code:
@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Programming Overview
@section An Overview of Guile Programming

Guile is designed as an extension language interpreter that is
straightforward to integrate with applications written in C (and C++).
The big win here for the application developer is that Guile
integration, as the Guile web page says, ``lowers your project's
hacktivation energy.''  Lowering the hacktivation energy means that you,
as the application developer, @emph{and your users}, reap the benefits
that flow from being able to extend the application in a high level
extension language rather than in plain old C.

In abstract terms, it's difficult to explain what this really means and
what the integration process involves, so instead let's begin by jumping
straight into an example of how you might integrate Guile into an
existing program, and what you could expect to gain by so doing.  With
that example under our belts, we'll then return to a more general
analysis of the arguments involved and the range of programming options
available.

@menu
* Extending Dia::               How one might extend Dia using Guile.
* Scheme vs C::                 Why Scheme is more hackable than C.
* Testbed Example::             Example: using Guile in a testbed.
* Programming Options::         Options for Guile programming.
* User Programming::            How about application users?
@end menu

@node Extending Dia
@subsection How One Might Extend Dia Using Guile

Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans (@uref{http://www.gnome.org/projects/dia/}).
This section conducts the thought
experiment of adding Guile to Dia.  In so doing, it aims to illustrate
several of the steps and considerations involved in adding Guile to
applications in general.

@menu
* Dia Objective::               Deciding why you want to add Guile.
* Dia Steps::                   Four steps required to add Guile.
* Dia Smobs::                   How to represent Dia data in Scheme.
* Dia Primitives::              Writing Guile primitives for Dia.
* Dia Hook::                    Providing a hook for Scheme evaluation.
* Dia Structure::               Overall structure for adding Guile.
* Dia Advanced::                Going further with Dia and Guile.
@end menu

@node Dia Objective
@subsubsection Deciding Why You Want to Add Guile

First off, you should understand why you want to add Guile to Dia at
all, and that means forming a picture of what Dia does and how it does
it.  So, what are the constituents of the Dia application?

@itemize @bullet
@item
Most importantly, the @dfn{application domain objects} --- in other
words, the concepts that differentiate Dia from another application such
as a word processor or spreadsheet: shapes, templates, connectors,
pages, plus the properties of all these things.

@item
The code that manages the graphical face of the application, including
the layout and display of the objects above.

@item
The code that handles input events, which indicate that the application
user is wanting to do something.
@end itemize

@noindent
(In other words, a textbook example of the @dfn{model - view -
controller} paradigm.)

Next question: how will Dia benefit once the Guile integration is
complete?  Several (positive!) answers are possible here, and the choice
is obviously up to the application developers.  Still, one answer is
that the main benefit will be the ability to manipulate Dia's
application domain objects from Scheme.

Suppose that Dia made a set of procedures available in Scheme,
representing the most basic operations on objects such as shapes,
connectors, and so on.  Using Scheme, the application user could then
write code that builds upon these basic operations to create more
complex procedures.  For example, given basic procedures to enumerate
the objects on a page, to determine whether an object is a square, and
to change the fill pattern of a single shape, the user can write a
Scheme procedure to change the fill pattern of all squares on the
current page:

@lisp
(define (change-squares'-fill-pattern new-pattern)
  (for-each-shape current-page
    (lambda (shape)
      (if (square? shape)
          (change-fill-pattern shape new-pattern)))))
@end lisp

@node Dia Steps
@subsubsection Four Steps Required to Add Guile

Assuming this objective, four steps are needed to achieve it.

First, you need a way of representing your application-specific objects
--- such as @code{shape} in the previous example --- when they are
passed into the Scheme world.  Unless your objects are so simple that
they map naturally into builtin Scheme data types like numbers and
strings, you will probably want to use Guile's @dfn{SMOB} interface to
create a new Scheme data type for your objects.

Second, you need to write code for the basic operations like
@code{for-each-shape} and @code{square?} such that they access and
manipulate your existing data structures correctly, and then make these
operations available as @dfn{primitives} on the Scheme level.

Third, you need to provide some mechanism within the Dia application
that a user can hook into to cause arbitrary Scheme code to be
evaluated.

Finally, you need to restructure your top-level application C code a
little so that it initializes the Guile interpreter correctly and
declares your @dfn{SMOBs} and @dfn{primitives} to the Scheme world.

The following subsections expand on these four points in turn.

@node Dia Smobs
@subsubsection How to Represent Dia Data in Scheme

For all but the most trivial applications, you will probably want to
allow some representation of your domain objects to exist on the Scheme
level.  This is where the idea of SMOBs comes in, and with it issues of
lifetime management and garbage collection.

To get more concrete about this, let's look again at the example we gave
earlier of how application users can use Guile to build higher-level
functions from the primitives that Dia itself provides.

@lisp
(define (change-squares'-fill-pattern new-pattern)
  (for-each-shape current-page
    (lambda (shape)
      (if (square? shape)
          (change-fill-pattern shape new-pattern)))))
@end lisp

Consider what is stored here in the variable @code{shape}.  For each
shape on the current page, the @code{for-each-shape} primitive calls
@code{(lambda (shape) @dots{})} with an argument representing that
shape.  Question is: how is that argument represented on the Scheme
level?  The issues are as follows.

@itemize @bullet
@item
Whatever the representation, it has to be decodable again by the C code
for the @code{square?} and @code{change-fill-pattern} primitives.  In
other words, a primitive like @code{square?} has somehow to be able to
turn the value that it receives back into something that points to the
underlying C structure describing a shape.

@item
The representation must also cope with Scheme code holding on to the
value for later use.  What happens if the Scheme code stores
@code{shape} in a global variable, but then that shape is deleted (in a
way that the Scheme code is not aware of), and later on some other
Scheme code uses that global variable again in a call to, say,
@code{square?}?

@item
The lifetime and memory allocation of objects that exist @emph{only} in
the Scheme world is managed automatically by Guile's garbage collector
using one simple rule: when there are no remaining references to an
object, the object is considered dead and so its memory is freed.  But
for objects that exist in both C and Scheme, the picture is more
complicated; in the case of Dia, where the @code{shape} argument passes
transiently in and out of the Scheme world, it would be quite wrong the
@strong{delete} the underlying C shape just because the Scheme code has
finished evaluation.  How do we avoid this happening?
@end itemize

One resolution of these issues is for the Scheme-level representation of
a shape to be a new, Scheme-specific C structure wrapped up as a SMOB.
The SMOB is what is passed into and out of Scheme code, and the
Scheme-specific C structure inside the SMOB points to Dia's underlying C
structure so that the code for primitives like @code{square?} can get at
it.

