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(A Common Mistake In Allocating Smobs): New section.

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Jim Blandy 1999-06-17 22:52:01 +00:00
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@ -18,7 +18,7 @@
@ifinfo @ifinfo
Data Representation in Guile Data Representation in Guile
Copyright (C) 1998 Free Software Foundation Copyright (C) 1998, 1999 Free Software Foundation
Permission is granted to make and distribute verbatim copies of Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice this manual provided the copyright notice and this permission notice
@ -46,7 +46,7 @@ by the Free Software Foundation.
@sp 10 @sp 10
@comment The title is printed in a large font. @comment The title is printed in a large font.
@title Data Representation in Guile @title Data Representation in Guile
@subtitle $Id: data-rep.texi,v 1.4 1999-04-17 15:16:18 jimb Exp $ @subtitle $Id: data-rep.texi,v 1.5 1999-06-17 22:52:01 jimb Exp $
@subtitle For use with Guile @value{VERSION} @subtitle For use with Guile @value{VERSION}
@author Jim Blandy @author Jim Blandy
@author Free Software Foundation @author Free Software Foundation
@ -77,6 +77,12 @@ by Free Software Foundation.
@headings double @headings double
@menu
* Data Representation in Scheme::
* How Guile does it::
* Defining New Types (Smobs)::
@end menu
@node Top, Data Representation in Scheme, (dir), (dir) @node Top, Data Representation in Scheme, (dir), (dir)
@top Data Representation in Guile @top Data Representation in Guile
@ -1107,6 +1113,7 @@ datatypes described here.)
* Creating Instances:: * Creating Instances::
* Typechecking:: * Typechecking::
* Garbage Collecting Smobs:: * Garbage Collecting Smobs::
* A Common Mistake In Allocating Smobs::
* Garbage Collecting Simple Smobs:: * Garbage Collecting Simple Smobs::
* A Complete Example:: * A Complete Example::
@end menu @end menu
@ -1340,7 +1347,7 @@ collection; see the description of @code{SCM_GCTYP16} in @ref{Garbage
Collecting Smobs}. Collecting Smobs}.
@node Garbage Collecting Smobs, Garbage Collecting Simple Smobs, Typechecking, Defining New Types (Smobs) @node Garbage Collecting Smobs, A Common Mistake In Allocating Smobs, Typechecking, Defining New Types (Smobs)
@subsection Garbage Collecting Smobs @subsection Garbage Collecting Smobs
Once a smob has been released to the tender mercies of the Scheme Once a smob has been released to the tender mercies of the Scheme
@ -1481,7 +1488,79 @@ very simple. Since collections occur at unpredictable times, it is easy
for any unusual activity to interfere with normal code. for any unusual activity to interfere with normal code.
@node Garbage Collecting Simple Smobs, A Complete Example, Garbage Collecting Smobs, Defining New Types (Smobs) @node A Common Mistake In Allocating Smobs, Garbage Collecting Simple Smobs, Garbage Collecting Smobs, Defining New Types (Smobs)
@subsection A Common Mistake In Allocating Smobs
When constructing new objects, you must be careful that the garbage
collector can always find any new objects you allocate. For example,
suppose we wrote the @code{make_image} function this way:
@example
SCM
make_image (SCM name, SCM s_width, SCM s_height)
@{
struct image *image;
SCM image_smob;
int width, height;
SCM_ASSERT (SCM_NIMP (name) && SCM_STRINGP (name), name,
SCM_ARG1, "make-image");
SCM_ASSERT (SCM_INUMP (s_width), s_width, SCM_ARG2, "make-image");
SCM_ASSERT (SCM_INUMP (s_height), s_height, SCM_ARG3, "make-image");
width = SCM_INUM (s_width);
height = SCM_INUM (s_height);
image = (struct image *) scm_must_malloc (sizeof (struct image), "image");
image->width = width;
image->height = height;
image->pixels = scm_must_malloc (width * height, "image pixels");
/* THESE TWO LINES HAVE CHANGED: */
image->name = scm_string_copy (name);
image->update_func = scm_make_gsubr (@dots{});
SCM_NEWCELL (image_smob);
SCM_SETCDR (image_smob, image);
SCM_SETCAR (image_smob, image_tag);
return image_smob;
@}
@end example
This code is incorrect. The calls to @code{scm_string_copy} and
@code{scm_make_gsubr} allocate fresh objects. Allocating any new object
may cause the garbage collector to run. If @code{scm_make_gsubr}
invokes a collection, the garbage collector has no way to discover that
@code{image->name} points to the new string object; the @code{image}
structure is not yet part of any Scheme object, so the garbage collector
will not traverse it. Since the garbage collector cannot find any
references to the new string object, it will free it, leaving
@code{image} pointing to a dead object.
A correct implementation might say, instead:
@example
image->name = SCM_BOOL_F;
image->update_func = SCM_BOOL_F;
SCM_NEWCELL (image_smob);
SCM_SETCDR (image_smob, image);
SCM_SETCAR (image_smob, image_tag);
image->name = scm_string_copy (name);
image->update_func = scm_make_gsubr (@dots{});
return image_smob;
@end example
Now, by the time we allocate the new string and function objects,
@code{image_smob} points to @code{image}. If the garbage collector
scans the stack, it will find a reference to @code{image_smob} and
traverse @code{image}, so any objects @code{image} points to will be
preserved.
@node Garbage Collecting Simple Smobs, A Complete Example, A Common Mistake In Allocating Smobs, Defining New Types (Smobs)
@subsection Garbage Collecting Simple Smobs @subsection Garbage Collecting Simple Smobs
It is often useful to define very simple smob types --- smobs which have It is often useful to define very simple smob types --- smobs which have