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\chapter{Storage Management\label{CHPTSMGMT}}

This chapter describes aspects of the storage management system and

procedures that may be used to control its operation.

\section{Garbage Collection\label{SECTSMGMTGC}}

Scheme objects such as pairs, strings, and procedures are

never explicitly deallocated by a Scheme program.

Instead, the \index{storage management}storage management system

automatically reclaims the

storage associated with an object once it proves the object is no longer

accessible.

In order to reclaim this storage, {\ChezScheme} employs a

\index{garbage collector}garbage

collector which runs periodically as a program runs.

Starting from a set of known \emph{roots}, e.g., the machine registers,

the garbage collector locates all accessible objects,

copies them (in most cases) in order to eliminate fragmentation

between accessible objects, and reclaims storage occupied by

inaccessible objects.

Collections are triggered automatically by the default collect-request

handler, which is invoked via a collect-request interrupt that occurs

after approximately $n$ bytes of storage have been allocated, where $n$ is

the value of the parameter

\index{\scheme{collect-trip-bytes}}\scheme{collect-trip-bytes}.

The default collect-request handler causes a collection by calling the

procedure \index{\scheme{collect}}\scheme{collect} without arguments.

The collect-request handler can be redefined by changing the value of the

parameter

\index{\scheme{collect-request-handler}}\scheme{collect-request-handler}.

A program can also cause a collection to occur between collect-request

interrupts by calling \scheme{collect} directly.

{\ChezScheme}'s collector is a \emph{generation-based} collector.

It segregates objects based on their age (roughly speaking, the

number of collections survived) and collects older objects less

frequently than younger objects.

Since younger objects tend to become inaccessible more quickly than

older objects, the result is that most collections take little

time.

The system also maintains a

\index{static generation}\emph{static} generation from

which storage is never reclaimed.

Objects are placed into the static generation only

when a heap is compacted (see

\index{\scheme{Scompact_heap}}\scheme{Scompact_heap} in

Section~\ref{SECTFOREIGNCLIB}) or when the target-generation argument to

\index{\scheme{collect}}\scheme{collect} is the symbol \scheme{static}.

Nonstatic generations are numbered starting at zero for the youngest

generation up through the current value of

\index{\scheme{collect-maximum-generation}}\scheme{collect-maximum-generation}.

The storage manager places newly allocated objects into generation 0.

During a generation 0 collection, objects in generation 0 that survive the

collection move, by default, to generation 1.

Similarly, during a generation 1 collection, objects in generations 0 and

1 that survive move to generation 2, and so on.

During the collection of the maximum nonstatic collection, all surviving

nonstatic objects move (possibly back) into the maximum nonstatic

generation.

With this mechanism, it is possible for an object to skip one or more

generations, but this is not likely to happen to many objects, and if the

objects become inaccessible, their storage is reclaimed eventually.

An internal counter, gc-trip, is maintained to control when each

generation is collected.

Each time \scheme{collect} is called without arguments (as from the default

collect-request handler), gc-trip is incremented by one.

With a collect-generation radix of $r$, the collected generation

is the highest numbered generation $g$ for which gc-trip is a

multiple of $r^g$.

If \scheme{collect-generation-radix} is set to 4, the system thus

collects generation 0 every time, generation 1 every 4 times, generation 2

every 16 times, and so on.

Each time \scheme{collect} is called with a single generation argument $g$,

generation $g$ is collected and

gc-trip is advanced to the next $r^g$ boundary, but not past the next

$r^{g+1}$ boundary, where $r$ is again the

value of \scheme{collect-generation-radix}.

If \scheme{collect} is called with a second generation argument,

$tg$, $tg$ determines the target generation.

When $g$ is the maximum nonstatic generation, $tg$ must be

$g$ or the symbol \scheme{static}.

Otherwise, $tg$ must be $g$ or $g+1$.

When the target generation is the symbol \scheme{static}, all data in

the nonstatic generations are moved to the static generation.

Objects in the static generation are never collected.

This is primarily useful after an application's permanent code and data

structures have been loaded and initialized, to reduce the overhead of

subsequent collections.

It is possible to make substantial adjustments in the collector's behavior

by setting the parameters described in this section.

It is even possible to completely override the collector's default strategy for

determining when each generation is collected by redefining the

collect-request handler to call \scheme{collect} with explicit $g$ and

$tg$ arguments.

