guile/doc/ref/api-memory.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000-2004, 2009, 2010, 2012-2016
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Memory Management
@section Memory Management and Garbage Collection

Guile uses a @emph{garbage collector} to manage most of its objects.
While the garbage collector is designed to be mostly invisible, you 
sometimes need to interact with it explicitly.

See @ref{Garbage Collection} for a general discussion of how garbage
collection relates to using Guile from C.

@menu
* Garbage Collection Functions::
* Memory Blocks::
* Weak References::
* Guardians::
@end menu


@node Garbage Collection Functions
@subsection Function related to Garbage Collection

@deffn {Scheme Procedure} gc
@deffnx {C Function} scm_gc ()
Finds all of the ``live'' @code{SCM} objects and reclaims for further
use those that are no longer accessible.  You normally don't need to
call this function explicitly.  Its functionality is invoked
automatically as needed.
@end deffn

@deftypefn {C Function} SCM scm_gc_protect_object (SCM @var{obj})
Protects @var{obj} from being freed by the garbage collector, when it
otherwise might be.  When you are done with the object, call
@code{scm_gc_unprotect_object} on the object. Calls to
@code{scm_gc_protect_object}/@code{scm_gc_unprotect_object} can be nested, and
the object remains protected until it has been unprotected as many times
as it was protected. It is an error to unprotect an object more times
than it has been protected. Returns the SCM object it was passed.

Note that storing @var{obj} in a C global variable has the same
effect@footnote{In Guile up to version 1.8, C global variables were not
visited by the garbage collector in the mark phase; hence,
@code{scm_gc_protect_object} was the only way in C to prevent a Scheme
object from being freed.}.
@end deftypefn

@deftypefn {C Function} SCM scm_gc_unprotect_object (SCM @var{obj})

Unprotects an object from the garbage collector which was protected by
@code{scm_gc_unprotect_object}. Returns the SCM object it was passed.
@end deftypefn

@deftypefn {C Function} SCM scm_permanent_object (SCM @var{obj})

Similar to @code{scm_gc_protect_object} in that it causes the
collector to always mark the object, except that it should not be
nested (only call @code{scm_permanent_object} on an object once), and
it has no corresponding unpermanent function. Once an object is
declared permanent, it will never be freed. Returns the SCM object it
was passed.
@end deftypefn

@c  NOTE: The varargs scm_remember_upto_here is deliberately not
@c  documented, because we don't think it can be implemented as a nice
@c  inline compiler directive or asm block.  New _3, _4 or whatever
@c  forms could certainly be added though, if needed.

@deftypefn {C Macro} void scm_remember_upto_here_1 (SCM obj)
@deftypefnx {C Macro} void scm_remember_upto_here_2 (SCM obj1, SCM obj2)
Create a reference to the given object or objects, so they're certain
to be present on the stack or in a register and hence will not be
freed by the garbage collector before this point.

Note that these functions can only be applied to ordinary C local
variables (ie.@: ``automatics'').  Objects held in global or static
variables or some malloced block or the like cannot be protected with
this mechanism.
@end deftypefn

@deffn {Scheme Procedure} gc-stats
@deffnx {C Function} scm_gc_stats ()
Return an association list of statistics about Guile's current
use of storage.
@end deffn

@deffn {Scheme Procedure} gc-live-object-stats
@deffnx {C Function} scm_gc_live_object_stats ()
Return an alist of statistics of the current live objects. 
@end deffn

@deftypefun void scm_gc_mark (SCM @var{x})
Mark the object @var{x}, and recurse on any objects @var{x} refers to.
If @var{x}'s mark bit is already set, return immediately.  This function
must only be called during the mark-phase of garbage collection,
typically from a smob @emph{mark} function.
@end deftypefun


@node Memory Blocks
@subsection Memory Blocks

@cindex automatically-managed memory
@cindex GC-managed memory
@cindex conservative garbage collection

In C programs, dynamic management of memory blocks is normally done
with the functions malloc, realloc, and free.  Guile has additional
functions for dynamic memory allocation that are integrated into the
garbage collector and the error reporting system.

