========================================================================
* README.QUICK
========================================================================

Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers
Copyright (c) 1991-1995 by Xerox Corporation.  All rights reserved.
Copyright (c) 1996-1999 by Silicon Graphics.  All rights reserved.
Copyright (c) 1999-2001 by Hewlett-Packard. All rights reserved.

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 files have other copyright holders. A few of the files needed
to use the GNU-style build procedure come with a modified GPL license
that appears not to significantly restrict use of the collector, though
use of those files for a purpose other than building the collector may
require the resulting code to be covered by the GPL.

For more details and the names of other contributors, see the
doc/README* files and include/gc.h.  This file describes typical use of
the collector on a machine that is already supported.

For the version number, see doc/README or version.h.

INSTALLATION:
Under UN*X, Linux:
Alternative 1 (the old way): type "make test" in this directory.
        Link against gc.a.  With the most recent GC distributions
        you may have to type "make -f Makefile.direct test" or
        copy Makefile.direct to Makefile first.

Alternative 2 (the new way): type
        "./configure --prefix=<dir>; make; make check; make install".
        Link against <dir>/lib/libgc.a or <dir>/lib/libgc.so.
        See README.autoconf for details

Under Windows 95, 98, Me, NT, or 2000:
copy the appropriate makefile to MAKEFILE, read it, and type "nmake test".
(Under Windows, this assumes you have Microsoft command-line tools
installed, and suitably configured.)
Read the machine specific README in the doc directory if one exists.

If you need thread support, you will need to follow the special
platform-dependent instructions (win32), or define GC_THREADS
as described in doc/README.macros, or possibly use
--enable-threads=posix when running the configure script.

If you wish to use the cord (structured string) library with the stand-alone
Makefile.direct, type "make cords", after copying to "Makefile".
(This requires an ANSI C compiler.  You may
need to redefine CC in the Makefile. The CORD_printf implementation in
cordprnt.c is known to be less than perfectly portable.  The rest of the
package should still work.)

If you wish to use the collector from C++, type "make c++", or use
--enable-cplusplus with the configure script.   With Makefile.direct,
these ones add further files to gc.a and to the include subdirectory.
With the alternate build process,this generates libgccpp.
See cord/cord.h and include/gc_cpp.h.

TYPICAL USE:
Include "gc.h" from the include subdirectory.  Link against the
appropriate library ("gc.a" under UN*X).  Replace calls to malloc
by calls to GC_MALLOC, and calls to realloc by calls to GC_REALLOC.
If the object is known to never contain pointers, use GC_MALLOC_ATOMIC
instead of GC_MALLOC.

Define GC_DEBUG before including gc.h for additional checking.

More documentation on the collector interface can be found at
http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcinterface.html,
in doc/README and other files in the doc directory, and in include/gc.h .

WARNINGS:

Do not store the only pointer to an object in memory allocated
with system malloc, since the collector usually does not scan
memory allocated in this way.

Use with threads may be supported on your system, but requires the
collector to be built with thread support.  See Makefile.  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.

========================================================================
* doc/README
========================================================================

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) 1999-2011 by Hewlett-Packard Development Company.

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.  The files included in the libatomic_ops distribution (included
here) use either the license below, or a similar MIT-style license,
or, for some files not actually used by the garbage-collector library, the
GPL.

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.)

The atomic_ops library contains some code that is covered by the GNU General
Public License, but is not needed by, nor linked into the collector library.
It is included here only because the atomic_ops distribution is, for
simplicity, included in its entirety.

This is version 7.2d 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 fewer 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 documentation 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, or GC_all_interior_pointers is otherwise
set, as is now the default.

Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention
of garbage objects, by requiring pointers from the heap to the beginning
of an object.  But this no longer appears to be a significant
issue for most programs occupying a small fraction of the possible
address space.

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 probability 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 collector operates silently
In the event of problems, this can usually be changed by defining the
GC_PRINT_STATS or GC_PRINT_VERBOSE_STATS environment variables.  This
will result in a few lines 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.)