To cope with an underlying shape being deleted while Scheme code is
still holding onto a Scheme shape value, the underlying C structure
should have a new field that points to the Scheme-specific SMOB.  When a
shape is deleted, the relevant code chains through to the
Scheme-specific structure and sets its pointer back to the underlying
structure to NULL.  Thus the SMOB value for the shape continues to
exist, but any primitive code that tries to use it will detect that the
underlying shape has been deleted because the underlying structure
pointer is NULL.

So, to summarize the steps involved in this resolution of the problem
(and assuming that the underlying C structure for a shape is
@code{struct dia_shape}):

@itemize @bullet
@item
Define a new Scheme-specific structure that @emph{points} to the
underlying C structure:

@lisp
struct dia_guile_shape
@{
  struct dia_shape * c_shape;   /* NULL => deleted */
@}
@end lisp

@item
Add a field to @code{struct dia_shape} that points to its @code{struct
dia_guile_shape} if it has one ---

@lisp
struct dia_shape
@{
  @dots{}
  struct dia_guile_shape * guile_shape;
@}
@end lisp

@noindent
--- so that C code can set @code{guile_shape->c_shape} to NULL when the
underlying shape is deleted.

@item
Wrap @code{struct dia_guile_shape} as a SMOB type.

@item
Whenever you need to represent a C shape onto the Scheme level, create a
SMOB instance for it, and pass that.

@item
In primitive code that receives a shape SMOB instance, check the
@code{c_shape} field when decoding it, to find out whether the
underlying C shape is still there.
@end itemize

As far as memory management is concerned, the SMOB values and their
Scheme-specific structures are under the control of the garbage
collector, whereas the underlying C structures are explicitly managed in
exactly the same way that Dia managed them before we thought of adding
Guile.

When the garbage collector decides to free a shape SMOB value, it calls
the @dfn{SMOB free} function that was specified when defining the shape
SMOB type.  To maintain the correctness of the @code{guile_shape} field
in the underlying C structure, this function should chain through to the
underlying C structure (if it still exists) and set its
@code{guile_shape} field to NULL.

For full documentation on defining and using SMOB types, see
@ref{Defining New Types (Smobs)}.

@node Dia Primitives
@subsubsection Writing Guile Primitives for Dia

Once the details of object representation are decided, writing the
primitive function code that you need is usually straightforward.

A primitive is simply a C function whose arguments and return value are
all of type @code{SCM}, and whose body does whatever you want it to do.
As an example, here is a possible implementation of the @code{square?}
primitive:

@lisp
#define FUNC_NAME "square?"
static SCM square_p (SCM shape)
@{
  struct dia_guile_shape * guile_shape;

  /* Check that arg is really a shape SMOB. */
  SCM_VALIDATE_SHAPE (SCM_ARG1, shape);

  /* Access Scheme-specific shape structure. */
  guile_shape = SCM_SMOB_DATA (shape);

  /* Find out if underlying shape exists and is a
     square; return answer as a Scheme boolean. */
  return scm_from_bool (guile_shape->c_shape &&
                        (guile_shape->c_shape->type == DIA_SQUARE));
@}
#undef FUNC_NAME
@end lisp

Notice how easy it is to chain through from the @code{SCM shape}
parameter that @code{square_p} receives --- which is a SMOB --- to the
Scheme-specific structure inside the SMOB, and thence to the underlying
C structure for the shape.

In this code, @code{SCM_SMOB_DATA} and @code{scm_from_bool} are from
the standard Guile API.  @code{SCM_VALIDATE_SHAPE} is a macro that you
should define as part of your SMOB definition: it checks that the
passed parameter is of the expected type.  This is needed to guard
against Scheme code using the @code{square?} procedure incorrectly, as
in @code{(square? "hello")}; Scheme's latent typing means that usage
errors like this must be caught at run time.

Having written the C code for your primitives, you need to make them
available as Scheme procedures by calling the @code{scm_c_define_gsubr}
function.  @code{scm_c_define_gsubr} (@pxref{Primitive Procedures}) takes arguments that
specify the Scheme-level name for the primitive and how many required,
optional and rest arguments it can accept.  The @code{square?} primitive
always requires exactly one argument, so the call to make it available
in Scheme reads like this:

@lisp
scm_c_define_gsubr ("square?", 1, 0, 0, square_p);
@end lisp

For where to put this call, see the subsection after next on the
structure of Guile-enabled code (@pxref{Dia Structure}).

@node Dia Hook
@subsubsection Providing a Hook for the Evaluation of Scheme Code

To make the Guile integration useful, you have to design some kind of
hook into your application that application users can use to cause their
Scheme code to be evaluated.

Technically, this is straightforward; you just have to decide on a
mechanism that is appropriate for your application.  Think of Emacs, for
example: when you type @kbd{@key{ESC} :}, you get a prompt where you can
type in any Elisp code, which Emacs will then evaluate.  Or, again like
Emacs, you could provide a mechanism (such as an init file) to allow
Scheme code to be associated with a particular key sequence, and
evaluate the code when that key sequence is entered.

In either case, once you have the Scheme code that you want to evaluate,
as a null terminated string, you can tell Guile to evaluate it by
calling the @code{scm_c_eval_string} function.

@node Dia Structure
@subsubsection Top-level Structure of Guile-enabled Dia

Let's assume that the pre-Guile Dia code looks structurally like this:

@itemize @bullet
@item
@code{main ()}

@itemize @bullet
@item
do lots of initialization and setup stuff
@item
enter Gtk main loop
@end itemize
@end itemize

When you add Guile to a program, one (rather technical) requirement is
that Guile's garbage collector needs to know where the bottom of the C
stack is.  The easiest way to ensure this is to use
@code{scm_boot_guile} like this:

@itemize @bullet
@item
@code{main ()}

@itemize @bullet
@item
do lots of initialization and setup stuff
@item
@code{scm_boot_guile (argc, argv, inner_main, NULL)}
@end itemize

@item
@code{inner_main ()}

@itemize @bullet
@item
define all SMOB types
@item
export primitives to Scheme using @code{scm_c_define_gsubr}
@item
enter Gtk main loop
@end itemize
@end itemize

In other words, you move the guts of what was previously in your
@code{main} function into a new function called @code{inner_main}, and
then add a @code{scm_boot_guile} call, with @code{inner_main} as a
parameter, to the end of @code{main}.

Assuming that you are using SMOBs and have written primitive code as
described in the preceding subsections, you also need to insert calls to
declare your new SMOBs and export the primitives to Scheme.  These
declarations must happen @emph{inside} the dynamic scope of the
@code{scm_boot_guile} call, but also @emph{before} any code is run that
could possibly use them --- the beginning of @code{inner_main} is an
ideal place for this.

