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-Copyright (c) 1988, 1989 Hans-J. Boehm, Alan J. Demers
-Copyright (c) 1991-1996 by Xerox Corporation.  All rights reserved.
-Copyright (c) 1996-1999 by Silicon Graphics.  All rights reserved.
-Copyright (c) 1999-2001 by Hewlett-Packard Company. All rights reserved.
-
-The file linux_threads.c is also
-Copyright (c) 1998 by Fergus Henderson.  All rights reserved.
-
-The files Makefile.am, and configure.in are
-Copyright (c) 2001 by Red Hat Inc. All rights reserved.
-
-Several files supporting GNU-style builds are copyrighted by the Free
-Software Foundation, and carry a different license from that given
-below.
-
-THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
-OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
-
-Permission is hereby granted to use or copy this program
-for any purpose,  provided the above notices are retained on all copies.
-Permission to modify the code and to distribute modified code is granted,
-provided the above notices are retained, and a notice that the code was
-modified is included with the above copyright notice.
-
-A few of the files needed to use the GNU-style build procedure come with
-slightly different licenses, though they are all similar in spirit.  A few
-are GPL'ed, but with an exception that should cover all uses in the
-collector.  (If you are concerned about such things, I recommend you look
-at the notice in config.guess or ltmain.sh.)
-
-This is version 6.1alpha5 of a conservative garbage collector for C and C++.
-
-You might find a more recent version of this at
-
-http://www.hpl.hp.com/personal/Hans_Boehm/gc
-
-OVERVIEW
-
-    This is intended to be a general purpose, garbage collecting storage
-allocator.  The algorithms used are described in:
-
-Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment",
-Software Practice & Experience, September 1988, pp. 807-820.
-
-Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection",
-Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design
-and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164.
-
-Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings
-of the ACM SIGPLAN '91 Conference on Programming Language Design and
-Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206.
-
-Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the
-2000 International Symposium on Memory Management.
-
-  Possible interactions between the collector and optimizing compilers are
-discussed in
-
-Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation",
-The Journal of C Language Translation 4, 2 (December 1992).
-
-and
-
-Boehm H., "Simple GC-safe Compilation", Proceedings
-of the ACM SIGPLAN '96 Conference on Programming Language Design and
-Implementation.
-
-(Some of these are also available from
-http://www.hpl.hp.com/personal/Hans_Boehm/papers/, among other places.)
-
-  Unlike the collector described in the second reference, this collector
-operates either with the mutator stopped during the entire collection
-(default) or incrementally during allocations.  (The latter is supported
-on only a few machines.)  On the most common platforms, it can be built
-with or without thread support.  On a few platforms, it can take advantage
-of a multiprocessor to speed up garbage collection.
-
-  Many of the ideas underlying the collector have previously been explored
-by others.  Notably, some of the run-time systems developed at Xerox PARC
-in the early 1980s conservatively scanned thread stacks to locate possible
-pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types
-to a Strongly-Typed Statically Checked, Concurrent Language"  Xerox PARC
-CSL 84-7).  Doug McIlroy wrote a simpler fully conservative collector that
-was part of version 8 UNIX (tm), but appears to not have received
-widespread use.
-
-  Rudimentary tools for use of the collector as a leak detector are included
-(see http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html),
-as is a fairly sophisticated string package "cord" that makes use of the
-collector.  (See doc/README.cords and H.-J. Boehm, R. Atkinson, and M. Plass,
-"Ropes: An Alternative to Strings", Software Practice and Experience 25, 12
-(December 1995), pp. 1315-1330.  This is very similar to the "rope" package
-in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.)
-
-Further collector documantation can be found at
-
-http://www.hpl.hp.com/personal/Hans_Boehm/gc
-
-
-GENERAL DESCRIPTION
-
-  This is a garbage collecting storage allocator that is intended to be
-used as a plug-in replacement for C's malloc.
-
-  Since the collector does not require pointers to be tagged, it does not
-attempt to ensure that all inaccessible storage is reclaimed.  However,
-in our experience, it is typically more successful at reclaiming unused
-memory than most C programs using explicit deallocation.  Unlike manually
-introduced leaks, the amount of unreclaimed memory typically stays
-bounded.
