then allow
it to continue propagating up to the next innermost handler.)

So, here are some examples of how to do all this in Scheme and in C.
For the purpose of these examples we assume that the captured stack
should be stored in a variable, so that it can be displayed or
arbitrarily processed later on.  In Scheme:

@lisp
(let ((captured-stack #f))
  (catch #t
         (lambda ()
           ;; Execute the code in which
           ;; you want to catch errors here.
           ...)
         (lambda (key . parameters)
           ;; Put the code which you want
           ;; to handle an error after the
           ;; stack has been unwound here.
           ...)
         (lambda (key . parameters)
           ;; Capture the stack here:
           (set! captured-stack (make-stack #t))))
  ...
  (if captured-stack
      (begin
        ;; Display or process the captured stack.
        ...))
  ...)
@end lisp

@noindent
And in C:

@lisp
SCM my_body_proc (void *body_data)
@{
  /* Execute the code in which
     you want to catch errors here. */
  ...
@}

SCM my_handler_proc (void *handler_data,
                     SCM key,
                     SCM parameters)
@{
  /* Put the code which you want
     to handle an error after the
     stack has been unwound here. */
  ...
@}

SCM my_preunwind_proc (void *handler_data,
                       SCM key,
                       SCM parameters)
@{
  /* Capture the stack here: */
  *(SCM *)handler_data = scm_make_stack (SCM_BOOL_T, SCM_EOL);
@}

@{
  SCM captured_stack = SCM_BOOL_F;
  ...
  scm_c_catch (SCM_BOOL_T,
               my_body_proc, body_data,
               my_handler_proc, handler_data,
               my_preunwind_proc, &captured_stack);
  ...
  if (captured_stack != SCM_BOOL_F)
  @{
    /* Display or process the captured stack. */
    ...
  @}
  ...
@}
@end lisp

Once you have a captured stack, you can interrogate and display its
details in any way that you want, using the @code{stack-@dots{}} and
@code{frame-@dots{}} API described in @ref{Stacks} and
@ref{Frames}.

If you want to print out a backtrace in the same format that the Guile
REPL does, you can use the @code{display-backtrace} procedure to do so.
You can also use @code{display-application} to display an individual
frame in the Guile REPL format.

@node Pre-Unwind Debugging
@subsubsection Pre-Unwind Debugging

Instead of saving a stack away and waiting for the @code{catch} to
return, you can handle errors directly, from within the pre-unwind
handler.

For example, to show a backtrace when an error is thrown, you might want
to use a procedure like this:

@lisp
(define (with-backtrace thunk)
  (with-throw-handler #t
                      thunk
                      (lambda args (backtrace))))
(with-backtrace (lambda () (error "Not a vegetable: tomato")))
@end lisp

Since we used @code{with-throw-handler} here, we didn't actually catch
the error. @xref{Throw Handlers}, for more information. However, we did
print out a context at the time of the error, using the built-in
procedure, @code{backtrace}.

@deffn {Scheme Procedure} backtrace [highlights]
@deffnx {C Function} scm_backtrace_with_highlights (highlights)
@deffnx {C Function} scm_backtrace ()
Display a backtrace of the current stack to the current output port.  If
@var{highlights} is given it should be a list; the elements of this list
will be highlighted wherever they appear in the backtrace.
@end deffn

The Guile REPL code (in @file{system/repl/repl.scm} and related files)
uses a @code{catch} with a pre-unwind handler to capture the stack when
an error occurs in an expression that was typed into the REPL, and debug
that stack interactively in the context of the error.

These procedures are available for use by user programs, in the
@code{(system repl error-handling)} module.

@lisp
(use-modules (system repl error-handling))
@end lisp

@deffn {Scheme Procedure} call-with-error-handling thunk @
       [#:on-error on-error='debug] [#:post-error post-error='catch] @
       [#:pass-keys pass-keys='(quit)] @
       [#:report-keys report-keys='(stack-overflow)] @
       [#:trap-handler trap-handler='debug]
Call a thunk in a context in which errors are handled.

There are five keyword arguments:

@table @var
@item on-error
Specifies what to do before the stack is unwound.

Valid options are @code{debug} (the default), which will enter a
debugger; @code{pass}, in which case nothing is done, and the exception
is rethrown; or a procedure, which will be the pre-unwind handler.

@item post-error
Specifies what to do after the stack is unwound.

