[Emacs-diffs] Changes to emacs/gc/doc/README [Boehm-versions]

[Dave Love]

Index: emacs/gc/doc/README
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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 6.2alpha6 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/Solaris,
+ (and even that support is not functional on the last Sun 3 release),
+ Linux, FreeBSD, NetBSD, IRIX 5&6, HP/UX, Win32 (not Win32S) and OSF/1
+ on DEC AXP machines plus perhaps a few others listed near the top
+ of dyn_load.c.  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.
+ Note that this may also be an issue with packed records/structs, if those
+ enforce less alignment for pointers.
+ 
+   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 address@hidden
+ 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 address@hidden  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).
+