Memory blocks that are associated with Scheme objects (for example a
foreign object) should be allocated with @code{scm_gc_malloc} or
@code{scm_gc_malloc_pointerless}.  These two functions will either
return a valid pointer or signal an error.  Memory blocks allocated this
way may be released explicitly; however, this is not strictly needed,
and we recommend @emph{not} calling @code{scm_gc_free}.  All memory
allocated with @code{scm_gc_malloc} or @code{scm_gc_malloc_pointerless}
is automatically reclaimed when the garbage collector no longer sees any
live reference to it@footnote{In Guile up to version 1.8, memory
allocated with @code{scm_gc_malloc} @emph{had} to be freed with
@code{scm_gc_free}.}.

When garbage collection occurs, Guile will visit the words in memory
allocated with @code{scm_gc_malloc}, looking for live pointers.  This
means that if @code{scm_gc_malloc}-allocated memory contains a pointer
to some other part of the memory, the garbage collector notices it and
prevents it from being reclaimed@footnote{In Guile up to 1.8, memory
allocated with @code{scm_gc_malloc} was @emph{not} visited by the
collector in the mark phase.  Consequently, the GC had to be told
explicitly about pointers to live objects contained in the memory block,
e.g., @i{via} SMOB mark functions (@pxref{Smobs,
@code{scm_set_smob_mark}})}.  Conversely, memory allocated with
@code{scm_gc_malloc_pointerless} is assumed to be ``pointer-less'' and
is not scanned for pointers.

For memory that is not associated with a Scheme object, you can use
@code{scm_malloc} instead of @code{malloc}.  Like
@code{scm_gc_malloc}, it will either return a valid pointer or signal
an error.  However, it will not assume that the new memory block can
be freed by a garbage collection.  The memory must be explicitly freed
with @code{free}.

There is also @code{scm_gc_realloc} and @code{scm_realloc}, to be used
in place of @code{realloc} when appropriate, and @code{scm_gc_calloc}
and @code{scm_calloc}, to be used in place of @code{calloc} when
appropriate.

The function @code{scm_dynwind_free} can be useful when memory should be
freed with libc's @code{free} when leaving a dynwind context,
@xref{Dynamic Wind}.

@deftypefn {C Function} {void *} scm_malloc (size_t @var{size})
@deftypefnx {C Function} {void *} scm_calloc (size_t @var{size})
Allocate @var{size} bytes of memory and return a pointer to it.  When
@var{size} is 0, return @code{NULL}.  When not enough memory is
available, signal an error.  This function runs the GC to free up some
memory when it deems it appropriate.

The memory is allocated by the libc @code{malloc} function and can be
freed with @code{free}.  There is no @code{scm_free} function to go
with @code{scm_malloc} to make it easier to pass memory back and forth
between different modules.  

The function @code{scm_calloc} is similar to @code{scm_malloc}, but
initializes the block of memory to zero as well.

These functions will (indirectly) call
@code{scm_gc_register_allocation}.
@end deftypefn

@deftypefn {C Function} {void *} scm_realloc (void *@var{mem}, size_t @var{new_size})
Change the size of the memory block at @var{mem} to @var{new_size} and
return its new location.  When @var{new_size} is 0, this is the same
as calling @code{free} on @var{mem} and @code{NULL} is returned.  When
@var{mem} is @code{NULL}, this function behaves like @code{scm_malloc}
and allocates a new block of size @var{new_size}.

When not enough memory is available, signal an error.  This function
runs the GC to free up some memory when it deems it appropriate.

This function will call @code{scm_gc_register_allocation}.
@end deftypefn




@deftypefn {C Function} {void *} scm_gc_malloc (size_t @var{size}, const char *@var{what})
@deftypefnx {C Function} {void *} scm_gc_malloc_pointerless (size_t @var{size}, const char *@var{what})
@deftypefnx {C Function} {void *} scm_gc_realloc (void *@var{mem}, size_t @var{old_size}, size_t @var{new_size}, const char *@var{what});
@deftypefnx {C Function} {void *} scm_gc_calloc (size_t @var{size}, const char *@var{what})
Allocate @var{size} bytes of automatically-managed memory.  The memory
is automatically freed when no longer referenced from any live memory
block.

When garbage collection occurs, Guile will visit the words in memory
allocated with @code{scm_gc_malloc} or @code{scm_gc_calloc}, looking for
pointers to other memory allocations that are managed by the GC.  In
contrast, memory allocated by @code{scm_gc_malloc_pointerless} is not
scanned for pointers.