  On most Un*x-like platforms, the collector can be built either using a
GNU autoconf-based build infrastructure (type "configure; make" in the
simplest case), or with a classic makefile by itself (type
"cp Makefile.direct Makefile; make").  Here we focus on the latter option.
On other platforms, typically only the latter option is available, though
with a different supplied Makefile.)

  For the Makefile.direct-based process, typing "make test" instead of "make"
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 a second to two to run on reasonable 2007 vintage desktops.  It may
use up to about 30MB 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.)

  Makefile.direct will generate a library gc.a which you should link against.
Typing "make cords" will add the cord library to gc.a.

  The GNU style build process understands the usual targets.  "Make check"
runs a number of tests.  "Make install" installs at least libgc, and libcord.
Try "./configure --help" to see the configuration options.  It is currently
not possible to exercise all combinations of build options this way.

  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.)

  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,
some porting suggestions are provided in the "porting.html" file.

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 normally be declared volatile to prevent
          interference from compiler optimizations.)  This is the recommended
          way to allocate anything that is likely to be larger than 100Kbytes
          or so.  (GC_malloc may result in failure to reclaim such objects.)

7)  GC_set_warn_proc(proc)
        - Can be used to redirect warnings from the collector.  Such warnings
          should be rare, and should not be ignored during code development.

8) GC_enable_incremental()
    - Enables generational and incremental collection.  Useful for large
      heaps on machines that provide access to page dirty information.
      Some dirty bit implementations may interfere with debugging
      (by catching address faults) and place restrictions on heap arguments
      to system calls (since write faults inside a system call may not be
      handled well).

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

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

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

  Some additional tuning is possible through the parameters defined
near the top of gc_priv.h.

  If only GC_malloc is intended to be used, it might be appropriate to define:

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

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

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

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

THE C++ INTERFACE TO THE ALLOCATOR:

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

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

USE AS LEAK DETECTOR:

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

DEBUGGING FACILITIES:

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

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

  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 probability 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 gc@linux.hpl.hp.com
for suggestions on how to fix your compiler.
  This is not a real-time collector.  In the standard configuration,
percentage of time required for collection should be constant across
heap sizes.  But collection pauses will increase for larger heaps.
They will decrease with the number of processors if parallel marking
is enabled.
(On 2007 vintage machines, GC times may be on the order of 5 msecs
per MB of accessible memory that needs to be scanned and processor.
Your mileage may vary.)  The incremental/generational collection facility
may help in some cases.
  Please address bug reports to gc@linux.hpl.hp.com.  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).

========================================================================
* doc/README.cords
========================================================================

Copyright (c) 1993-1994 by Xerox Corporation.  All rights reserved.

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.

Please send bug reports to Hans-J. Boehm (Hans.Boehm@hp.com) or,
preferred, to GC mailing list (gc@linux.hpl.hp.com).

This is a string packages that uses a tree-based representation.
See cord.h for a description of the functions provided.  Ec.h describes
"extensible cords", which are essentially output streams that write
to a cord.  These allow for efficient construction of cords without
requiring a bound on the size of a cord.

More details on the data structure can be found in

Boehm, Atkinson, and Plass, "Ropes: An Alternative to Strings",
Software Practice and Experience 25, 12, December 1995, pp. 1315-1330.

A fundamentally similar "rope" data structure is also part of SGI's standard
template library implementation, and its descendents, which include the
GNU C++ library.  That uses reference counting by default.
There is a short description of that data structure at
http://reality.sgi.com/boehm/ropeimpl.html .  (The more official location
http://www.sgi.com/tech/stl/ropeimpl.html is missing a figure.)

All of these are descendents of the "ropes" in Xerox Cedar.

de.c is a very dumb text editor that illustrates the use of cords.
It maintains a list of file versions.  Each version is simply a
cord representing the file contents.  Nonetheless, standard
editing operations are efficient, even on very large files.
(Its 3 line "user manual" can be obtained by invoking it without
arguments.  Note that ^R^N and ^R^P move the cursor by
almost a screen.  It does not understand tabs, which will show
up as highlighred "I"s.  Use the UNIX "expand" program first.)
To build the editor, type "make cord/de" in the gc directory.