@node Dia Advanced
@subsubsection Going Further with Dia and Guile

The steps described so far implement an initial Guile integration that
already gives a lot of additional power to Dia application users.  But
there are further steps that you could take, and it's interesting to
consider a few of these.

In general, you could progressively move more of Dia's source code from
C into Scheme.  This might make the code more maintainable and
extensible, and it could open the door to new programming paradigms that
are tricky to effect in C but straightforward in Scheme.

A specific example of this is that you could use the guile-gtk package,
which provides Scheme-level procedures for most of the Gtk+ library, to
move the code that lays out and displays Dia objects from C to Scheme.

As you follow this path, it naturally becomes less useful to maintain a
distinction between Dia's original non-Guile-related source code, and
its later code implementing SMOBs and primitives for the Scheme world.

For example, suppose that the original source code had a
@code{dia_change_fill_pattern} function:

@lisp
void dia_change_fill_pattern (struct dia_shape * shape,
                              struct dia_pattern * pattern)
@{
  /* real pattern change work */
@}
@end lisp

During initial Guile integration, you add a @code{change_fill_pattern}
primitive for Scheme purposes, which accesses the underlying structures
from its SMOB values and uses @code{dia_change_fill_pattern} to do the
real work:

@lisp
SCM change_fill_pattern (SCM shape, SCM pattern)
@{
  struct dia_shape * d_shape;
  struct dia_pattern * d_pattern;

  @dots{}

  dia_change_fill_pattern (d_shape, d_pattern);

  return SCM_UNSPECIFIED;
@}
@end lisp

At this point, it makes sense to keep @code{dia_change_fill_pattern} and
@code{change_fill_pattern} separate, because
@code{dia_change_fill_pattern} can also be called without going through
Scheme at all, say because the user clicks a button which causes a
C-registered Gtk+ callback to be called.

But, if the code for creating buttons and registering their callbacks is
moved into Scheme (using guile-gtk), it may become true that
@code{dia_change_fill_pattern} can no longer be called other than
through Scheme.  In which case, it makes sense to abolish it and move
its contents directly into @code{change_fill_pattern}, like this:

@lisp
SCM change_fill_pattern (SCM shape, SCM pattern)
@{
  struct dia_shape * d_shape;
  struct dia_pattern * d_pattern;

  @dots{}

  /* real pattern change work */

  return SCM_UNSPECIFIED;
@}
@end lisp

So further Guile integration progressively @emph{reduces} the amount of
functional C code that you have to maintain over the long term.

A similar argument applies to data representation.  In the discussion of
SMOBs earlier, issues arose because of the different memory management
and lifetime models that normally apply to data structures in C and in
Scheme.  However, with further Guile integration, you can resolve this
issue in a more radical way by allowing all your data structures to be
under the control of the garbage collector, and kept alive by references
from the Scheme world.  Instead of maintaining an array or linked list
of shapes in C, you would instead maintain a list in Scheme.

Rather like the coalescing of @code{dia_change_fill_pattern} and
@code{change_fill_pattern}, the practical upshot of such a change is
that you would no longer have to keep the @code{dia_shape} and
@code{dia_guile_shape} structures separate, and so wouldn't need to
worry about the pointers between them.  Instead, you could change the
SMOB definition to wrap the @code{dia_shape} structure directly, and
send @code{dia_guile_shape} off to the scrap yard.  Cut out the middle
man!

Finally, we come to the holy grail of Guile's free software / extension
language approach.  Once you have a Scheme representation for
interesting Dia data types like shapes, and a handy bunch of primitives
for manipulating them, it suddenly becomes clear that you have a bundle
of functionality that could have far-ranging use beyond Dia itself.  In
other words, the data types and primitives could now become a library,
and Dia becomes just one of the many possible applications using that
library --- albeit, at this early stage, a rather important one!

In this model, Guile becomes just the glue that binds everything
together.  Imagine an application that usefully combined functionality
from Dia, Gnumeric and GnuCash --- it's tricky right now, because no
such application yet exists; but it'll happen some day @dots{}

@node Scheme vs C
@subsection Why Scheme is More Hackable Than C

Underlying Guile's value proposition is the assumption that programming
in a high level language, specifically Guile's implementation of Scheme,
is necessarily better in some way than programming in C.  What do we
mean by this claim, and how can we be so sure?

One class of advantages applies not only to Scheme, but more generally
to any interpretable, high level, scripting language, such as Emacs
Lisp, Python, Ruby, or @TeX{}'s macro language.  Common features of all
such languages, when compared to C, are that:

@itemize @bullet
@item
They lend themselves to rapid and experimental development cycles,
owing usually to a combination of their interpretability and the
integrated development environment in which they are used.

@item
They free developers from some of the low level bookkeeping tasks
associated with C programming, notably memory management.

@item
They provide high level features such as container objects and exception
handling that make common programming tasks easier.
@end itemize

In the case of Scheme, particular features that make programming easier
--- and more fun! --- are its powerful mechanisms for abstracting parts
of programs (closures --- @pxref{About Closure}) and for iteration
(@pxref{while do}).

The evidence in support of this argument is empirical: the huge amount
of code that has been written in extension languages for applications
that support this mechanism.  Most notable are extensions written in
Emacs Lisp for GNU Emacs, in @TeX{}'s macro language for @TeX{}, and in
Script-Fu for the Gimp, but there is increasingly now a significant code
eco-system for Guile-based applications as well, such as Lilypond and
GnuCash.  It is close to inconceivable that similar amounts of
functionality could have been added to these applications just by
writing new code in their base implementation languages.

@node Testbed Example
@subsection Example: Using Guile for an Application Testbed

As an example of what this means in practice, imagine writing a testbed
for an application that is tested by submitting various requests (via a
C interface) and validating the output received.  Suppose further that
the application keeps an idea of its current state, and that the
``correct'' output for a given request may depend on the current
application state.  A complete ``white box''@footnote{A @dfn{white box}
test plan is one that incorporates knowledge of the internal design of
the application under test.} test plan for this application would aim to
submit all possible requests in each distinguishable state, and validate
the output for all request/state combinations.

To write all this test code in C would be very tedious.  Suppose instead
that the testbed code adds a single new C function, to submit an
arbitrary request and return the response, and then uses Guile to export
this function as a Scheme procedure.  The rest of the testbed can then
be written in Scheme, and so benefits from all the advantages of
programming in Scheme that were described in the previous section.

(In this particular example, there is an additional benefit of writing
most of the testbed in Scheme.  A common problem for white box testing
is that mistakes and mistaken assumptions in the application under test
can easily be reproduced in the testbed code.  It is more difficult to
copy mistakes like this when the testbed is written in a different
language from the application.)