-
-  In the following, an "object" is defined to be a region of memory allocated
-by the routines described below.  
-
-  Any objects not intended to be collected must be pointed to either
-from other such accessible objects, or from the registers,
-stack, data, or statically allocated bss segments.  Pointers from
-the stack or registers may point to anywhere inside an object.
-The same is true for heap pointers if the collector is compiled with
- ALL_INTERIOR_POINTERS defined, as is now the default.
-
-Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention
-of garbage objects, by requiring pointers from the heap to to the beginning
-of an object.  But this no longer appears to be a significant
-issue for most programs.
-
-There are a number of routines which modify the pointer recognition
-algorithm.  GC_register_displacement allows certain interior pointers
-to be recognized even if ALL_INTERIOR_POINTERS is nor defined.
-GC_malloc_ignore_off_page allows some pointers into the middle of large objects
-to be disregarded, greatly reducing the probablility of accidental
-retention of large objects.  For most purposes it seems best to compile
-with ALL_INTERIOR_POINTERS and to use GC_malloc_ignore_off_page if
-you get collector warnings from allocations of very large objects.
-See README.debugging for details.
-
-  WARNING: pointers inside memory allocated by the standard "malloc" are not
-seen by the garbage collector.  Thus objects pointed to only from such a
-region may be prematurely deallocated.  It is thus suggested that the
-standard "malloc" be used only for memory regions, such as I/O buffers, that
-are guaranteed not to contain pointers to garbage collectable memory.
-Pointers in C language automatic, static, or register variables,
-are correctly recognized.  (Note that GC_malloc_uncollectable has semantics
-similar to standard malloc, but allocates objects that are traced by the
-collector.)
-
-  WARNING: the collector does not always know how to find pointers in data
-areas that are associated with dynamic libraries.  This is easy to
-remedy IF you know how to find those data areas on your operating
-system (see GC_add_roots).  Code for doing this under SunOS, IRIX 5.X and 6.X,
-HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default.  (See
-README.win32 for win32 details.)  On other systems pointers from dynamic
-library data areas may not be considered by the collector.
-If you're writing a program that depends on the collector scanning
-dynamic library data areas, it may be a good idea to include at least
-one call to GC_is_visible() to ensure that those areas are visible
-to the collector.
-
-  Note that the garbage collector does not need to be informed of shared
-read-only data.  However if the shared library mechanism can introduce
-discontiguous data areas that may contain pointers, then the collector does
-need to be informed.
-
-  Signal processing for most signals may be deferred during collection,
-and during uninterruptible parts of the allocation process.
-Like standard ANSI C mallocs, by default it is unsafe to invoke
-malloc (and other GC routines) from a signal handler while another
-malloc call may be in progress. Removing -DNO_SIGNALS from Makefile
-attempts to remedy that.  But that may not be reliable with a compiler that
-substantially reorders memory operations inside GC_malloc.
-
-  The allocator/collector can also be configured for thread-safe operation.
-(Full signal safety can also be achieved, but only at the cost of two system
-calls per malloc, which is usually unacceptable.)
-WARNING: the collector does not guarantee to scan thread-local storage
-(e.g. of the kind accessed with pthread_getspecific()).  The collector
-does scan thread stacks, though, so generally the best solution is to
-ensure that any pointers stored in thread-local storage are also
-stored on the thread's stack for the duration of their lifetime.
-(This is arguably a longstanding bug, but it hasn't been fixed yet.)
-
-INSTALLATION AND PORTABILITY
-
-  As distributed, the macro SILENT is defined in Makefile.
-In the event of problems, this can be removed to obtain a moderate
-amount of descriptive output for each collection.
-(The given statistics exhibit a few peculiarities.
-Things don't appear to add up for a variety of reasons, most notably
-fragmentation losses.  These are probably much more significant for the
-contrived program "test.c" than for your application.)