For example, the programmer can redefine the handler to treat the maximum

nonstatic generation as a static generation over a long period of time

by calling \scheme{collect} with explicit $g$ and $tg$ arguments

that are never equal to the maximum nonstatic generation during that

period of time.

Additional information on {\ChezScheme}'s collector can be found in the

report ``Don't stop the {BiBOP}: Flexible and efficient

storage management for dynamically typed languages''~\cite{Dybvig:sm}.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect}{\categoryprocedure}{(collect)}

\formdef{collect}{\categoryprocedure}{(collect \var{g})}

\formdef{collect}{\categoryprocedure}{(collect \var{g} \var{tg})}

\returns unspecified

\listlibraries

\endentryheader

\noindent

\var{g} must be a nonnegative fixnum no greater than the

maximum nonstatic generation, i.e., the

value returned by \scheme{collect-maximum-generation}.

If \var{g} is the maximum nonstatic generation,

\var{tg} must be a fixnum equal to \var{g} or the symbol

\scheme{static}.

Otherwise, \var{tg} must be a fixnum equal to or one

greater than \var{g}.

This procedure causes the storage manager to perform a garbage collection.

\scheme{collect} is invoked periodically via the collect-request

handler, but it may also be called explicitly to force collection at a

particular time, e.g., before timing a computation.

In the threaded versions of {\ChezScheme}, the thread that invokes

\scheme{collect} must be the only active thread.

The system determines which generations to collect, based on \var{g} and

\var{tg} if provided, as described in the lead-in to this section.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect-notify}{\categoryglobalparameter}{collect-notify}

\listlibraries

\endentryheader

\noindent

If \scheme{collect-notify} is set to a true value, the collector prints

a message whenever a collection is run.

\scheme{collect-notify} is set to \scheme{#f} by default.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect-trip-bytes}{\categoryglobalparameter}{collect-trip-bytes}

\listlibraries

\endentryheader

\noindent

This parameter determines the approximate amount of storage that is

allowed to be allocated between garbage collections.

Its value must be a positive fixnum.

{\ChezScheme} allocates memory internally in large chunks and

subdivides these chunks via inline operations for efficiency.

The storage manager determines whether to request a collection only

once per large chunk allocated.

Furthermore, some time may elapse between when a collection is

requested by the storage manager and when the collect request is

honored, especially if interrupts are temporarily disabled via

\index{\scheme{with-interrupts-disabled}}\scheme{with-interrupts-disabled}

or \index{\scheme{disable-interrupts}}\scheme{disable-interrupts}.

Thus, \scheme{collect-trip-bytes} is an approximate measure only.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect-generation-radix}{\categoryglobalparameter}{collect-generation-radix}

\listlibraries

\endentryheader

\noindent

This parameter determines how often each generation is collected

when \scheme{collect} is invoked without arguments, as by the default

collect-request handler.

Its value must be a positive fixnum.

Generations are collected once every $r^g$ times a collection occurs,

where $r$ is the

value of \scheme{collect-generation-radix} and $g$ is the generation

number.

Setting \scheme{collect-generation-radix} to one forces all generations

to be collected each time a collection occurs.

Setting \scheme{collect-generation-radix} to a very large number

effectively delays collection of older generations indefinitely.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect-maximum-generation}{\categoryglobalparameter}{collect-maximum-generation}

\listlibraries

\endentryheader

This parameter determines the maximum nonstatic generation, hence the

total number of generations, currently in use.

Its value is an exact integer in the range 1 through 254.

When set to 1, only two nonstatic generations are used; when set to 2,

three nonstatic generations are used, and so on.

When set to 254, 255 nonstatic generations are used, plus the single

static generation for a total of 256 generations.

Increasing the number of generations effectively decreases how often old

objects are collected, potentially decreasing collection overhead but

potentially increasing the number of inaccessible objects retained in the

system and thus the total amount of memory required.

%----------------------------------------------------------------------------

\entryheader

\formdef{collect-request-handler}{\categoryglobalparameter}{collect-request-handler}

\listlibraries

\endentryheader

\noindent

The value of \scheme{collect-request-handler} must be a procedure.