This package assumes an ANSI C compiler such as gcc.  It will
not compile with an old-style K&R compiler.

Note that CORD_printf iand friends use C functions with variable numbers
of arguments in non-standard-conforming ways.  This code is known to
break on some platforms, notably PowerPC.  It should be possible to
build the remainder of the library (everything but cordprnt.c) on
any platform that supports the collector.

========================================================================
* libatomic_ops/COPYING, libatomic_ops/doc/COPYING
========================================================================

		    GNU GENERAL PUBLIC LICENSE
		       Version 2, June 1991

 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 Everyone is permitted to copy and distribute verbatim copies
 of this license document, but changing it is not allowed.

			    Preamble

  The licenses for most software are designed to take away your
freedom to share and change it.  By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software--to make sure the software is free for all its users.  This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it.  (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.)  You can apply it to
your programs, too.

  When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.

  To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.

  For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have.  You must make sure that they, too, receive or can get the
source code.  And you must show them these terms so they know their
rights.

  We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.

  Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software.  If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.

  Finally, any free program is threatened constantly by software
patents.  We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary.  To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.

  The precise terms and conditions for copying, distribution and
modification follow.

		    GNU GENERAL PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License.  The "Program", below,
refers to any such program or work, and a "work based on the Program"
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language.  (Hereinafter, translation is included without limitation in
the term "modification".)  Each licensee is addressed as "you".

Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope.  The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.

  1. You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.

  2. You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

    a) You must cause the modified files to carry prominent notices
    stating that you changed the files and the date of any change.

    b) You must cause any work that you distribute or publish, that in
    whole or in part contains or is derived from the Program or any
    part thereof, to be licensed as a whole at no charge to all third
    parties under the terms of this License.

    c) If the modified program normally reads commands interactively
    when run, you must cause it, when started running for such
    interactive use in the most ordinary way, to print or display an
    announcement including an appropriate copyright notice and a
    notice that there is no warranty (or else, saying that you provide
    a warranty) and that users may redistribute the program under
    these conditions, and telling the user how to view a copy of this
    License.  (Exception: if the Program itself is interactive but
    does not normally print such an announcement, your work based on
    the Program is not required to print an announcement.)

These requirements apply to the modified work as a whole.  If
identifiable sections of that work are not derived from the Program,
and can be reasonably considered independent and separate works in
themselves, then this License, and its terms, do not apply to those
sections when you distribute them as separate works.  But when you
distribute the same sections as part of a whole which is a work based
on the Program, the distribution of the whole must be on the terms of
this License, whose permissions for other licensees extend to the
entire whole, and thus to each and every part regardless of who wrote it.

Thus, it is not the intent of this section to claim rights or contest
your rights to work written entirely by you; rather, the intent is to
exercise the right to control the distribution of derivative or
collective works based on the Program.

In addition, mere aggregation of another work not based on the Program
with the Program (or with a work based on the Program) on a volume of
a storage or distribution medium does not bring the other work under
the scope of this License.

  3. You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms of
Sections 1 and 2 above provided that you also do one of the following:

    a) Accompany it with the complete corresponding machine-readable
    source code, which must be distributed under the terms of Sections
    1 and 2 above on a medium customarily used for software interchange; or,

    b) Accompany it with a written offer, valid for at least three
    years, to give any third party, for a charge no more than your
    cost of physically performing source distribution, a complete
    machine-readable copy of the corresponding source code, to be
    distributed under the terms of Sections 1 and 2 above on a medium
    customarily used for software interchange; or,

    c) Accompany it with the information you received as to the offer
    to distribute corresponding source code.  (This alternative is
    allowed only for noncommercial distribution and only if you
    received the program in object code or executable form with such
    an offer, in accord with Subsection b above.)

The source code for a work means the preferred form of the work for
making modifications to it.  For an executable work, complete source
code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to
control compilation and installation of the executable.  However, as a
special exception, the source code distributed need not include
anything that is normally distributed (in either source or binary
form) with the major components (compiler, kernel, and so on) of the
operating system on which the executable runs, unless that component
itself accompanies the executable.