Valid options are @code{catch} (the default), which will silently catch
errors, returning the unspecified value; @code{report}, which prints out
a description of the error (via @code{display-error}), and then returns
the unspecified value; or a procedure, which will be the catch handler.

@item trap-handler
Specifies a trap handler: what to do when a breakpoint is hit.

Valid options are @code{debug}, which will enter the debugger;
@code{pass}, which does nothing; or @code{disabled}, which disables
traps entirely.  @xref{Traps}, for more information.

@item pass-keys
A set of keys to ignore, as a list.

@item report-keys
A set of keys to always report even if the post-error handler is
@code{catch}, as a list.
@end table
@end deffn

@node Stack Overflow
@subsubsection Stack Overflow

@cindex overflow, stack
@cindex stack overflow
Every time a Scheme program makes a call that is not in tail position,
it pushes a new frame onto the stack.  Returning a value from a function
pops the top frame off the stack.  Stack frames take up memory, and as
nobody has an infinite amount of memory, deep recursion could cause
Guile to run out of memory.  Running out of stack memory is called
@dfn{stack overflow}.

@subsubheading Stack Limits

Most languages have a terrible stack overflow story.  For example, in C,
if you use too much stack, your program will exhibit ``undefined
behavior'', which if you are lucky means that it will crash.  It's
especially bad in C, as you neither know ahead of time how much stack
your functions use, nor the stack limit imposed by the user's system,
and the stack limit is often quite small relative to the total memory
size.

Managed languages like Python have a better error story, as they are
defined to raise an exception on stack overflow -- but like C, Python
and most dynamic languages still have a fixed stack size limit that is
usually much smaller than the heap.

Arbitrary stack limits would have an unfortunate effect on Guile
programs.  For example, the following implementation of the inner loop
of @code{map} is clean and elegant:

@example
(define (map f l)
  (if (pair? l)
      (cons (f (car l))
            (map f (cdr l)))
      '()))
@end example

However, if there were a stack limit, that would limit the size of lists
that can be processed with this @code{map}.  Eventually, you would have
to rewrite it to use iteration with an accumulator:

@example
(define (map f l)
  (let lp ((l l) (out '()))
    (if (pair? l)
        (lp (cdr l) (cons (f (car l)) out))
        (reverse out))))
@end example

This second version is sadly not as clear, and it also allocates more
heap memory (once to build the list in reverse, and then again to
reverse the list).  You would be tempted to use the destructive
@code{reverse!} to save memory and time, but then your code would not be
continuation-safe -- if @var{f} returned again after the map had
finished, it would see an @var{out} list that had already been
reversed.  The recursive @code{map} has none of these problems.

Guile has no stack limit for Scheme code.  When a thread makes its first
Guile call, a small stack is allocated -- just one page of memory.
Whenever that memory limit would be reached, Guile arranges to grow the
stack by a factor of two.  When garbage collection happens, Guile
arranges to return the unused part of the stack to the operating system,
but without causing the stack to shrink.  In this way, the stack can
grow to consume up to all memory available to the Guile process, and
when the recursive computation eventually finishes, that stack memory is
returned to the system.

@subsubheading Exceptional Situations

Of course, it's still possible to run out of stack memory.  The most
common cause of this is program bugs that cause unbounded recursion, as
in:

@example
(define (faulty-map f l)
  (if (pair? l)
      (cons (f (car l)) (faulty-map f l))
      '()))
@end example

Did you spot the bug?  The recursive call to @code{faulty-map} recursed
on @var{l}, not @code{(cdr @var{l})}.  Running this program would cause
Guile to use up all memory in your system, and eventually Guile would
fail to grow the stack.  At that point you have a problem: Guile needs
to raise an exception to unwind the stack and return memory to the
system, but the user might have throw handlers in place (@pxref{Throw
Handlers}) that want to run before the stack is unwound, and we don't
have any stack in which to run them.

Therefore in this case, Guile throws an unwind-only exception that does
not run pre-unwind handlers.  Because this is such an odd case, Guile
prints out a message on the console, in case the user was expecting to
be able to get a backtrace from any pre-unwind handler.

@subsubheading Runaway Recursion

Still, this failure mode is not so nice.  If you are running an
environment in which you are interactively building a program while it
is running, such as at a REPL, you might want to impose an artificial
stack limit on the part of your program that you are building to detect
accidental runaway recursion.  For that purpose, there is
@code{call-with-stack-overflow-handler}, from @code{(system vm vm)}.