The procedure is invoked without arguments whenever the

system determines that a collection should occur, i.e., some time after

an amount of storage determined by the parameter

\scheme{collect-trip-bytes} has been allocated since the last

collection.

By default, \scheme{collect-request-handler} simply invokes

\scheme{collect} without arguments.

Automatic collection may be disabled by setting

\scheme{collect-request-handler} to a procedure that does nothing,

e.g.:

\schemedisplay

(collect-request-handler void)

\endschemedisplay

Collection can also be temporarily disabled using

\scheme{critical-section}, which prevents any interrupts from

occurring.

%----------------------------------------------------------------------------

\entryheader

\formdef{release-minimum-generation}{\categoryglobalparameter}{release-minimum-generation}

\listlibraries

\endentryheader

This parameter's value must be between 0 and the value of

\scheme{collect-maximum-generation}, inclusive, and defaults to the

value of \scheme{collect-maximum-generation}.

As new data is allocated and collections occur, the storage-management

system automatically requests additional virtual memory address space

from the operating system.

Correspondingly, in the event the heap shrinks significantly, the system

attempts to return some of the virtual-memory previously obtained from

the operating system back to the operating system.

By default, the system attempts to do so only after a collection that

targets the maximum nonstatic generation.

The system can be asked to do so after collections

targeting younger generations as well by altering the value

\scheme{release-minimum-generation} to something less than the value

of \scheme{collect-maximum-generation}.

When the generation to which the parameter is set, or any older

generation, is the target generation of a collection, the storage

management system attempts to return unneeded virtual memory to the

operating system following the collection.

When \scheme{collect-maximum-generation} is set to a new value \var{g},

\scheme{release-minimum-generation} is implicitly set to \var{g} as well

if (a) the two parameters have the same value before the change, or (b)

\scheme{release-minimum-generation} has a value greater than \var{g}.

%----------------------------------------------------------------------------

\entryheader

\formdef{heap-reserve-ratio}{\categoryglobalparameter}{heap-reserve-ratio}

\listlibraries

\endentryheader

This parameter determines the approximate amount of memory reserved (not

returned to the O/S as described in the entry for \scheme{release-minimum-generation})

in proportion to the amount currently occupied, excluding areas

of memory that have been made static.

Its value must be an inexact nonnegative flonum value; if set to an exact

real value, the exact value is converted to an inexact value.

The default value, 1.0, reserves one page of memory for each currently

occupied nonstatic page.

Setting it to a smaller value may result in a smaller average virtual

memory footprint, while setting it to a larger value may result in fewer

calls into the operating system to request and free memory space.

\section{Weak Pairs and Guardians\label{SECTGUARDWEAKPAIRS}}

\index{weak pairs}\index{weak pointers}\emph{Weak pairs} allow programs

to maintain \emph{weak pointers} to objects.

A weak pointer to an object does not prevent the object from being

reclaimed by the storage management system, but it does remain valid as

long as the object is otherwise accessible in the system.

\index{guardians}\emph{Guardians}

allow programs to protect objects from deallocation

by the garbage collector and to determine when the objects would

otherwise have been deallocated.

Weak pairs and guardians allow programs to retain

information about objects in separate data structures (such as hash

tables) without concern that maintaining this information will cause

the objects to remain indefinitely in the system.

In addition, guardians allow objects to be saved from deallocation

indefinitely so that they can be reused or so that clean-up or other

actions can be performed using the data stored within the objects.

The implementation of guardians and weak pairs used by {\ChezScheme}

is described in~\cite{Dybvig:guardians}.

%----------------------------------------------------------------------------

\entryheader\label{desc:weak-cons}

\formdef{weak-cons}{\categoryprocedure}{(weak-cons \var{obj_1} \var{obj_2})}

\returns a new weak pair

\listlibraries

\endentryheader

\noindent

\var{obj_1} becomes the car and \var{obj_2} becomes the cdr of the

new pair.

Weak pairs are indistinguishable from ordinary pairs in all but two ways:

\begin{itemize}

\item weak pairs can be distinguished from pairs using the

\scheme{weak-pair?} predicate, and

\item weak pairs maintain a weak pointer to the object in the

car of the pair.

\end{itemize}

\noindent

The weak pointer in the car of a weak pair is just like a normal

pointer as long as the object to which it points is accessible through

a normal (nonweak) pointer somewhere in the system.

