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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.
+
+The files config.guess and a few others are copyrighted by the Free
+Software Foundation.
+
+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 6.0 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).
+