@example
(use-module (system vm vm))
@end example

@deffn {Scheme Procedure} call-with-stack-overflow-handler limit thunk handler
Call @var{thunk} in an environment in which the stack limit has been
reduced to @var{limit} additional words.  If the limit is reached,
@var{handler} (a thunk) will be invoked in the dynamic environment of
the error.  For the extent of the call to @var{handler}, the stack limit
and handler are restored to the values that were in place when
@code{call-with-stack-overflow-handler} was called.

Usually, @var{handler} should raise an exception or abort to an outer
prompt.  However if @var{handler} does return, it should return a number
of additional words of stack space to allow to the inner environment.
@end deffn

A stack overflow handler may only ever ``credit'' the inner thunk with
stack space that was available when the handler was instated.  When
Guile first starts, there is no stack limit in place, so the outer
handler may allow the inner thunk an arbitrary amount of space, but any
nested stack overflow handler will not be able to consume more than its
limit.

Unlike the unwind-only exception that is thrown if Guile is unable to
grow its stack, any exception thrown by a stack overflow handler might
invoke pre-unwind handlers.  Indeed, the stack overflow handler is
itself a pre-unwind handler of sorts.  If the code imposing the stack
limit wants to protect itself against malicious pre-unwind handlers from
the inner thunk, it should abort to a prompt of its own making instead
of throwing an exception that might be caught by the inner thunk.

@subsubheading C Stack Usage

It is also possible for Guile to run out of space on the C stack.  If
you call a primitive procedure which then calls a Scheme procedure in a
loop, you will consume C stack space.  Guile tries to detect excessive
consumption of C stack space, throwing an error when you have hit 80% of
the process' available stack (as allocated by the operating system), or
160 kilowords in the absence of a strict limit.

For example, looping through @code{call-with-vm}, a primitive that calls
a thunk, gives us the following:

@lisp
scheme@@(guile-user)> (use-modules (system vm vm))
scheme@@(guile-user)> (let lp () (call-with-vm lp))
ERROR: Stack overflow
@end lisp

Unfortunately, that's all the information we get.  Overrunning the C
stack will throw an unwind-only exception, because it's not safe to
do very much when you are close to the C stack limit.

If you get an error like this, you can either try rewriting your code to
use less stack space, or increase the maximum stack size.  To increase
the maximum stack size, use @code{debug-set!}, for example:

@lisp
(debug-set! stack 200000)
@end lisp

The next section describes @code{debug-set!} more thoroughly.  Of course
the best thing is to have your code operate without so much resource
consumption by avoiding loops through C trampolines.


@node Debug Options
@subsubsection Debug options

The behavior of the @code{backtrace} procedure and of the default error
handler can be parameterized via the debug options.

@cindex options - debug
@cindex debug options
@deffn {Scheme Procedure} debug-options [setting]
Display the current settings of the debug options.  If @var{setting} is
omitted, only a short form of the current read options is printed.
Otherwise if @var{setting} is the symbol @code{help}, a complete options
description is displayed.
@end deffn

The set of available options, and their default values, may be had by
invoking @code{debug-options} at the prompt.

@smallexample
scheme@@(guile-user)>
backwards       no      Display backtrace in anti-chronological order.
width           79      Maximal width of backtrace.
depth           20      Maximal length of printed backtrace.
backtrace       yes     Show backtrace on error.
stack           1048576 Stack size limit (measured in words;
                        0 = no check). 
show-file-name  #t      Show file names and line numbers in backtraces
                        when not `#f'.  A value of `base' displays only
                        base names, while `#t' displays full names. 
warn-deprecated no      Warn when deprecated features are used.
@end smallexample

The boolean options may be toggled with @code{debug-enable} and
@code{debug-disable}. The non-boolean options must be set using
@code{debug-set!}.

@deffn {Scheme Procedure} debug-enable option-name
@deffnx {Scheme Procedure} debug-disable option-name
@deffnx {Scheme Syntax} debug-set! option-name value
Modify the debug options.  @code{debug-enable} should be used with boolean
options and switches them on, @code{debug-disable} switches them off.

@code{debug-set!} can be used to set an option to a specific value.  Due
to historical oddities, it is a macro that expects an unquoted option
name.
@end deffn


@node Traps
@subsection Traps

@cindex Traps
@cindex VM hooks
@cindex Breakpoints
@cindex Trace
@cindex Tracing
@cindex Code coverage
@cindex Profiling
Guile's virtual machine can be configured to call out at key points to
arbitrary user-specified procedures.

