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Boost provides free peer-reviewed portable C++ source libraries. We emphasize libraries that work

collector doesn't already understand them.)  On some machines, it may
be desirable to set GC_stacktop to a good approximation of the stack base. 
(This enhances code portability on HP PA machines, since there is no
good way for the collector to compute this value.)  Client code may include
"gc.h", which defines all of the following, plus many others.

1)  GC_malloc(nbytes)
    - allocate an object of size nbytes.  Unlike malloc, the object is
      cleared before being returned to the user.  Gc_malloc will
      invoke the garbage collector when it determines this to be appropriate.
      GC_malloc may return 0 if it is unable to acquire sufficient
      space from the operating system.  This is the most probable
      consequence of running out of space.  Other possible consequences
      are that a function call will fail due to lack of stack space,
      or that the collector will fail in other ways because it cannot
      maintain its internal data structures, or that a crucial system
      process will fail and take down the machine.  Most of these
      possibilities are independent of the malloc implementation.

2)  GC_malloc_atomic(nbytes)
    - allocate an object of size nbytes that is guaranteed not to contain any
      pointers.  The returned object is not guaranteed to be cleared.
      (Can always be replaced by GC_malloc, but results in faster collection
      times.  The collector will probably run faster if large character
      arrays, etc. are allocated with GC_malloc_atomic than if they are
      statically allocated.)

3)  GC_realloc(object, new_size)
    - change the size of object to be new_size.  Returns a pointer to the
      new object, which may, or may not, be the same as the pointer to
      the old object.  The new object is taken to be atomic iff the old one
      was.  If the new object is composite and larger than the original object,
      then the newly added bytes are cleared (we hope).  This is very likely
      to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h.
      Even then, it is likely to recycle the old object only if the object
      is grown in small additive increments (which, we claim, is generally bad
      coding practice.)

4)  GC_free(object)
    - explicitly deallocate an object returned by GC_malloc or
      GC_malloc_atomic.  Not necessary, but can be used to minimize
      collections if performance is critical.  Probably a performance
      loss for very small objects (<= 8 bytes).

5)  GC_expand_hp(bytes)
    - Explicitly increase the heap size.  (This is normally done automatically
      if a garbage collection failed to GC_reclaim enough memory.  Explicit
      calls to GC_expand_hp may prevent unnecessarily frequent collections at
      program startup.)

6)  GC_malloc_ignore_off_page(bytes)
	- identical to GC_malloc, but the client promises to keep a pointer to
	  the somewhere within the first 256 bytes of the object while it is
	  live.  (This pointer should nortmally be declared volatile to prevent
	  interference from compiler optimizations.)  This is the recommended
	  way to allocate anything that is likely to be larger than 100Kbytes
	  or so.  (GC_malloc may result in failure to reclaim such objects.)

7)  GC_set_warn_proc(proc)
	- Can be used to redirect warnings from the collector.  Such warnings
	  should be rare, and should not be ignored during code development.
      
8) GC_enable_incremental()
    - Enables generational and incremental collection.  Useful for large
      heaps on machines that provide access to page dirty information.
      Some dirty bit implementations may interfere with debugging
      (by catching address faults) and place restrictions on heap arguments
      to system calls (since write faults inside a system call may not be
      handled well).

9) Several routines to allow for registration of finalization code.
   User supplied finalization code may be invoked when an object becomes
   unreachable.  To call (*f)(obj, x) when obj becomes inaccessible, use
	GC_register_finalizer(obj, f, x, 0, 0);
   For more sophisticated uses, and for finalization ordering issues,
   see gc.h.

  The global variable GC_free_space_divisor may be adjusted up from its
default value of 4 to use less space and more collection time, or down for
the opposite effect.  Setting it to 1 or 0 will effectively disable collections
and cause all allocations to simply grow the heap.

  The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect
the amount of memory allocated by the above routines that should not be
considered as a candidate for collection.  Careless use may, of course, result
in excessive memory consumption.

