summaryrefslogtreecommitdiff
path: root/doc/README
diff options
context:
space:
mode:
Diffstat (limited to 'doc/README')
-rw-r--r--doc/README617
1 files changed, 617 insertions, 0 deletions
diff --git a/doc/README b/doc/README
new file mode 100644
index 0000000..c6b48c3
--- /dev/null
+++ b/doc/README
@@ -0,0 +1,617 @@
+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).
+