In principle, these @dfn{hooks} allow Scheme code to implement any model
it chooses for examining the evaluation stack as program execution
proceeds, and for suspending execution to be resumed later.

VM hooks are very low-level, though, and so Guile also has a library of
higher-level @dfn{traps} on top of the VM hooks. A trap is an execution
condition that, when fulfilled, will fire a handler. For example, Guile
defines a trap that fires when control reaches a certain source
location.

Finally, Guile also defines a third level of abstractions: per-thread
@dfn{trap states}. A trap state exists to give names to traps, and to
hold on to the set of traps so that they can be enabled, disabled, or
removed. The trap state infrastructure defines the most useful
abstractions for most cases. For example, Guile's REPL uses trap state
functions to set breakpoints and tracepoints.

The following subsections describe all this in detail, for both the
user wanting to use traps, and the developer interested in
understanding how the interface hangs together.


@menu
* VM Hooks::                Modifying Guile's virtual machine.
* Trap Interface::          Traps are on or off.
* Low-Level Traps::         The various kinds of low-level traps.
* Tracing Traps::           Traps to trace procedure calls and returns.
* Trap States::             One state (per thread) to bind them.
* High-Level Traps::        The highest-level trap interface. Use this.
@end menu


@node VM Hooks
@subsubsection VM Hooks

Everything that runs in Guile runs on its virtual machine, a C program
that defines a number of operations that Scheme programs can
perform.

Note that there are multiple VM ``engines'' for Guile. Only some of them
have support for hooks compiled in. Normally the deal is that you get
hooks if you are running interactively, and otherwise they are disabled,
as they do have some overhead (about 10 or 20 percent).

To ensure that you are running with hooks, pass @code{--debug} to Guile
when running your program, or otherwise use the @code{call-with-vm} and
@code{set-vm-engine!}  procedures to ensure that you are running in a VM
with the @code{debug} engine.

To digress, Guile's VM has 4 different hooks that can be fired at
different times.  For implementation reasons, these hooks are not
actually implemented with first-class Scheme hooks (@pxref{Hooks}); they
are managed using an ad-hoc interface.

VM hooks are called with one argument: the current frame.
@xref{Frames}.  Since these hooks may be fired very frequently, Guile
does a terrible thing: it allocates the frames on the C stack instead of
the garbage-collected heap.

The upshot here is that the frames are only valid within the dynamic
extent of the call to the hook. If a hook procedure keeps a reference to
the frame outside the extent of the hook, bad things will happen.

The interface to hooks is provided by the @code{(system vm vm)} module:

@example
(use-modules (system vm vm))
@end example

@noindent
All of these functions implicitly act on the VM for the current thread
only.

@deffn {Scheme Procedure} vm-add-next-hook! f
Arrange to call @var{f} when before an instruction is retired (and
executed).
@end deffn

@deffn {Scheme Procedure} vm-add-apply-hook! f
Arrange to call @var{f} whenever a procedure is applied.  The frame
locals will be the callee, followed by the arguments to the call.

Note that procedure application is somewhat orthogonal to continuation
pushes and pops.  To know whether a call is a tail call or not, with
respect to the frame previously in place, check the value of the frame
pointer compared the previous frame pointer.
@end deffn

@deffn {Scheme Procedure} vm-add-return-hook! f
Arrange to call @var{f} before returning from a frame.  The values in
the frame will be the frame's return values.

Note that it's possible to return from an ``inner'' frame: one that was
not immediately proceeded by a call with that frame pointer.  In that
case, it corresponds to a non-local control flow jump, either because of
applying a composable continuation or because of restoring a saved
undelimited continuation.
@end deffn

@deffn {Scheme Procedure} vm-add-abort-hook!
Arrange to call @var{f} after aborting to a prompt.  @xref{Prompts}.

Unfortunately, the values passed to the prompt handler are not easily
available to @var{f}.
@end deffn

@deffn {Scheme Procedure} vm-remove-next-hook! f
@deffnx {Scheme Procedure} vm-remove-apply-hook! f
@deffnx {Scheme Procedure} vm-remove-return-hook! f
@deffnx {Scheme Procedure} vm-remove-abort-hook! f
Remove @var{f} from the corresponding VM hook for the current thread.
@end deffn

@cindex VM trace level
These hooks do impose a performance penalty, if they are on. Obviously,
the @code{vm-next-hook} has quite an impact, performance-wise. Therefore
Guile exposes a single, heavy-handed knob to turn hooks on or off, the
@dfn{VM trace level}. If the trace level is positive, hooks run;
otherwise they don't.

