guile/doc/ref/libguile-smobs.texi

@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 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 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}.  For more information on
printing, see @ref{Port Data}.

@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 Instances
@subsection Creating 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 competely 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