If at some point the garbage collector recognizes that there are no

nonweak pointers to the object, however, it replaces each weak pointer

to the object with the ``broken weak-pointer'' object, \scheme{#!bwp},

and discards the object.

The cdr field of a weak pair is \emph{not} a weak pointer, so

weak pairs may be used to form lists of weakly held objects.

These lists may be manipulated using ordinary list-processing

operations such as \scheme{length}, \scheme{map}, and \scheme{assv}.

(Procedures like \scheme{map} that produce list structure always

produce lists formed from nonweak pairs, however, even when their input

lists are formed from weak pairs.)

Weak pairs may be altered using \scheme{set-car!} and \scheme{set-cdr!}; after

a \scheme{set-car!} the car field contains a weak pointer to the new

object in place of the old object.

Weak pairs are especially useful for building association pairs

in association lists or hash tables.

Weak pairs are printed in the same manner as ordinary pairs; there

is no reader syntax for weak pairs.

As a result, weak pairs become normal pairs when they are written

and then read.

\schemedisplay

(define x (cons 'a 'b))

(define p (weak-cons x '()))

(car p) ;=> (a . b)

(define x (cons 'a 'b))

(define p (weak-cons x '()))

(set! x '*)

(collect)

(car p) ;=> #!bwp

\endschemedisplay

\noindent

The latter example above may in fact return \scheme{(a . b)} if a

garbage collection promoting the pair into an older generation occurs

prior to the assignment of \scheme{x} to \scheme{*}.

It may be necessary to force an older generation collection to allow

the object to be reclaimed.

The storage management system guarantees only that the object

will be reclaimed eventually once all nonweak pointers to it are

dropped, but makes no guarantees about when this will occur.

%----------------------------------------------------------------------------

\entryheader

\formdef{weak-pair?}{\categoryprocedure}{(weak-pair? \var{obj})}

\returns \scheme{#t} if obj is a weak pair, \scheme{#f} otherwise

\listlibraries

\endentryheader

\schemedisplay

(weak-pair? (weak-cons 'a 'b)) ;=> #t

(weak-pair? (cons 'a 'b)) ;=> #f

(weak-pair? "oops") ;=> #f

\endschemedisplay

%----------------------------------------------------------------------------

\entryheader

\formdef{bwp-object?}{\categoryprocedure}{(bwp-object? \var{obj})}

\returns \scheme{#t} if obj is the broken weak-pair object, \scheme{#f} otherwise

\listlibraries

\endentryheader

\schemedisplay

(bwp-object? #!bwp) ;=> #t

(bwp-object? 'bwp) ;=> #f

(define x (cons 'a 'b))

(define p (weak-cons x '()))

(set! x '*)

(collect (collect-maximum-generation))

(car p) ;=> #!bwp

(bwp-object? (car p)) ;=> #t

\endschemedisplay

%----------------------------------------------------------------------------

\entryheader

\formdef{make-guardian}{\categoryprocedure}{(make-guardian)}

\returns a new guardian

\listlibraries

\endentryheader

\noindent

Guardians are represented by procedures that encapsulate groups of

objects registered for preservation.

When a guardian is created, the group of registered objects is empty.

An object is registered with a guardian by passing the object as an

argument to the guardian:

\schemedisplay

(define G (make-guardian))

(define x (cons 'aaa 'bbb))

x ;=> (aaa . bbb)

(G x)

\endschemedisplay

It is also possible to specify a ``representative'' object when

registering an object.

Continuing the above example:

\schemedisplay

(define y (cons 'ccc 'ddd))

y ;=> (ccc . ddd)

(G y 'rep)

\endschemedisplay

The group of registered objects associated with a guardian is logically

subdivided into two disjoint subgroups: a subgroup referred to

as ``accessible'' objects, and one referred to ``inaccessible'' objects.

Inaccessible objects are objects that have been proven to be

inaccessible (except through the guardian mechanism itself or through

the car field of a weak pair), and

accessible objects are objects that have not been proven so.

The word ``proven'' is important here: it may be that some objects in

the accessible group are indeed inaccessible but

that this has not yet been proven.

This proof may not be made in some cases until long after the object

actually becomes inaccessible (in the current implementation, until a

garbage collection of the generation containing the object occurs).