For convenience, when the VM fires a hook, it does so with the trap
level temporarily set to 0.  That way the hooks don't fire while you're
handling a hook.  The trace level is restored to whatever it was once the hook
procedure finishes.

@deffn {Scheme Procedure} vm-trace-level
Retrieve the ``trace level'' of the VM. If positive, the trace hooks
associated with @var{vm} will be run. The initial trace level is 0.
@end deffn

@deffn {Scheme Procedure} set-vm-trace-level! level
Set the ``trace level'' of the VM.
@end deffn

@xref{A Virtual Machine for Guile}, for more information on Guile's
virtual machine.

@node Trap Interface
@subsubsection Trap Interface

The capabilities provided by hooks are great, but hooks alone rarely
correspond to what users want to do.

For example, if a user wants to break when and if control reaches a
certain source location, how do you do it?  If you install a ``next''
hook, you get unacceptable overhead for the execution of the entire
program. It would be possible to install an ``apply'' hook, then if the
procedure encompasses those source locations, install a ``next'' hook,
but already you're talking about one concept that might be implemented
by a varying number of lower-level concepts.

It's best to be clear about things and define one abstraction for all
such conditions: the @dfn{trap}.

Considering the myriad capabilities offered by the hooks though, there
is only a minimum of functionality shared by all traps. Guile's current
take is to reduce this to the absolute minimum, and have the only
standard interface of a trap be ``turn yourself on'' or ``turn yourself
off''.

This interface sounds a bit strange, but it is useful to procedurally
compose higher-level traps from lower-level building blocks. For
example, Guile defines a trap that calls one handler when control enters
a procedure, and another when control leaves the procedure. Given that
trap, one can define a trap that adds to the next-hook only when within
a given procedure. Building further, one can define a trap that fires
when control reaches particular instructions within a procedure.

Or of course you can stop at any of these intermediate levels. For
example, one might only be interested in calls to a given procedure. But
the point is that a simple enable/disable interface is all the
commonality that exists between the various kinds of traps, and
furthermore that such an interface serves to allow ``higher-level''
traps to be composed from more primitive ones.

Specifically, a trap, in Guile, is a procedure. When a trap is created,
by convention the trap is enabled; therefore, the procedure that is the
trap will, when called, disable the trap, and return a procedure that
will enable the trap, and so on.

Trap procedures take one optional argument: the current frame. (A trap
may want to add to different sets of hooks depending on the frame that
is current at enable-time.)

If this all sounds very complicated, it's because it is. Some of it is
essential, but probably most of it is not. The advantage of using this
minimal interface is that composability is more lexically apparent than
when, for example, using a stateful interface based on GOOPS. But
perhaps this reflects the cognitive limitations of the programmer who
made the current interface more than anything else.

@node Low-Level Traps
@subsubsection Low-Level Traps

To summarize the last sections, traps are enabled or disabled, and when
they are enabled, they add to various VM hooks.

Note, however, that @emph{traps do not increase the VM trace level}. So
if you create a trap, it will be enabled, but unless something else
increases the VM's trace level (@pxref{VM Hooks}), the trap will not
fire.  It turns out that getting the VM trace level right is tricky
without a global view of what traps are enabled.  @xref{Trap States},
for Guile's answer to this problem.

Traps are created by calling procedures. Most of these procedures share
a set of common keyword arguments, so rather than document them
separately, we discuss them all together here:

@table @code
@item #:vm
The VM to instrument. Defaults to the current thread's VM.
@item #:current-frame
For traps that enable more hooks depending on their dynamic context,
this argument gives the current frame that the trap is running in.
Defaults to @code{#f}.
@end table

To have access to these procedures, you'll need to have imported the
@code{(system vm traps)} module:

@lisp
(use-modules (system vm traps))
@end lisp

@deffn {Scheme Procedure} trap-at-procedure-call proc handler @
       [#:vm]
A trap that calls @var{handler} when @var{proc} is applied.
@end deffn                

@deffn {Scheme Procedure} trap-in-procedure proc @
       enter-handler exit-handler [#:current-frame] [#:vm]
A trap that calls @var{enter-handler} when control enters @var{proc},
and @var{exit-handler} when control leaves @var{proc}.