-
-  Note that typing "make test" will automatically build the collector
-and then run setjmp_test and gctest. Setjmp_test will give you information
-about configuring the collector, which is useful primarily if you have
-a machine that's not already supported.  Gctest is a somewhat superficial
-test of collector functionality.  Failure is indicated by a core dump or
-a message to the effect that the collector is broken.  Gctest takes about 
-35 seconds to run on a SPARCstation 2. It may use up to 8 MB of memory.  (The
-multi-threaded version will use more.  64-bit versions may use more.)
-"Make test" will also, as its last step, attempt to build and test the
-"cord" string library.  This will fail without an ANSI C compiler, but
-the garbage collector itself should still be usable.
-
-  The Makefile will generate a library gc.a which you should link against.
-Typing "make cords" will add the cord library to gc.a.
-Note that this requires an ANSI C compiler.
-
-  It is suggested that if you need to replace a piece of the collector
-(e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the
-ld command line, rather than replacing the one in gc.a.  (This will
-generate numerous warnings under some versions of AIX, but it still
-works.)
-
-  All include files that need to be used by clients will be put in the
-include subdirectory.  (Normally this is just gc.h.  "Make cords" adds
-"cord.h" and "ec.h".)
-
-  The collector currently is designed to run essentially unmodified on
-machines that use a flat 32-bit or 64-bit address space.
-That includes the vast majority of Workstations and X86 (X >= 3) PCs.
-(The list here was deleted because it was getting too long and constantly
-out of date.)
-  It does NOT run under plain 16-bit DOS or Windows 3.X.  There are however
-various packages (e.g. win32s, djgpp) that allow flat 32-bit address
-applications to run under those systemsif the have at least an 80386 processor,
-and several of those are compatible with the collector.
-
-  In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile
-or equivalent is supplied.  Many of these have separate README.system
-files.
-
-  Dynamic libraries are completely supported only under SunOS
-(and even that support is not functional on the last Sun 3 release),
-Linux, IRIX 5&6, HP-PA, Win32 (not Win32S) and OSF/1 on DEC AXP machines.
-On other machines we recommend that you do one of the following:
-
-  1) Add dynamic library support (and send us the code).
-  2) Use static versions of the libraries.
-  3) Arrange for dynamic libraries to use the standard malloc.
-     This is still dangerous if the library stores a pointer to a
-     garbage collected object.  But nearly all standard interfaces
-     prohibit this, because they deal correctly with pointers
-     to stack allocated objects.  (Strtok is an exception.  Don't
-     use it.)
-
-  In all cases we assume that pointer alignment is consistent with that
-enforced by the standard C compilers.  If you use a nonstandard compiler
-you may have to adjust the alignment parameters defined in gc_priv.h.
-
-  A port to a machine that is not byte addressed, or does not use 32 bit
-or 64 bit addresses will require a major effort.  A port to plain MSDOS
-or win16 is hard.
-
-  For machines not already mentioned, or for nonstandard compilers, the
-following are likely to require change:
-
-1.  The parameters in gcconfig.h.
-      The parameters that will usually require adjustment are
-   STACKBOTTOM,  ALIGNMENT and DATASTART.  Setjmp_test
-   prints its guesses of the first two.
-      DATASTART should be an expression for computing the
-   address of the beginning of the data segment.  This can often be
-   &etext.  But some memory management units require that there be
-   some unmapped space between the text and the data segment.  Thus
-   it may be more complicated.   On UNIX systems, this is rarely
-   documented.  But the adb "$m" command may be helpful.  (Note
-   that DATASTART will usually be a function of &etext.  Thus a
-   single experiment is usually insufficient.)
-     STACKBOTTOM is used to initialize GC_stackbottom, which
-   should be a sufficient approximation to the coldest stack address.
-   On some machines, it is difficult to obtain such a value that is
-   valid across a variety of MMUs, OS releases, etc.  A number of
-   alternatives exist for using the collector in spite of this.  See the
-   discussion in gcconfig.h immediately preceding the various
-   definitions of STACKBOTTOM.
-   
-2.  mach_dep.c.
-      The most important routine here is one to mark from registers.