Objects registered with a guardian are initially placed in the accessible

group and are moved into the inaccessible group at some point after they

become inaccessible.

Objects in the inaccessible group are retrieved by invoking the guardian

without arguments.

If there are no objects in the inaccessible group, the guardian returns

\scheme{#f}.

Continuing the above example:

\schemedisplay

(G) ;=> #f

(set! x #f)

(set! y #f)

(collect)

(G) ;=> (aaa . bbb) ; \var{this might come out second}

(G) ;=> rep ; \var{and this first}

(G) ;=> #f

\endschemedisplay

\noindent

The initial call to \scheme{G} returns \scheme{#f}, since the pairs bound

to \scheme{x} and \scheme{y} are the

only object registered with \scheme{G}, and the pairs are still accessible

through those binding.

When \scheme{collect} is called, the objects shift into the inaccessible group.

The two calls to \scheme{G} therefore return the pair previously bound to

\scheme{x} and the representative of the pair previously bound to \scheme{y},

though perhaps in the other order from the one shown.

(As noted above for weak pairs, the call to collect may not actually be

sufficient to prove the object inaccessible, if the object has

migrated into an older generation.)

Although an object registered without a representative and returned from

a guardian has been proven otherwise

inaccessible (except possibly via the car field of a weak pair), it has

not yet been reclaimed by the storage management system and will not be

reclaimed until after the last nonweak pointer to it within or outside

of the guardian system has been dropped.

In fact, objects that have been retrieved from a guardian have no

special status in this or in any other regard.

This feature circumvents the problems that might otherwise arise with

shared or cyclic structure.

A shared or cyclic structure consisting of inaccessible objects is

preserved in its entirety, and each piece registered for preservation

with any guardian is placed in the inaccessible set for that guardian.

The programmer then has complete control over the order in which pieces

of the structure are processed.

An object may be registered with a guardian more than once, in which

case it will be retrievable more than once:

\schemedisplay

(define G (make-guardian))

(define x (cons 'aaa 'bbb))

(G x)

(G x)

(set! x #f)

(collect)

(G) ;=> (aaa . bbb)

(G) ;=> (aaa . bbb)

\endschemedisplay

\noindent

It may also be registered with more than one guardian, and guardians

themselves can be registered with other guardians.

An object that has been registered with a guardian without a

representative and placed in

the car field of a weak pair remains in the car field of the

weak pair until after it has been returned from the guardian and

dropped by the program or until the guardian itself is dropped.

\schemedisplay

(define G (make-guardian))

(define x (cons 'aaa 'bbb))

(define p (weak-cons x '()))

(G x)

(set! x #f)

(collect)

(set! y (G))

y ;=> (aaa . bbb)

(car p) ;=> (aaa . bbb)

(set! y #f)

(collect 1)

(car p) ;=> #!bwp

\endschemedisplay

\noindent

(The first collector call above would

promote the object at least into generation~1, requiring the second

collector call to be a generation~1 collection.

This can also be forced by invoking \scheme{collect} several times.)

On the other hand, if a representative (other than the object itself)

is specified, the guarded object is dropped from the car field of the

weak pair at the same time as the representative becomes available

from the guardian.

\schemedisplay

(define G (make-guardian))

(define x (cons 'aaa 'bbb))

(define p (weak-cons x '()))

(G x 'rep)

(set! x #f)

(collect)

(G) ;=> rep

(car p) ;=> #!bwp

\endschemedisplay

The following example illustrates that the object is deallocated and

the car field of the weak pointer set to \scheme{#!bwp} when the guardian

itself is dropped:

\schemedisplay

(define G (make-guardian))

(define x (cons 'aaa 'bbb))

(define p (weak-cons x '()))

(G x)

(set! x #f)

(set! G #f)

(collect)

(car p) ;=> #!bwp

\endschemedisplay

The example below demonstrates how guardians might be used to

deallocate external storage, such as storage managed by the C library

``malloc'' and ``free'' operations.

\schemedisplay

(define malloc

(let ([malloc-guardian (make-guardian)])

(lambda (size)

; first free any storage that has been dropped. to avoid long

; delays, it might be better to deallocate no more than, say,

; ten objects for each one allocated

(let f ()

(let ([x (malloc-guardian)])

(when x

(do-free x)

(f))))

; then allocate and register the new storage

(let ([x (do-malloc size)])

(malloc-guardian x)

x))))

\endschemedisplay

\noindent

\scheme{do-malloc} must return a Scheme object ``header'' encapsulating a pointer to the

external storage (perhaps as an unsigned integer), and all access to the

external storage must be made through this header.