Control can enter a procedure via:
@itemize
@item
A procedure call.
@item
A return to a procedure's frame on the stack.
@item
A continuation returning directly to an application of this procedure.
@end itemize

Control can leave a procedure via:
@itemize
@item
A normal return from the procedure.
@item
An application of another procedure.
@item
An invocation of a continuation.
@item
An abort.
@end itemize
@end deffn

@deffn {Scheme Procedure} trap-instructions-in-procedure proc @
       next-handler exit-handler [#:current-frame] [#:vm]
A trap that calls @var{next-handler} for every instruction executed in
@var{proc}, and @var{exit-handler} when execution leaves @var{proc}.
@end deffn

@deffn {Scheme Procedure} trap-at-procedure-ip-in-range proc range @
       handler [#:current-frame] [#:vm]
A trap that calls @var{handler} when execution enters a range of
instructions in @var{proc}. @var{range} is a simple of pairs,
@code{((@var{start} . @var{end}) ...)}. The @var{start} addresses are
inclusive, and @var{end} addresses are exclusive.
@end deffn

@deffn {Scheme Procedure} trap-at-source-location file user-line handler @
       [#:current-frame] [#:vm]
A trap that fires when control reaches a given source location.  The
@var{user-line} parameter is one-indexed, as a user counts lines,
instead of zero-indexed, as Guile counts lines.
@end deffn

@deffn {Scheme Procedure} trap-frame-finish frame @
       return-handler abort-handler [#:vm]
A trap that fires when control leaves the given frame. @var{frame}
should be a live frame in the current continuation. @var{return-handler}
will be called on a normal return, and @var{abort-handler} on a nonlocal
exit.
@end deffn

@deffn {Scheme Procedure} trap-in-dynamic-extent proc @
       enter-handler return-handler abort-handler [#:vm]
A more traditional dynamic-wind trap, which fires @var{enter-handler}
when control enters @var{proc}, @var{return-handler} on a normal return,
and @var{abort-handler} on a nonlocal exit.

Note that rewinds are not handled, so there is no rewind handler.
@end deffn

@deffn {Scheme Procedure} trap-calls-in-dynamic-extent proc @
       apply-handler return-handler [#:current-frame] [#:vm]
A trap that calls @var{apply-handler} every time a procedure is applied,
and @var{return-handler} for returns, but only during the dynamic extent
of an application of @var{proc}.
@end deffn

@deffn {Scheme Procedure} trap-instructions-in-dynamic-extent proc @
       next-handler [#:current-frame] [#:vm]
A trap that calls @var{next-handler} for all retired instructions within
the dynamic extent of a call to @var{proc}.
@end deffn

@deffn {Scheme Procedure} trap-calls-to-procedure proc @
       apply-handler return-handler [#:vm]
A trap that calls @var{apply-handler} whenever @var{proc} is applied,
and @var{return-handler} when it returns, but with an additional
argument, the call depth.

That is to say, the handlers will get two arguments: the frame in
question, and the call depth (a non-negative integer).
@end deffn

@deffn {Scheme Procedure} trap-matching-instructions frame-pred handler [#:vm]
A trap that calls @var{frame-pred} at every instruction, and if
@var{frame-pred} returns a true value, calls @var{handler} on the
frame.
@end deffn

@node Tracing Traps
@subsubsection Tracing Traps

The @code{(system vm trace)} module defines a number of traps for
tracing of procedure applications.  When a procedure is @dfn{traced}, it
means that every call to that procedure is reported to the user during a
program run.  The idea is that you can mark a collection of procedures
for tracing, and Guile will subsequently print out a line of the form

@lisp
|  |  (@var{procedure} @var{args} @dots{})
@end lisp

whenever a marked procedure is about to be applied to its arguments.
This can help a programmer determine whether a function is being called
at the wrong time or with the wrong set of arguments.

In addition, the indentation of the output is useful for demonstrating
how the traced applications are or are not tail recursive with respect
to each other.  Thus, a trace of a non-tail recursive factorial
implementation looks like this:

@lisp
scheme@@(guile-user)> (define (fact1 n) 
                       (if (zero? n) 1
                           (* n (fact1 (1- n)))))
scheme@@(guile-user)> ,trace (fact1 4)
trace: (fact1 4)
trace: |  (fact1 3)
trace: |  |  (fact1 2)
trace: |  |  |  (fact1 1)
trace: |  |  |  |  (fact1 0)
trace: |  |  |  |  1
trace: |  |  |  1
trace: |  |  2
trace: |  6
trace: 24
@end lisp

While a typical tail recursive implementation would look more like this:

