path: root/README.md

# Boehm-Demers-Weiser Garbage Collector

This is version 7.5.0 (next release development) of a conservative garbage
collector for C and C++.

You might find a more recent version
[here](http://www.hboehm.info/gc/), or
[here](https://github.com/ivmai/bdwgc).


## 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.

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](http://www.hboehm.info/gc/leak.html) are included,
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
[here](http://www.hboehm.info/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 doc/debugging.html 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 collectible 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 doc/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.

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 Unix-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
`make -f Makefile.direct`).

Please note that the collector source repository does not contain configure
and similar auto-generated files, thus the full procedure of autoconf-based
build of `master` branch of the collector (using `master` branch of
libatomic_ops source repository as well) could look like:

    git clone git://github.com/ivmai/bdwgc.git
    cd bdwgc
    git clone git://github.com/ivmai/libatomic_ops.git
    autoreconf -vif
    automake --add-missing
    ./configure
    make
    make check

Below we focus on the collector build using classic 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 doc/porting.html.


## 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 if and only if 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 100 Kbytes
       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 it
default value of 3 to use less space and more collection time, or down for
the opposite effect.  Setting it to 1 will almost 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 symbolic stack
trace by invoking program "foo" with "tools/callprocs.sh foo".  It 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 pointer-free
(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.

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 [here](mailto:bdwgc@lists.opendylan.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).


## Copyright & Warranty

 * 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.