-    The distributed file includes a generic hack (based on setjmp) that
-    happens to work on many machines, and may work on yours.  Try
-    compiling and running setjmp_t.c to see whether it has a chance of
-    working.  (This is not correct C, so don't blame your compiler if it
-    doesn't work.  Based on limited experience, register window machines
-    are likely to cause trouble.  If your version of setjmp claims that
-    all accessible variables, including registers, have the value they
-    had at the time of the longjmp, it also will not work.  Vanilla 4.2 BSD
-    on Vaxen makes such a claim.  SunOS does not.)
-      If your compiler does not allow in-line assembly code, or if you prefer
-    not to use such a facility, mach_dep.c may be replaced by a .s file
-    (as we did for the MIPS machine and the PC/RT).
-      At this point enough architectures are supported by mach_dep.c
-    that you will rarely need to do more than adjust for assembler
-    syntax.
-
-3.  os_dep.c (and gc_priv.h).
-  	  Several kinds of operating system dependent routines reside here.
-  	Many are optional.  Several are invoked only through corresponding
-  	macros in gc_priv.h, which may also be redefined as appropriate.
-      The routine GC_register_data_segments is crucial.  It registers static
-    data areas that must be traversed by the collector. (User calls to
-    GC_add_roots may sometimes be used for similar effect.)
-      Routines to obtain memory from the OS also reside here.
-    Alternatively this can be done entirely by the macro GET_MEM
-    defined in gc_priv.h.  Routines to disable and reenable signals
-    also reside here if they are need by the macros DISABLE_SIGNALS
-    and ENABLE_SIGNALS defined in gc_priv.h.
-      In a multithreaded environment, the macros LOCK and UNLOCK
-    in gc_priv.h will need to be suitably redefined.
-      The incremental collector requires page dirty information, which
-    is acquired through routines defined in os_dep.c.  Unless directed
-    otherwise by gcconfig.h, these are implemented as stubs that simply
-    treat all pages as dirty.  (This of course makes the incremental
-    collector much less useful.)
-
-4.  dyn_load.c
-	This provides a routine that allows the collector to scan data
-	segments associated with dynamic libraries.  Often it is not
-	necessary to provide this routine unless user-written dynamic
-	libraries are used.
-
-  For a different version of UN*X or different machines using the
-Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture,
-it should frequently suffice to change definitions in gcconfig.h.
-
-
-THE C INTERFACE TO THE ALLOCATOR
-
-  The following routines are intended to be directly called by the user.
-Note that usually only GC_malloc is necessary.  GC_clear_roots and GC_add_roots
-calls may be required if the collector has to trace from nonstandard places
-(e.g. from dynamic library data areas on a machine on which the 
-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.
-
-Cautions:
-1. Arrays allocated without new placement syntax are
-allocated as uncollectable objects.  They are traced by the
-collector, but will not be reclaimed.
-
-2. Failure to use "make c++" in combination with (1) will
-result in arrays allocated using the default new operator.
-This is likely to result in disaster without linker warnings.
-
-3. If your compiler supports an overloaded new[] operator,
-then gc_cpp.cc and gc_cpp.h should be suitably modified.
-
-4. Many current C++ compilers have deficiencies that
-break some of the functionality.  See the comments in gc_cpp.h
-for suggested workarounds.
-
-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.
-  Productive use of this facility normally involves redefining report_leak
-to do something more intelligent.  This typically requires annotating
-objects with additional information (e.g. creation time stack trace) that
-identifies their origin.  Such code is typically not very portable, and is
-not included here, except on SPARC machines.
-  If all objects are allocated with GC_DEBUG_MALLOC (see next section),
-then the default version of report_leak will report the source file
-and line number at which the leaked object was allocated.  This may
-sometimes be sufficient.  (On SPARC/SUNOS4 machines, it will also report
-a cryptic stack trace.  This can often be turned 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 boehm@acm.org
-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.
-(On SPARCstation 2s collection times will be on the order of 300 msecs
-per MB of accessible memory that needs to be scanned.  Your mileage
-may vary.)  The incremental/generational collection facility helps,
-but is portable only if "stubborn" allocation is used.
-  Please address bug reports to boehm@acm.org.  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).
-