@lisp
scheme@@(guile-user)> (define (facti acc n)
                       (if (zero? n) acc
                           (facti (* n acc) (1- n))))
scheme@@(guile-user)> (define (fact2 n) (facti 1 n))
scheme@@(guile-user)> ,trace (fact2 4)
trace: (fact2 4)
trace: (facti 1 4)
trace: (facti 4 3)
trace: (facti 12 2)
trace: (facti 24 1)
trace: (facti 24 0)
trace: 24
@end lisp

The low-level traps below (@pxref{Low-Level Traps}) share some common
options:

@table @code
@item #:width
The maximum width of trace output. Trace printouts will try not to
exceed this column, but for highly nested procedure calls, it may be
unavoidable. Defaults to 80.
@item #:vm
The VM on which to add the traps. Defaults to the current thread's VM.
@item #:prefix
A string to print out before each trace line. As seen above in the
examples, defaults to @code{"trace: "}.
@end table

To have access to these procedures, you'll need to have imported the
@code{(system vm trace)} module:

@lisp
(use-modules (system vm trace))
@end lisp

@deffn {Scheme Procedure} trace-calls-to-procedure proc @
       [#:width] [#:vm] [#:prefix]
Print a trace at applications of and returns from @var{proc}.
@end deffn

@deffn {Scheme Procedure} trace-calls-in-procedure proc @
       [#:width] [#:vm] [#:prefix]
Print a trace at all applications and returns within the dynamic extent
of calls to @var{proc}.
@end deffn

@deffn {Scheme Procedure} trace-instructions-in-procedure proc [#:width] [#:vm]
Print a trace at all instructions executed in the dynamic extent of
calls to @var{proc}.
@end deffn

In addition, Guile defines a procedure to call a thunk, tracing all
procedure calls and returns within the thunk.

@deffn {Scheme Procedure} call-with-trace thunk [#:calls?=#t] @
                          [#:instructions?=#f] @
                          [#:width=80]
Call @var{thunk}, tracing all execution within its dynamic extent.

If @var{calls?} is true, Guile will print a brief report at each
procedure call and return, as given above.

If @var{instructions?} is true, Guile will also print a message each
time an instruction is executed.  This is a lot of output, but it is
sometimes useful when doing low-level optimization.

Note that because this procedure manipulates the VM trace level
directly, it doesn't compose well with traps at the REPL.
@end deffn

@xref{Profile Commands}, for more information on tracing at the REPL.

@node Trap States
@subsubsection Trap States

When multiple traps are present in a system, we begin to have a
bookkeeping problem. How are they named? How does one disable, enable,
or delete them?

Guile's answer to this is to keep an implicit per-thread @dfn{trap
state}. The trap state object is not exposed to the user; rather, API
that works on trap states fetches the current trap state from the
dynamic environment.

Traps are identified by integers. A trap can be enabled, disabled, or
removed, and can have an associated user-visible name.

These procedures have their own module:

@lisp
(use-modules (system vm trap-state))
@end lisp

@deffn {Scheme Procedure} add-trap! trap name
Add a trap to the current trap state, associating the given @var{name}
with it. Returns a fresh trap identifier (an integer).

Note that usually the more specific functions detailed in
@ref{High-Level Traps} are used in preference to this one.
@end deffn

@deffn {Scheme Procedure} list-traps
List the current set of traps, both enabled and disabled. Returns a list
of integers.
@end deffn

@deffn {Scheme Procedure} trap-name idx
Returns the name associated with trap @var{idx}, or @code{#f} if there
is no such trap.
@end deffn

@deffn {Scheme Procedure} trap-enabled? idx
Returns @code{#t} if trap @var{idx} is present and enabled, or @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} enable-trap! idx
Enables trap @var{idx}.
@end deffn

@deffn {Scheme Procedure} disable-trap! idx
Disables trap @var{idx}.
@end deffn

@deffn {Scheme Procedure} delete-trap! idx
Removes trap @var{idx}, disabling it first, if necessary.
@end deffn

@node High-Level Traps
@subsubsection High-Level Traps

The low-level trap API allows one to make traps that call procedures,
and the trap state API allows one to keep track of what traps are
there.  But neither of these APIs directly helps you when you want to
set a breakpoint, because it's unclear what to do when the trap fires.
Do you enter a debugger, or mail a summary of the situation to your
great-aunt, or what?

So for the common case in which you just want to install breakpoints,
and then have them all result in calls to one parameterizable procedure,
we have the high-level trap interface.