@node Programming Options
@subsection A Choice of Programming Options

The preceding arguments and example point to a model of Guile
programming that is applicable in many cases.  According to this model,
Guile programming involves a balance between C and Scheme programming,
with the aim being to extract the greatest possible Scheme level benefit
from the least amount of C level work.

The C level work required in this model usually consists of packaging
and exporting functions and application objects such that they can be
seen and manipulated on the Scheme level.  To help with this, Guile's C
language interface includes utility features that aim to make this kind
of integration very easy for the application developer.  These features
are documented later in this part of the manual: see REFFIXME.

This model, though, is really just one of a range of possible
programming options.  If all of the functionality that you need is
available from Scheme, you could choose instead to write your whole
application in Scheme (or one of the other high level languages that
Guile supports through translation), and simply use Guile as an
interpreter for Scheme.  (In the future, we hope that Guile will also be
able to compile Scheme code, so lessening the performance gap between C
and Scheme code.)  Or, at the other end of the C--Scheme scale, you
could write the majority of your application in C, and only call out to
Guile occasionally for specific actions such as reading a configuration
file or executing a user-specified extension.  The choices boil down to
two basic questions:

@itemize @bullet
@item
Which parts of the application do you write in C, and which in Scheme
(or another high level translated language)?

@item
How do you design the interface between the C and Scheme parts of your
application?
@end itemize

These are of course design questions, and the right design for any given
application will always depend upon the particular requirements that you
are trying to meet.  In the context of Guile, however, there are some
generally applicable considerations that can help you when designing
your answers.

@menu
* Available Functionality::     What functionality is already available?
* Basic Constraints::           Functional and performance constraints.
* Style Choices::               Your preferred programming style.
* Program Control::             What controls program execution?
@end menu

@node Available Functionality
@subsubsection What Functionality is Already Available?

Suppose, for the sake of argument, that you would prefer to write your
whole application in Scheme.  Then the API available to you consists of:

@itemize @bullet
@item
standard Scheme

@item
plus the extensions to standard Scheme provided by
Guile in its core distribution

@item
plus any additional functionality that you or others have packaged so
that it can be loaded as a Guile Scheme module.
@end itemize

A module in the last category can either be a pure Scheme module --- in
other words a collection of utility procedures coded in Scheme --- or a
module that provides a Scheme interface to an extension library coded in
C --- in other words a nice package where someone else has done the work
of wrapping up some useful C code for you.  The set of available modules
is growing quickly and already includes such useful examples as
@code{(gtk gtk)}, which makes Gtk+ drawing functions available in
Scheme, and @code{(database postgres)}, which provides SQL access to a
Postgres database.

Given the growing collection of pre-existing modules, it is quite
feasible that your application could be implemented by combining a
selection of these modules together with new application code written in
Scheme.

If this approach is not enough, because the functionality that your
application needs is not already available in this form, and it is
impossible to write the new functionality in Scheme, you will need to
write some C code.  If the required function is already available in C
(e.g. in a library), all you need is a little glue to connect it to the
world of Guile.  If not, you need both to write the basic code and to
plumb it into Guile.

In either case, two general considerations are important.  Firstly, what
is the interface by which the functionality is presented to the Scheme
world?  Does the interface consist only of function calls (for example,
a simple drawing interface), or does it need to include @dfn{objects} of
some kind that can be passed between C and Scheme and manipulated by
both worlds.  Secondly, how does the lifetime and memory management of
objects in the C code relate to the garbage collection governed approach
of Scheme objects?  In the case where the basic C code is not already
written, most of the difficulties of memory management can be avoided by
using Guile's C interface features from the start.

For the full documentation on writing C code for Guile and connecting
existing C code to the Guile world, see REFFIXME.

@node Basic Constraints
@subsubsection Functional and Performance Constraints

@node Style Choices
@subsubsection Your Preferred Programming Style

@node Program Control
@subsubsection What Controls Program Execution?

@node User Programming
@subsection How About Application Users?

So far we have considered what Guile programming means for an
application developer.  But what if you are instead @emph{using} an
existing Guile-based application, and want to know what your
options are for programming and extending this application?

The answer to this question varies from one application to another,
because the options available depend inevitably on whether the
application developer has provided any hooks for you to hang your own
code on and, if there are such hooks, what they allow you to
do.@footnote{Of course, in the world of free software, you always have
the freedom to modify the application's source code to your own
requirements.  Here we are concerned with the extension options that the
application has provided for without your needing to modify its source
code.}  For example@dots{}

@itemize @bullet
@item
If the application permits you to load and execute any Guile code, the
world is your oyster.  You can extend the application in any way that
you choose.

@item
A more cautious application might allow you to load and execute Guile
code, but only in a @dfn{safe} environment, where the interface
available is restricted by the application from the standard Guile API.

@item
Or a really fearful application might not provide a hook to really
execute user code at all, but just use Scheme syntax as a convenient way
for users to specify application data or configuration options.
@end itemize

In the last two cases, what you can do is, by definition, restricted by
the application, and you should refer to the application's own manual to
find out your options.

The most well known example of the first case is Emacs, with its
extension language Emacs Lisp: as well as being a text editor, Emacs
supports the loading and execution of arbitrary Emacs Lisp code.  The
result of such openness has been dramatic: Emacs now benefits from
user-contributed Emacs Lisp libraries that extend the basic editing
function to do everything from reading news to psychoanalysis and
playing adventure games.  The only limitation is that extensions are
restricted to the functionality provided by Emacs's built-in set of
primitive operations.  For example, you can interact and display data by
manipulating the contents of an Emacs buffer, but you can't pop-up and
draw a window with a layout that is totally different to the Emacs
standard.

This situation with a Guile application that supports the loading of
arbitrary user code is similar, except perhaps even more so, because
Guile also supports the loading of extension libraries written in C.
This last point enables user code to add new primitive operations to
Guile, and so to bypass the limitation present in Emacs Lisp.

At this point, the distinction between an application developer and an
application user becomes rather blurred.  Instead of seeing yourself as
a user extending an application, you could equally well say that you are
developing a new application of your own using some of the primitive
functionality provided by the original application.  As such, all the
discussions of the preceding sections of this chapter are relevant to
how you can proceed with developing your extension.

@c Local Variables:
@c TeX-master: "guile.texi"
@c End:


code:
@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2010
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Defining New Types (Smobs)
@section Defining New Types (Smobs)

@dfn{Smobs} are Guile's mechanism for adding new primitive types to
the system.  The term ``smob'' was coined by Aubrey Jaffer, who says
it comes from ``small object'', referring to the fact that they are
quite limited in size: they can hold just one pointer to a larger
memory block plus 16 extra bits.