In particular, care must be taken that no pointers to the external storage

exist outside of Scheme after the corresponding header has been

dropped.

\scheme{do-free} must deallocate the external storage using the encapsulated

pointer.

Both primitives can be defined in terms of \scheme{foreign-alloc}

and \scheme{foreign-free} or the C-library ``malloc'' and ``free''

operators, imported as foreign procedures. (See

Chapter~\ref{CHPTFOREIGN}.)

If it is undesirable to wait until \scheme{malloc} is called to free dropped

storage previously allocated by \scheme{malloc}, a collect-request handler

can be used instead to check for and free dropped storage, as shown below.

\schemedisplay

(define malloc)

(let ([malloc-guardian (make-guardian)])

(set! malloc

(lambda (size)

; allocate and register the new storage

(let ([x (do-malloc size)])

(malloc-guardian x)

x)))

(collect-request-handler

(lambda ()

; first, invoke the collector

(collect)

; then free any storage that has been dropped

(let f ()

(let ([x (malloc-guardian)])

(when x

(do-free x)

(f)))))))

\endschemedisplay

%% for testing:

% (define do-malloc (lambda (x) (list x)))

% (define do-free (lambda (x) (printf "freeing ~s~%" (car x))))

% (define a (malloc 1))

% (malloc 10)

% (let f () (cons f f) (f))

With a bit of refactoring, it would be possible to register

the encapsulated foreign address as a representative with

each header, in which \scheme{do-free} would take just the

foreign address as an argument.

This would allow the header to be dropped from the Scheme

heap as soon as it becomes inaccessible.

\section{Locking Objects\label{SECTSMGMTLOCKING}}

All pointers from C variables or data structures to Scheme objects

should generally be discarded before entry (or reentry) into Scheme.

When this guideline cannot be followed, the object may be

\emph{locked} via \scheme{lock-object} or via the equivalent

C library procedure \index{\scheme{Slock_object}}\scheme{Slock_object}

(Section~\ref{SECTFOREIGNCLIB}).

%----------------------------------------------------------------------------

\entryheader

\formdef{lock-object}{\categoryprocedure}{(lock-object \var{obj})}

\returns unspecified

\listlibraries

\endentryheader

\noindent

Locking an object prevents the storage manager from reclaiming or

relocating the object.

Locking should be used sparingly, as it introduces memory fragmentation

and increases storage management overhead.

Locking can also lead to accidental retention of storage if objects

are not unlocked.

Objects may be unlocked via \scheme{unlock-object} or the equivalent

C library procedure

\index{\scheme{Sunlock_object}}\scheme{Sunlock_object}.

Locking immediate values, such as fixnums, booleans, and characters,

or objects that have been made static is unnecessary but harmless.

%----------------------------------------------------------------------------

\entryheader

\formdef{unlock-object}{\categoryprocedure}{(unlock-object \var{obj})}

\returns unspecified

\listlibraries

\endentryheader

\noindent

An object may be locked more than once by successive calls to

\scheme{lock-object}, \scheme{Slock_object}, or both, in which case it must

be unlocked by an equal number of calls to

\scheme{unlock-object} or \scheme{Sunlock_object} before it is

truly unlocked.

An object contained within a locked object, such as an object in the

car of a locked pair, need not also be locked unless a separate C

pointer to the object exists.

That is, if the inner object is accessed only via an indirection of the

outer object, it should be left unlocked so that the collector is free

to relocate it during collection.

Unlocking immediate values, such as fixnums, booleans, and characters,

or objects that have been made static is unnecessary and ineffective but harmless.

%----------------------------------------------------------------------------

\entryheader

\formdef{locked-object?}{\categoryprocedure}{(locked-object? \var{obj})}

\returns \scheme{#t} if \var{obj} is locked, immediate, or static

\listlibraries

\endentryheader

\noindent

This predicate returns true if \var{obj} cannot be relocated or reclaimed

by the collector, including immediate values, such as fixnums,

booleans, and characters, and objects that have been made static.