If distribution of executable or object code is made by offering
access to copy from a designated place, then offering equivalent
access to copy the source code from the same place counts as
distribution of the source code, even though third parties are not
compelled to copy the source along with the object code.

  4. You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.
However, parties who have received copies, or rights, from you under
this License will not have their licenses terminated so long as such
parties remain in full compliance.

  5. You are not required to accept this License, since you have not
signed it.  However, nothing else grants you permission to modify or
distribute the Program or its derivative works.  These actions are
prohibited by law if you do not accept this License.  Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so, and
all its terms and conditions for copying, distributing or modifying
the Program or works based on it.

  6. Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions.  You may not impose any further
restrictions on the recipients' exercise of the rights granted herein.
You are not responsible for enforcing compliance by third parties to
this License.

  7. If, as a consequence of a court judgment or allegation of patent
infringement or for any other reason (not limited to patent issues),
conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot
distribute so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you
may not distribute the Program at all.  For example, if a patent
license would not permit royalty-free redistribution of the Program by
all those who receive copies directly or indirectly through you, then
the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.

If any portion of this section is held invalid or unenforceable under
any particular circumstance, the balance of the section is intended to
apply and the section as a whole is intended to apply in other
circumstances.

It is not the purpose of this section to induce you to infringe any
patents or other property right claims or to contest validity of any
such claims; this section has the sole purpose of protecting the
integrity of the free software distribution system, which is
implemented by public license practices.  Many people have made
generous contributions to the wide range of software distributed
through that system in reliance on consistent application of that
system; it is up to the author/donor to decide if he or she is willing
to distribute software through any other system and a licensee cannot
impose that choice.

This section is intended to make thoroughly clear what is believed to
be a consequence of the rest of this License.

  8. If the distribution and/or use of the Program is restricted in
certain countries either by patents or by copyrighted interfaces, the
original copyright holder who places the Program under this License
may add an explicit geographical distribution limitation excluding
those countries, so that distribution is permitted only in or among
countries not thus excluded.  In such case, this License incorporates
the limitation as if written in the body of this License.

  9. The Free Software Foundation may publish revised and/or new versions
of the General Public License from time to time.  Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.

Each version is given a distinguishing version number.  If the Program
specifies a version number of this License which applies to it and "any
later version", you have the option of following the terms and conditions
either of that version or of any later version published by the Free
Software Foundation.  If the Program does not specify a version number of
this License, you may choose any version ever published by the Free Software
Foundation.

  10. If you wish to incorporate parts of the Program into other free
programs whose distribution conditions are different, write to the author
to ask for permission.  For software which is copyrighted by the Free
Software Foundation, write to the Free Software Foundation; we sometimes
make exceptions for this.  Our decision will be guided by the two goals
of preserving the free status of all derivatives of our free software and
of promoting the sharing and reuse of software generally.

			    NO WARRANTY

  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
REPAIR OR CORRECTION.

  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.

		     END OF TERMS AND CONDITIONS

	    How to Apply These Terms to Your New Programs

  If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.

  To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
convey the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.

    <one line to give the program's name and a brief idea of what it does.>
    Copyright (C) <year>  <name of author>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA


Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this
when it starts in an interactive mode:

    Gnomovision version 69, Copyright (C) year  name of author
    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
    This is free software, and you are welcome to redistribute it
    under certain conditions; type `show c' for details.

The hypothetical commands `show w' and `show c' should show the appropriate
parts of the General Public License.  Of course, the commands you use may
be called something other than `show w' and `show c'; they could even be
mouse-clicks or menu items--whatever suits your program.

You should also get your employer (if you work as a programmer) or your
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  Here is a sample; alter the names:

  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
  `Gnomovision' (which makes passes at compilers) written by James Hacker.

  <signature of Ty Coon>, 1 April 1989
  Ty Coon, President of Vice

This General Public License does not permit incorporating your program into
proprietary programs.  If your program is a subroutine library, you may
consider it more useful to permit linking proprietary applications with the
library.  If this is what you want to do, use the GNU Library General
Public License instead of this License.