To define a new smob type, the programmer provides Guile with some
essential information about the type --- how to print it, how to
garbage collect it, and so on --- and Guile allocates a fresh type tag
for it.  The programmer can then use @code{scm_c_define_gsubr} to make
a set of C functions visible to Scheme code that create and operate on
these objects.

(You can find a complete version of the example code used in this
section in the Guile distribution, in @file{doc/example-smob}.  That
directory includes a makefile and a suitable @code{main} function, so
you can build a complete interactive Guile shell, extended with the
datatypes described here.)

@menu
* Describing a New Type::
* Creating Smob Instances::
* Type checking::
* Garbage Collecting Smobs::
* Garbage Collecting Simple Smobs::
* Remembering During Operations::
* Double Smobs::
* The Complete Example::
@end menu

@node Describing a New Type
@subsection Describing a New Type

To define a new type, the programmer must write four functions to
manage instances of the type:

@table @code
@item mark
Guile will apply this function to each instance of the new type it
encounters during garbage collection.  This function is responsible for
telling the collector about any other @code{SCM} values that the object
has stored.  The default smob mark function does nothing.
@xref{Garbage Collecting Smobs}, for more details.

@item free
Guile will apply this function to each instance of the new type that is
to be deallocated.  The function should release all resources held by
the object.  This is analogous to the Java finalization method-- it is
invoked at an unspecified time (when garbage collection occurs) after
the object is dead.  The default free function frees the smob data (if
the size of the struct passed to @code{scm_make_smob_type} is non-zero)
using @code{scm_gc_free}.  @xref{Garbage Collecting Smobs}, for more
details.

This function operates while the heap is in an inconsistent state and
must therefore be careful.  @xref{Smobs}, for details about what this
function is allowed to do.

@item print
Guile will apply this function to each instance of the new type to print
the value, as for @code{display} or @code{write}.  The default print
function prints @code{#<NAME ADDRESS>} where @code{NAME} is the first
argument passed to @code{scm_make_smob_type}.

@item equalp
If Scheme code asks the @code{equal?} function to compare two instances
of the same smob type, Guile calls this function.  It should return
@code{SCM_BOOL_T} if @var{a} and @var{b} should be considered
@code{equal?}, or @code{SCM_BOOL_F} otherwise.  If @code{equalp} is
@code{NULL}, @code{equal?} will assume that two instances of this type are
never @code{equal?} unless they are @code{eq?}.

@end table

To actually register the new smob type, call @code{scm_make_smob_type}.
It returns a value of type @code{scm_t_bits} which identifies the new
smob type.

The four special functions described above are registered by calling
one of @code{scm_set_smob_mark}, @code{scm_set_smob_free},
@code{scm_set_smob_print}, or @code{scm_set_smob_equalp}, as
appropriate.  Each function is intended to be used at most once per
type, and the call should be placed immediately following the call to
@code{scm_make_smob_type}.

There can only be at most 256 different smob types in the system.
Instead of registering a huge number of smob types (for example, one
for each relevant C struct in your application), it is sometimes
better to register just one and implement a second layer of type
dispatching on top of it.  This second layer might use the 16 extra
bits to extend its type, for example.

Here is how one might declare and register a new type representing
eight-bit gray-scale images:

@example
#include <libguile.h>

struct image @{
  int width, height;
  char *pixels;

  /* The name of this image */
  SCM name;

  /* A function to call when this image is
     modified, e.g., to update the screen,
     or SCM_BOOL_F if no action necessary */
  SCM update_func;
@};

static scm_t_bits image_tag;

void
init_image_type (void)
@{
  image_tag = scm_make_smob_type ("image", sizeof (struct image));
  scm_set_smob_mark (image_tag, mark_image);
  scm_set_smob_free (image_tag, free_image);
  scm_set_smob_print (image_tag, print_image);
@}
@end example

@node Creating Smob Instances
@subsection Creating Smob Instances

Normally, smobs can have one @emph{immediate} word of data.  This word
stores either a pointer to an additional memory block that holds the
real data, or it might hold the data itself when it fits.  The word is
large enough for a @code{SCM} value, a pointer to @code{void}, or an
integer that fits into a @code{size_t} or @code{ssize_t}.

You can also create smobs that have two or three immediate words, and
when these words suffice to store all data, it is more efficient to use
these super-sized smobs instead of using a normal smob plus a memory
block.  @xref{Double Smobs}, for their discussion.

Guile provides functions for managing memory which are often helpful
when implementing smobs.  @xref{Memory Blocks}.

To retrieve the immediate word of a smob, you use the macro
@code{SCM_SMOB_DATA}.  It can be set with @code{SCM_SET_SMOB_DATA}.
The 16 extra bits can be accessed with @code{SCM_SMOB_FLAGS} and
@code{SCM_SET_SMOB_FLAGS}.

The two macros @code{SCM_SMOB_DATA} and @code{SCM_SET_SMOB_DATA} treat
the immediate word as if it were of type @code{scm_t_bits}, which is
an unsigned integer type large enough to hold a pointer to
@code{void}.  Thus you can use these macros to store arbitrary
pointers in the smob word.

When you want to store a @code{SCM} value directly in the immediate
word of a smob, you should use the macros @code{SCM_SMOB_OBJECT} and
@code{SCM_SET_SMOB_OBJECT} to access it.

Creating a smob instance can be tricky when it consists of multiple
steps that allocate resources and might fail.  It is recommended that
you go about creating a smob in the following way:

@itemize
@item
Allocate the memory block for holding the data with
@code{scm_gc_malloc}.
@item
Initialize it to a valid state without calling any functions that might
cause a non-local exits.  For example, initialize pointers to NULL.
Also, do not store @code{SCM} values in it that must be protected.
Initialize these fields with @code{SCM_BOOL_F}.

A valid state is one that can be safely acted upon by the @emph{mark}
and @emph{free} functions of your smob type.
@item
Create the smob using @code{SCM_NEWSMOB}, passing it the initialized
memory block.  (This step will always succeed.)
@item
Complete the initialization of the memory block by, for example,
allocating additional resources and making it point to them.
@end itemize

This procedure ensures that the smob is in a valid state as soon as it
exists, that all resources that are allocated for the smob are
properly associated with it so that they can be properly freed, and
that no @code{SCM} values that need to be protected are stored in it
while the smob does not yet completely exist and thus can not protect
them.