The @code{scm_gc_realloc} call preserves the ``pointerlessness'' of the
memory area pointed to by @var{mem}.  Note that you need to pass the old
size of a reallocated memory block as well.  See below for a motivation.
@end deftypefn


@deftypefn {C Function} void scm_gc_free (void *@var{mem}, size_t @var{size}, const char *@var{what})
Explicitly free the memory block pointed to by @var{mem}, which was
previously allocated by one of the above @code{scm_gc} functions.  This
function is almost always unnecessary, except for codebases that still
need to compile on Guile 1.8.

Note that you need to explicitly pass the @var{size} parameter.  This
is done since it should normally be easy to provide this parameter
(for memory that is associated with GC controlled objects) and help keep
the memory management overhead very low.  However, in Guile 2.x,
@var{size} is always ignored.
@end deftypefn


@deftypefn {C Function} void scm_gc_register_allocation (size_t @var{size})
Informs the garbage collector that @var{size} bytes have been allocated,
which the collector would otherwise not have known about.

In general, Scheme will decide to collect garbage only after some amount
of memory has been allocated.  Calling this function will make the
Scheme garbage collector know about more allocation, and thus run more
often (as appropriate).

It is especially important to call this function when large unmanaged
allocations, like images, may be freed by small Scheme allocations, like
foreign objects.
@end deftypefn


@deftypefn {C Function} void scm_dynwind_free (void *mem)
Equivalent to @code{scm_dynwind_unwind_handler (free, @var{mem},
SCM_F_WIND_EXPLICITLY)}.  That is, the memory block at @var{mem} will be
freed (using @code{free} from the C library) when the current dynwind is
left.
@end deftypefn

@deffn {Scheme Procedure} malloc-stats
Return an alist ((@var{what} . @var{n}) ...) describing number
of malloced objects.
@var{what} is the second argument to @code{scm_gc_malloc},
@var{n} is the number of objects of that type currently
allocated.

This function is only available if the @code{GUILE_DEBUG_MALLOC}
preprocessor macro was defined when Guile was compiled.
@end deffn


@node Weak References
@subsection Weak References

[FIXME: This chapter is based on Mikael Djurfeldt's answer to a
question by Michael Livshin. Any mistakes are not theirs, of course. ]

Weak references let you attach bookkeeping information to data so that
the additional information automatically disappears when the original
data is no longer in use and gets garbage collected. In a weak key hash,
the hash entry for that key disappears as soon as the key is no longer
referenced from anywhere else. For weak value hashes, the same happens
as soon as the value is no longer in use. Entries in a doubly weak hash
disappear when either the key or the value are not used anywhere else
anymore.

Object properties offer the same kind of functionality as weak key
hashes in many situations. (@pxref{Object Properties})

Here's an example (a little bit strained perhaps, but one of the
examples is actually used in Guile):

Assume that you're implementing a debugging system where you want to
associate information about filename and position of source code
expressions with the expressions themselves.

Hashtables can be used for that, but if you use ordinary hash tables
it will be impossible for the scheme interpreter to "forget" old
source when, for example, a file is reloaded.

To implement the mapping from source code expressions to positional
information it is necessary to use weak-key tables since we don't want
the expressions to be remembered just because they are in our table.

To implement a mapping from source file line numbers to source code
expressions you would use a weak-value table.

To implement a mapping from source code expressions to the procedures
they constitute a doubly-weak table has to be used.

@menu
* Weak hash tables::
* Weak vectors::
@end menu


@node Weak hash tables
@subsubsection Weak hash tables

@deffn {Scheme Procedure} make-weak-key-hash-table [size]
@deffnx {Scheme Procedure} make-weak-value-hash-table [size]
@deffnx {Scheme Procedure} make-doubly-weak-hash-table [size]
@deffnx {C Function} scm_make_weak_key_hash_table (size)
@deffnx {C Function} scm_make_weak_value_hash_table (size)
@deffnx {C Function} scm_make_doubly_weak_hash_table (size)
Return a weak hash table with @var{size} buckets. As with any
hash table, choosing a good size for the table requires some
caution.