Perhaps we should have started this section with this interface, as it's
clearly the one most people should use.  But as its capabilities and
limitations proceed from the lower layers, we felt that the
character-building exercise of building a mental model might be helpful.

These procedures share a module with trap states:

@lisp
(use-modules (system vm trap-state))
@end lisp

@deffn {Scheme Procedure} with-default-trap-handler handler thunk
Call @var{thunk} in a dynamic context in which @var{handler} is the
current trap handler.

Additionally, during the execution of @var{thunk}, the VM trace level
(@pxref{VM Hooks}) is set to the number of enabled traps. This ensures
that traps will in fact fire.

@var{handler} may be @code{#f}, in which case VM hooks are not enabled
as they otherwise would be, as there is nothing to handle the traps.
@end deffn

The trace-level-setting behavior of @code{with-default-trap-handler} is
one of its more useful aspects, but if you are willing to forgo that,
and just want to install a global trap handler, there's a function for
that too:

@deffn {Scheme Procedure} install-trap-handler! handler
Set the current thread's trap handler to @var{handler}.
@end deffn

Trap handlers are called when traps installed by procedures from this
module fire.  The current ``consumer'' of this API is Guile's REPL, but
one might easily imagine other trap handlers being used to integrate
with other debugging tools.

@cindex Breakpoints
@cindex Setting breakpoints
@deffn {Scheme Procedure} add-trap-at-procedure-call! proc
Install a trap that will fire when @var{proc} is called.

This is a breakpoint.
@end deffn

@cindex Tracepoints
@cindex Setting tracepoints
@deffn {Scheme Procedure} add-trace-at-procedure-call! proc
Install a trap that will print a tracing message when @var{proc} is
called. @xref{Tracing Traps}, for more information.

This is a tracepoint.
@end deffn

@deffn {Scheme Procedure} add-trap-at-source-location! file user-line
Install a trap that will fire when control reaches the given source
location. @var{user-line} is one-indexed, as users count lines, instead
of zero-indexed, as Guile counts lines.

This is a source breakpoint.
@end deffn

@deffn {Scheme Procedure} add-ephemeral-trap-at-frame-finish! frame handler
Install a trap that will call @var{handler} when @var{frame} finishes
executing. The trap will be removed from the trap state after firing, or
on nonlocal exit.

This is a finish trap, used to implement the ``finish'' REPL command.
@end deffn

@deffn {Scheme Procedure} add-ephemeral-stepping-trap! frame handler [#:into?] [#:instruction?]
Install a trap that will call @var{handler} after stepping to a
different source line or instruction.  The trap will be removed from the
trap state after firing, or on nonlocal exit.

If @var{instruction?} is false (the default), the trap will fire when
control reaches a new source line. Otherwise it will fire when control
reaches a new instruction.

Additionally, if @var{into?} is false (not the default), the trap will
only fire for frames at or prior to the given frame. If @var{into?} is
true (the default), the trap may step into nested procedure
invocations.

This is a stepping trap, used to implement the ``step'', ``next'',
``step-instruction'', and ``next-instruction'' REPL commands.
@end deffn

@node GDB Support
@subsection GDB Support

@cindex GDB support

Sometimes, you may find it necessary to debug Guile applications at the
C level.  Doing so can be tedious, in particular because the debugger is
oblivious to Guile's @code{SCM} type, and thus unable to display
@code{SCM} values in any meaningful way:

@example
(gdb) frame
#0  scm_display (obj=0xf04310, port=0x6f9f30) at print.c:1437
@end example

To address that, Guile comes with an extension of the GNU Debugger (GDB)
that contains a ``pretty-printer'' for @code{SCM} values.  With this GDB
extension, the C frame in the example above shows up like this:

@example
(gdb) frame
#0  scm_display (obj=("hello" GDB!), port=#<port file 6f9f30>) at print.c:1437
@end example

@noindent
Here GDB was able to decode the list pointed to by @var{obj}, and to
print it using Scheme's read syntax.

That extension is a @code{.scm} file installed alongside the
@file{libguile} shared library.  When GDB 7.8 or later is installed and
compiled with support for extensions written in Guile, the extension is
automatically loaded when debugging a program linked against
@file{libguile} (@pxref{Auto-loading,,, gdb, Debugging with GDB}).  Note
that the directory where @file{libguile} is installed must be among
GDB's auto-loading ``safe directories'' (@pxref{Auto-loading safe
path,,, gdb, Debugging with GDB}).


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