Continuing the example from above, if the global variable
@code{image_tag} contains a tag returned by @code{scm_make_smob_type},
here is how we could construct a smob whose immediate word contains a
pointer to a freshly allocated @code{struct image}:

@example
SCM
make_image (SCM name, SCM s_width, SCM s_height)
@{
  SCM smob;
  struct image *image;
  int width = scm_to_int (s_width);
  int height = scm_to_int (s_height);

  /* Step 1: Allocate the memory block.
   */
  image = (struct image *)
     scm_gc_malloc (sizeof (struct image), "image");

  /* Step 2: Initialize it with straight code.
   */
  image->width = width;
  image->height = height;
  image->pixels = NULL;
  image->name = SCM_BOOL_F;
  image->update_func = SCM_BOOL_F;

  /* Step 3: Create the smob.
   */
  SCM_NEWSMOB (smob, image_tag, image);

  /* Step 4: Finish the initialization.
   */
  image->name = name;
  image->pixels =
     scm_gc_malloc (width * height, "image pixels");

  return smob;
@}
@end example

Let us look at what might happen when @code{make_image} is called.

The conversions of @var{s_width} and @var{s_height} to @code{int}s might
fail and signal an error, thus causing a non-local exit.  This is not a
problem since no resources have been allocated yet that would have to be
freed.

The allocation of @var{image} in step 1 might fail, but this is likewise
no problem.

Step 2 can not exit non-locally.  At the end of it, the @var{image}
struct is in a valid state for the @code{mark_image} and
@code{free_image} functions (see below).

Step 3 can not exit non-locally either.  This is guaranteed by Guile.
After it, @var{smob} contains a valid smob that is properly initialized
and protected, and in turn can properly protect the Scheme values in its
@var{image} struct.

But before the smob is completely created, @code{SCM_NEWSMOB} might
cause the garbage collector to run.  During this garbage collection, the
@code{SCM} values in the @var{image} struct would be invisible to Guile.
It only gets to know about them via the @code{mark_image} function, but
that function can not yet do its job since the smob has not been created
yet.  Thus, it is important to not store @code{SCM} values in the
@var{image} struct until after the smob has been created.

Step 4, finally, might fail and cause a non-local exit.  In that case,
the complete creation of the smob has not been successful, but it does
nevertheless exist in a valid state.  It will eventually be freed by
the garbage collector, and all the resources that have been allocated
for it will be correctly freed by @code{free_image}.

@node Type checking
@subsection Type checking

Functions that operate on smobs should check that the passed
@code{SCM} value indeed is a suitable smob before accessing its data.
They can do this with @code{scm_assert_smob_type}.

For example, here is a simple function that operates on an image smob,
and checks the type of its argument.

@example
SCM
clear_image (SCM image_smob)
@{
  int area;
  struct image *image;

  scm_assert_smob_type (image_tag, image_smob);

  image = (struct image *) SCM_SMOB_DATA (image_smob);
  area = image->width * image->height;
  memset (image->pixels, 0, area);

  /* Invoke the image's update function.
   */
  if (scm_is_true (image->update_func))
    scm_call_0 (image->update_func);

  scm_remember_upto_here_1 (image_smob);

  return SCM_UNSPECIFIED;
@}
@end example

See @ref{Remembering During Operations} for an explanation of the call
to @code{scm_remember_upto_here_1}.

@node Garbage Collecting Smobs
@subsection Garbage Collecting Smobs

Once a smob has been released to the tender mercies of the Scheme
system, it must be prepared to survive garbage collection.  Guile calls
the @emph{mark} and @emph{free} functions of the smob to manage this.

As described in more detail elsewhere (@pxref{Conservative GC}), every
object in the Scheme system has a @dfn{mark bit}, which the garbage
collector uses to tell live objects from dead ones.  When collection
starts, every object's mark bit is clear.  The collector traces pointers
through the heap, starting from objects known to be live, and sets the
mark bit on each object it encounters.  When it can find no more
unmarked objects, the collector walks all objects, live and dead, frees
those whose mark bits are still clear, and clears the mark bit on the
others.

The two main portions of the collection are called the @dfn{mark phase},
during which the collector marks live objects, and the @dfn{sweep
phase}, during which the collector frees all unmarked objects.

The mark bit of a smob lives in a special memory region.  When the
collector encounters a smob, it sets the smob's mark bit, and uses the
smob's type tag to find the appropriate @emph{mark} function for that
smob.  It then calls this @emph{mark} function, passing it the smob as
its only argument.

The @emph{mark} function is responsible for marking any other Scheme
objects the smob refers to.  If it does not do so, the objects' mark
bits will still be clear when the collector begins to sweep, and the
collector will free them.  If this occurs, it will probably break, or at
least confuse, any code operating on the smob; the smob's @code{SCM}
values will have become dangling references.

To mark an arbitrary Scheme object, the @emph{mark} function calls
@code{scm_gc_mark}.

Thus, here is how we might write @code{mark_image}:

@example
@group
SCM
mark_image (SCM image_smob)
@{
  /* Mark the image's name and update function.  */
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_gc_mark (image->name);
  scm_gc_mark (image->update_func);

  return SCM_BOOL_F;
@}
@end group
@end example

Note that, even though the image's @code{update_func} could be an
arbitrarily complex structure (representing a procedure and any values
enclosed in its environment), @code{scm_gc_mark} will recurse as
necessary to mark all its components.  Because @code{scm_gc_mark} sets
an object's mark bit before it recurses, it is not confused by
circular structures.

As an optimization, the collector will mark whatever value is returned
by the @emph{mark} function; this helps limit depth of recursion during
the mark phase.  Thus, the code above should really be written as:
@example
@group
SCM
mark_image (SCM image_smob)
@{
  /* Mark the image's name and update function.  */
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_gc_mark (image->name);
  return image->update_func;
@}
@end group
@end example

Finally, when the collector encounters an unmarked smob during the sweep
phase, it uses the smob's tag to find the appropriate @emph{free}
function for the smob.  It then calls that function, passing it the smob
as its only argument.

The @emph{free} function must release any resources used by the smob.
However, it must not free objects managed by the collector; the
collector will take care of them.  For historical reasons, the return
type of the @emph{free} function should be @code{size_t}, an unsigned
integral type; the @emph{free} function should always return zero.

Here is how we might write the @code{free_image} function for the image
smob type:
@example
size_t
free_image (SCM image_smob)
@{
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_gc_free (image->pixels,
               image->width * image->height,
               "image pixels");
  scm_gc_free (image, sizeof (struct image), "image");

  return 0;
@}
@end example

During the sweep phase, the garbage collector will clear the mark bits
on all live objects.  The code which implements a smob need not do this
itself.

There is no way for smob code to be notified when collection is
complete.