You can modify weak hash tables in exactly the same way you would modify
regular hash tables, with the exception of the routines that act on
handles.  Weak tables have a different implementation behind the scenes
that doesn't have handles.  @pxref{Hash Tables}, for more on
@code{hashq-ref} et al.
@end deffn

Note that in a weak-key hash table, the reference to the value is
strong.  This means that if the value references the key, even
indirectly, the key will never be collected, which can lead to a memory
leak.  The reverse is true for weak value tables.

@deffn {Scheme Procedure} weak-key-hash-table? obj
@deffnx {Scheme Procedure} weak-value-hash-table? obj
@deffnx {Scheme Procedure} doubly-weak-hash-table? obj
@deffnx {C Function} scm_weak_key_hash_table_p (obj)
@deffnx {C Function} scm_weak_value_hash_table_p (obj)
@deffnx {C Function} scm_doubly_weak_hash_table_p (obj)
Return @code{#t} if @var{obj} is the specified weak hash
table. Note that a doubly weak hash table is neither a weak key
nor a weak value hash table.
@end deffn

@node Weak vectors
@subsubsection Weak vectors

@deffn {Scheme Procedure} make-weak-vector size [fill]
@deffnx {C Function} scm_make_weak_vector (size, fill)
Return a weak vector with @var{size} elements. If the optional
argument @var{fill} is given, all entries in the vector will be
set to @var{fill}. The default value for @var{fill} is the
empty list.
@end deffn

@deffn {Scheme Procedure} weak-vector elem @dots{}
@deffnx {Scheme Procedure} list->weak-vector l
@deffnx {C Function} scm_weak_vector (l)
Construct a weak vector from a list: @code{weak-vector} uses
the list of its arguments while @code{list->weak-vector} uses
its only argument @var{l} (a list) to construct a weak vector
the same way @code{list->vector} would.
@end deffn

@deffn {Scheme Procedure} weak-vector? obj
@deffnx {C Function} scm_weak_vector_p (obj)
Return @code{#t} if @var{obj} is a weak vector.
@end deffn

@deffn {Scheme Procedure} weak-vector-ref wvect k
@deffnx {C Function} scm_weak_vector_ref (wvect, k)
Return the @var{k}th element of the weak vector @var{wvect}, or
@code{#f} if that element has been collected.
@end deffn

@deffn {Scheme Procedure} weak-vector-set! wvect k elt
@deffnx {C Function} scm_weak_vector_set_x (wvect, k, elt)
Set the @var{k}th element of the weak vector @var{wvect} to @var{elt}.
@end deffn


@node Guardians
@subsection Guardians

Guardians provide a way to be notified about objects that would
otherwise be collected as garbage.  Guarding them prevents the objects
from being collected and cleanup actions can be performed on them, for
example.

See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in
a Generation-Based Garbage Collector".  ACM SIGPLAN Conference on
Programming Language Design and Implementation, June 1993.

@deffn {Scheme Procedure} make-guardian
@deffnx {C Function} scm_make_guardian ()
Create a new guardian.  A guardian protects a set of objects from
garbage collection, allowing a program to apply cleanup or other
actions.

@code{make-guardian} returns a procedure representing the guardian.
Calling the guardian procedure with an argument adds the argument to
the guardian's set of protected objects.  Calling the guardian
procedure without an argument returns one of the protected objects
which are ready for garbage collection, or @code{#f} if no such object
is available.  Objects which are returned in this way are removed from
the guardian.

You can put a single object into a guardian more than once and you can
put a single object into more than one guardian.  The object will then
be returned multiple times by the guardian procedures.

An object is eligible to be returned from a guardian when it is no
longer referenced from outside any guardian.

There is no guarantee about the order in which objects are returned
from a guardian.  If you want to impose an order on finalization
actions, for example, you can do that by keeping objects alive in some
global data structure until they are no longer needed for finalizing
other objects.

Being an element in a weak vector, a key in a hash table with weak
keys, or a value in a hash table with weak values does not prevent an
object from being returned by a guardian.  But as long as an object
can be returned from a guardian it will not be removed from such a
weak vector or hash table.  In other words, a weak link does not
prevent an object from being considered collectable, but being inside
a guardian prevents a weak link from being broken.

A key in a weak key hash table can be thought of as having a strong
reference to its associated value as long as the key is accessible.
Consequently, when the key is only accessible from within a guardian,
the reference from the key to the value is also considered to be
coming from within a guardian.  Thus, if there is no other reference
to the value, it is eligible to be returned from a guardian.
@end deffn


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