  Some additional tuning is possible through the parameters defined
near the top of gc_priv.h.
  
  If only GC_malloc is intended to be used, it might be appropriate to define:

#define malloc(n) GC_malloc(n)
#define calloc(m,n) GC_malloc((m)*(n))

  For small pieces of VERY allocation intensive code, gc_inl.h
includes some allocation macros that may be used in place of GC_malloc
and friends.

  All externally visible names in the garbage collector start with "GC_".
To avoid name conflicts, client code should avoid this prefix, except when
accessing garbage collector routines or variables.

  There are provisions for allocation with explicit type information.
This is rarely necessary.  Details can be found in gc_typed.h.

THE C++ INTERFACE TO THE ALLOCATOR:

  The Ellis-Hull C++ interface to the collector is included in
the collector distribution.  If you intend to use this, type
"make c++" after the initial build of the collector is complete.
See gc_cpp.h for the definition of the interface.  This interface
tries to approximate the Ellis-Detlefs C++ garbage collection
proposal without compiler changes.

  Very often it will also be necessary to use gc_allocator.h and the
allocator declared there to construct STL data structures.  Otherwise
subobjects of STL data structures wil be allcoated using a system
allocator, and objects they refer to may be prematurely collected.

USE AS LEAK DETECTOR:

  The collector may be used to track down leaks in C programs that are
intended to run with malloc/free (e.g. code with extreme real-time or
portability constraints).  To do so define FIND_LEAK in Makefile
This will cause the collector to invoke the report_leak
routine defined near the top of reclaim.c whenever an inaccessible
object is found that has not been explicitly freed.  Such objects will
also be automatically reclaimed.
  If all objects are allocated with GC_DEBUG_MALLOC (see next section), then
the default version of report_leak will report at least the source file and
line number at which the leaked object was allocated.  This may sometimes be
sufficient.  (On a few machines, it will also report a cryptic stack trace.
If this is not symbolic, it can somethimes be called into a sympolic stack
trace by invoking program "foo" with "callprocs foo".  Callprocs is a short
shell script that invokes adb to expand program counter values to symbolic
addresses.  It was largely supplied by Scott Schwartz.)
  Note that the debugging facilities described in the next section can
sometimes be slightly LESS effective in leak finding mode, since in
leak finding mode, GC_debug_free actually results in reuse of the object.
(Otherwise the object is simply marked invalid.)  Also note that the test
program is not designed to run meaningfully in FIND_LEAK mode.
Use "make gc.a" to build the collector.

DEBUGGING FACILITIES:

  The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc,
and GC_debug_free provide an alternate interface to the collector, which
provides some help with memory overwrite errors, and the like.
Objects allocated in this way are annotated with additional
information.  Some of this information is checked during garbage
collections, and detected inconsistencies are reported to stderr.

  Simple cases of writing past the end of an allocated object should
be caught if the object is explicitly deallocated, or if the
collector is invoked while the object is live.  The first deallocation
of an object will clear the debugging info associated with an
object, so accidentally repeated calls to GC_debug_free will report the
deallocation of an object without debugging information.  Out of
memory errors will be reported to stderr, in addition to returning
NIL.

  GC_debug_malloc checking  during garbage collection is enabled
with the first call to GC_debug_malloc.  This will result in some
slowdown during collections.  If frequent heap checks are desired,
this can be achieved by explicitly invoking GC_gcollect, e.g. from
the debugger.

  GC_debug_malloc allocated objects should not be passed to GC_realloc
or GC_free, and conversely.  It is however acceptable to allocate only
some objects with GC_debug_malloc, and to use GC_malloc for other objects,
provided the two pools are kept distinct.  In this case, there is a very
low probablility that GC_malloc allocated objects may be misidentified as
having been overwritten.  This should happen with probability at most
one in 2**32.  This probability is zero if GC_debug_malloc is never called.

  GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two
additional trailing arguments, a string and an integer.  These are not
interpreted by the allocator.  They are stored in the object (the string is
not copied).  If an error involving the object is detected, they are printed.

  The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and
GC_REGISTER_FINALIZER are also provided.  These require the same arguments
as the corresponding (nondebugging) routines.  If gc.h is included
with GC_DEBUG defined, they call the debugging versions of these
functions, passing the current file name and line number as the two
extra arguments, where appropriate.  If gc.h is included without GC_DEBUG
defined, then all these macros will instead be defined to their nondebugging
equivalents.  (GC_REGISTER_FINALIZER is necessary, since pointers to
objects with debugging information are really pointers to a displacement
of 16 bytes form the object beginning, and some translation is necessary
when finalization routines are invoked.  For details, about what's stored
in the header, see the definition of the type oh in debug_malloc.c)

INCREMENTAL/GENERATIONAL COLLECTION:

The collector normally interrupts client code for the duration of 
a garbage collection mark phase.  This may be unacceptable if interactive
response is needed for programs with large heaps.  The collector
can also run in a "generational" mode, in which it usually attempts to
collect only objects allocated since the last garbage collection.
Furthermore, in this mode, garbage collections run mostly incrementally,
with a small amount of work performed in response to each of a large number of
GC_malloc requests.

This mode is enabled by a call to GC_enable_incremental().

Incremental and generational collection is effective in reducing
pause times only if the collector has some way to tell which objects
or pages have been recently modified.  The collector uses two sources
of information:

1. Information provided by the VM system.  This may be provided in
one of several forms.  Under Solaris 2.X (and potentially under other
similar systems) information on dirty pages can be read from the
/proc file system.  Under other systems (currently SunOS4.X) it is
possible to write-protect the heap, and catch the resulting faults.
On these systems we require that system calls writing to the heap
(other than read) be handled specially by client code.
See os_dep.c for details.

2. Information supplied by the programmer.  We define "stubborn"
objects to be objects that are rarely changed.  Such an object
can be allocated (and enabled for writing) with GC_malloc_stubborn.
Once it has been initialized, the collector should be informed with
a call to GC_end_stubborn_change.  Subsequent writes that store
pointers into the object must be preceded by a call to
GC_change_stubborn.

This mechanism performs best for objects that are written only for
initialization, and such that only one stubborn object is writable
at once.  It is typically not worth using for short-lived
objects.  Stubborn objects are treated less efficiently than pointerfree
(atomic) objects.

A rough rule of thumb is that, in the absence of VM information, garbage
collection pauses are proportional to the amount of pointerful storage
plus the amount of modified "stubborn" storage that is reachable during
the collection.  

Initial allocation of stubborn objects takes longer than allocation
of other objects, since other data structures need to be maintained.

We recommend against random use of stubborn objects in client
code, since bugs caused by inappropriate writes to stubborn objects
are likely to be very infrequently observed and hard to trace.  
However, their use may be appropriate in a few carefully written
library routines that do not make the objects themselves available
for writing by client code.


BUGS:

  Any memory that does not have a recognizable pointer to it will be
reclaimed.  Exclusive-or'ing forward and backward links in a list
doesn't cut it.
  Some C optimizers may lose the last undisguised pointer to a memory
object as a consequence of clever optimizations.  This has almost
never been observed in practice.  Send mail to [e-mail]
for suggestions on how to fix your compiler.
  This is not a real-time collector.  In the standard configuration,
percentage of time required for collection should be constant across
heap sizes.  But collection pauses will increase for larger heaps.
They will decrease with the number of processors if parallel marking
is enabled.
(On 2007 vintage machines, GC times may be on the order of 5 msecs
per MB of accessible memory that needs to be scanned and processor.
Your mileage may vary.)  The incremental/generational collection facility
may help in some cases.
  Please address bug reports to [e-mail].  If you are
contemplating a major addition, you might also send mail to ask whether
it's already been done (or whether we tried and discarded it).