It is usually a good idea to minimize the amount of processing done
during garbage collection; keep the @emph{mark} and @emph{free}
functions very simple.  Since collections occur at unpredictable times,
it is easy for any unusual activity to interfere with normal code.

@node Garbage Collecting Simple Smobs
@subsection Garbage Collecting Simple Smobs

It is often useful to define very simple smob types --- smobs which have
no data to mark, other than the cell itself, or smobs whose immediate
data word is simply an ordinary Scheme object, to be marked recursively.
Guile provides some functions to handle these common cases; you can use
this function as your smob type's @emph{mark} function, if your smob's
structure is simple enough.

If the smob refers to no other Scheme objects, then no action is
necessary; the garbage collector has already marked the smob cell
itself.  In that case, you can use zero as your mark function.

If the smob refers to exactly one other Scheme object via its first
immediate word, you can use @code{scm_markcdr} as its mark function.
Its definition is simply:

@smallexample
SCM
scm_markcdr (SCM obj)
@{
  return SCM_SMOB_OBJECT (obj);
@}
@end smallexample

@node Remembering During Operations
@subsection Remembering During Operations
@cindex remembering

It's important that a smob is visible to the garbage collector
whenever its contents are being accessed.  Otherwise it could be freed
while code is still using it.

For example, consider a procedure to convert image data to a list of
pixel values.

@example
SCM
image_to_list (SCM image_smob)
@{
  struct image *image;
  SCM lst;
  int i;

  scm_assert_smob_type (image_tag, image_smob);

  image = (struct image *) SCM_SMOB_DATA (image_smob);
  lst = SCM_EOL;
  for (i = image->width * image->height - 1; i >= 0; i--)
    lst = scm_cons (scm_from_char (image->pixels[i]), lst);

  scm_remember_upto_here_1 (image_smob);
  return lst;
@}
@end example

In the loop, only the @code{image} pointer is used and the C compiler
has no reason to keep the @code{image_smob} value anywhere.  If
@code{scm_cons} results in a garbage collection, @code{image_smob} might
not be on the stack or anywhere else and could be freed, leaving the
loop accessing freed data.  The use of @code{scm_remember_upto_here_1}
prevents this, by creating a reference to @code{image_smob} after all
data accesses.

There's no need to do the same for @code{lst}, since that's the return
value and the compiler will certainly keep it in a register or
somewhere throughout the routine.

The @code{clear_image} example previously shown (@pxref{Type checking})
also used @code{scm_remember_upto_here_1} for this reason.

It's only in quite rare circumstances that a missing
@code{scm_remember_upto_here_1} will bite, but when it happens the
consequences are serious.  Fortunately the rule is simple: whenever
calling a Guile library function or doing something that might, ensure
that the @code{SCM} of a smob is referenced past all accesses to its
insides.  Do this by adding an @code{scm_remember_upto_here_1} if
there are no other references.

In a multi-threaded program, the rule is the same.  As far as a given
thread is concerned, a garbage collection still only occurs within a
Guile library function, not at an arbitrary time.  (Guile waits for all
threads to reach one of its library functions, and holds them there
while the collector runs.)

@node Double Smobs
@subsection Double Smobs

Smobs are called smob because they are small: they normally have only
room for one @code{void*} or @code{SCM} value plus 16 bits.  The
reason for this is that smobs are directly implemented by using the
low-level, two-word cells of Guile that are also used to implement
pairs, for example.  (@pxref{Data Representation} for the
details.)  One word of the two-word cells is used for
@code{SCM_SMOB_DATA} (or @code{SCM_SMOB_OBJECT}), the other contains
the 16-bit type tag and the 16 extra bits.

In addition to the fundamental two-word cells, Guile also has
four-word cells, which are appropriately called @dfn{double cells}.
You can use them for @dfn{double smobs} and get two more immediate
words of type @code{scm_t_bits}.

A double smob is created with @code{SCM_NEWSMOB2} or
@code{SCM_NEWSMOB3} instead of @code{SCM_NEWSMOB}.  Its immediate
words can be retrieved as @code{scm_t_bits} with
@code{SCM_SMOB_DATA_2} and @code{SCM_SMOB_DATA_3} in addition to
@code{SCM_SMOB_DATA}.  Unsurprisingly, the words can be set to
@code{scm_t_bits} values with @code{SCM_SET_SMOB_DATA_2} and
@code{SCM_SET_SMOB_DATA_3}.

Of course there are also @code{SCM_SMOB_OBJECT_2},
@code{SCM_SMOB_OBJECT_3}, @code{SCM_SET_SMOB_OBJECT_2}, and
@code{SCM_SET_SMOB_OBJECT_3}.

@node The Complete Example
@subsection The Complete Example

Here is the complete text of the implementation of the image datatype,
as presented in the sections above.  We also provide a definition for
the smob's @emph{print} function, and make some objects and functions
static, to clarify exactly what the surrounding code is using.

As mentioned above, you can find this code in the Guile distribution, in
@file{doc/example-smob}.  That directory includes a makefile and a
suitable @code{main} function, so you can build a complete interactive
Guile shell, extended with the datatypes described here.)

@example
/* file "image-type.c" */

#include <stdlib.h>
#include <libguile.h>

static scm_t_bits image_tag;

struct image @{
  int width, height;
  char *pixels;

  /* The name of this image */
  SCM name;

  /* A function to call when this image is
     modified, e.g., to update the screen,
     or SCM_BOOL_F if no action necessary */
  SCM update_func;
@};

static SCM
make_image (SCM name, SCM s_width, SCM s_height)
@{
  SCM smob;
  struct image *image;
  int width = scm_to_int (s_width);
  int height = scm_to_int (s_height);

  /* Step 1: Allocate the memory block.
   */
  image = (struct image *)
     scm_gc_malloc (sizeof (struct image), "image");

  /* Step 2: Initialize it with straight code.
   */
  image->width = width;
  image->height = height;
  image->pixels = NULL;
  image->name = SCM_BOOL_F;
  image->update_func = SCM_BOOL_F;

  /* Step 3: Create the smob.
   */
  SCM_NEWSMOB (smob, image_tag, image);

  /* Step 4: Finish the initialization.
   */
  image->name = name;
  image->pixels =
     scm_gc_malloc (width * height, "image pixels");

  return smob;
@}

SCM
clear_image (SCM image_smob)
@{
  int area;
  struct image *image;

  scm_assert_smob_type (image_tag, image_smob);

  image = (struct image *) SCM_SMOB_DATA (image_smob);
  area = image->width * image->height;
  memset (image->pixels, 0, area);

  /* Invoke the image's update function.
   */
  if (scm_is_true (image->update_func))
    scm_call_0 (image->update_func);

  scm_remember_upto_here_1 (image_smob);

  return SCM_UNSPECIFIED;
@}

static SCM
mark_image (SCM image_smob)
@{
  /* Mark the image's name and update function.  */
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_gc_mark (image->name);
  return image->update_func;
@}

static size_t
free_image (SCM image_smob)
@{
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_gc_free (image->pixels,
               image->width * image->height,
               "image pixels");
  scm_gc_free (image, sizeof (struct image), "image");

  return 0;
@}

static int
print_image (SCM image_smob, SCM port, scm_print_state *pstate)
@{
  struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);

  scm_puts ("#<image ", port);
  scm_display (image->name, port);
  scm_puts (">", port);

  /* non-zero means success */
  return 1;
@}

void
init_image_type (void)
@{
  image_tag = scm_make_smob_type ("image", sizeof (struct image));
  scm_set_smob_mark (image_tag, mark_image);
  scm_set_smob_free (image_tag, free_image);
  scm_set_smob_print (image_tag, print_image);

  scm_c_define_gsubr ("clear-image", 1, 0, 0, clear_image);
  scm_c_define_gsubr ("make-image", 3, 0, 0, make_image);
@}
@end example

Here is a sample build and interaction with the code from the
@file{example-smob} directory, on the author's machine:

@example
zwingli:example-smob$ make CC=gcc
gcc `guile-config compile`   -c image-type.c -o image-type.o
gcc `guile-config compile`   -c myguile.c -o myguile.o
gcc image-type.o myguile.o `guile-config link` -o myguile
zwingli:example-smob$ ./myguile
guile> make-image
#<primitive-procedure make-image>
guile> (define i (make-image "Whistler's Mother" 100 100))
guile> i
#<image Whistler's Mother>
guile> (clear-image i)
guile> (clear-image 4)
ERROR: In procedure clear-image in expression (clear-image 4):
ERROR: Wrong type (expecting image): 4
ABORT: (wrong-type-arg)

Type "(backtrace)" to get more information.
guile>
@end example

license: mit


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

@node Function Snarfing
@section Function Snarfing

When writing C code for use with Guile, you typically define a set of
C functions, and then make some of them visible to the Scheme world by
calling @code{scm_c_define_gsubr} or related functions.  If you have
many functions to publish, it can sometimes be annoying to keep the
list of calls to @code{scm_c_define_gsubr} in sync with the list of
function definitions.

Guile provides the @code{guile-snarf} program to manage this problem.
Using this tool, you can keep all the information needed to define the
function alongside the function definition itself; @code{guile-snarf}
will extract this information from your source code, and automatically
generate a file of calls to @code{scm_c_define_gsubr} which you can
@code{#include} into an initialization function.

The snarfing mechanism works for many kind of initialization actions,
not just for collecting calls to @code{scm_c_define_gsubr}.  For a
full list of what can be done, @xref{Snarfing Macros}.

@cindex guile-snarf invocation
@cindex guile-snarf example

The @code{guile-snarf} program is invoked like this:

@smallexample
guile-snarf [-o @var{outfile}] [@var{cpp-args} ...]
@end smallexample

This command will extract initialization actions to @var{outfile}.
When no @var{outfile} has been specified or when @var{outfile} is
@code{-}, standard output will be used.  The C preprocessor is called
with @var{cpp-args} (which usually include an input file) and the
output is filtered to extract the initialization actions.

If there are errors during processing, @var{outfile} is deleted and the
program exits with non-zero status.

During snarfing, the pre-processor macro @code{SCM_MAGIC_SNARFER} is
defined.  You could use this to avoid including snarfer output files
that don't yet exist by writing code like this:

@smallexample
#ifndef SCM_MAGIC_SNARFER
#include "foo.x"
#endif
@end smallexample

Here is how you might define the Scheme function @code{clear-image},
implemented by the C function @code{clear_image}:

@example
@group
#include <libguile.h>

SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
            (SCM image_smob),
            "Clear the image.")
@{
  /* C code to clear the image in @code{image_smob}... */
@}

void
init_image_type ()
@{
#include "image-type.x"
@}
@end group
@end example

The @code{SCM_DEFINE} declaration says that the C function
@code{clear_image} implements a Scheme function called
@code{clear-image}, which takes one required argument (of type
@code{SCM} and named @code{image_smob}), no optional arguments, and no
rest argument.  The string @code{"Clear the image."} provides a short
help text for the function, it is called a @dfn{docstring}.

For historical reasons, the @code{SCM_DEFINE} macro also defines a
static array of characters named @code{s_clear_image}, initialized to
the string "clear-image".  You shouldn't use this array, but you might
need to be aware that it exists.

Assuming the text above lives in a file named @file{image-type.c}, you
will need to execute the following command to prepare this file for
compilation:

@example
guile-snarf -o image-type.x image-type.c
@end example

This scans @file{image-type.c} for @code{SCM_DEFINE}
declarations, and writes to @file{image-type.x} the output:

@example
scm_c_define_gsubr ("clear-image", 1, 0, 0, (SCM (*)() ) clear_image);
@end example

When compiled normally, @code{SCM_DEFINE} is a macro which expands to
the function header for @code{clear_image}.

Note that the output file name matches the @code{#include} from the
input file.  Also, you still need to provide all the same information
you would if you were using @code{scm_c_define_gsubr} yourself, but you
can place the information near the function definition itself, so it is
less likely to become incorrect or out-of-date.

If you have many files that @code{guile-snarf} must process, you should
consider using a fragment like the following in your Makefile:

@example
snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
.SUFFIXES: .x
.c.x:
	guile-snarf -o $@@ $< $(snarfcppopts)
@end example

This tells make to run @code{guile-snarf} to produce each needed
@file{.x} file from the corresponding @file{.c} file.

The program @code{guile-snarf} passes its command-line arguments
directly to the C preprocessor, which it uses to extract the
information it needs from the source code. this means you can pass
normal compilation flags to @code{guile-snarf} to define preprocessor
symbols, add header file directories, and so on.


code:
;; make-texinfo.scm -- document a set of scheme modules as texinfo
;; Copyright (C) 2006,2007,2009  Andy Wingo <wingo at pobox dot com>

;; This program is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.
;;
;; This program is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License
;; along with this program.  If not, see <http://www.gnu.org/licenses/>.

(use-modules (texinfo reflection)
             (texinfo serialize))

(define (main config-scm)
  (load config-scm)
  (display
   (stexi->texi
    (package-stexi-documentation-for-include
     (map car *modules*)
     (map cdr *modules*)))))

(apply main (cdr (command-line)))