mirror of
https://github.com/autc04/Retro68.git
synced 2024-11-28 05:51:04 +00:00
249 lines
12 KiB
HTML
249 lines
12 KiB
HTML
<!DOCTYPE HTML>
|
|
<HEAD>
|
|
<TITLE>Garbage Collector Interface</TITLE>
|
|
</HEAD>
|
|
<BODY>
|
|
<H1>C Interface</h1>
|
|
On many platforms, a single-threaded garbage collector library can be built
|
|
to act as a plug-in malloc replacement.
|
|
(Build with <TT>-DREDIRECT_MALLOC=GC_malloc -DIGNORE_FREE</tt>.)
|
|
This is often the best way to deal with third-party libraries
|
|
which leak or prematurely free objects. <TT>-DREDIRECT_MALLOC</tt> is intended
|
|
primarily as an easy way to adapt old code, not for new development.
|
|
<P>
|
|
New code should use the interface discussed below.
|
|
<P>
|
|
Code must be linked against the GC library. On most UNIX platforms,
|
|
depending on how the collector is built, this will be <TT>gc.a</tt>
|
|
or <TT>libgc.{a,so}</tt>.
|
|
<P>
|
|
The following describes the standard C interface to the garbage collector.
|
|
It is not a complete definition of the interface. It describes only the
|
|
most commonly used functionality, approximately in decreasing order of
|
|
frequency of use.
|
|
The full interface is described in
|
|
<A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a>
|
|
or <TT>gc.h</tt> in the distribution.
|
|
<P>
|
|
Clients should include <TT>gc.h</tt>.
|
|
<P>
|
|
In the case of multithreaded code,
|
|
<TT>gc.h</tt> should be included after the threads header file, and
|
|
after defining the appropriate <TT>GC_</tt><I>XXXX</i><TT>_THREADS</tt> macro.
|
|
(For 6.2alpha4 and later, simply defining <TT>GC_THREADS</tt> should suffice.)
|
|
The header file <TT>gc.h</tt> must be included
|
|
in files that use either GC or threads primitives, since threads primitives
|
|
will be redefined to cooperate with the GC on many platforms.
|
|
<DL>
|
|
<DT> <B>void * GC_MALLOC(size_t <I>nbytes</i>)</b>
|
|
<DD>
|
|
Allocates and clears <I>nbytes</i> of storage.
|
|
Requires (amortized) time proportional to <I>nbytes</i>.
|
|
The resulting object will be automatically deallocated when unreferenced.
|
|
References from objects allocated with the system malloc are usually not
|
|
considered by the collector. (See <TT>GC_MALLOC_UNCOLLECTABLE</tt>, however.)
|
|
<TT>GC_MALLOC</tt> is a macro which invokes <TT>GC_malloc</tt> by default or,
|
|
if <TT>GC_DEBUG</tt>
|
|
is defined before <TT>gc.h</tt> is included, a debugging version that checks
|
|
occasionally for overwrite errors, and the like.
|
|
<DT> <B>void * GC_MALLOC_ATOMIC(size_t <I>nbytes</i>)</b>
|
|
<DD>
|
|
Allocates <I>nbytes</i> of storage.
|
|
Requires (amortized) time proportional to <I>nbytes</i>.
|
|
The resulting object will be automatically deallocated when unreferenced.
|
|
The client promises that the resulting object will never contain any pointers.
|
|
The memory is not cleared.
|
|
This is the preferred way to allocate strings, floating point arrays,
|
|
bitmaps, etc.
|
|
More precise information about pointer locations can be communicated to the
|
|
collector using the interface in
|
|
<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc_typedh.txt">gc_typed.h</a> in the distribution.
|
|
<DT> <B>void * GC_MALLOC_UNCOLLECTABLE(size_t <I>nbytes</i>)</b>
|
|
<DD>
|
|
Identical to <TT>GC_MALLOC</tt>,
|
|
except that the resulting object is not automatically
|
|
deallocated. Unlike the system-provided malloc, the collector does
|
|
scan the object for pointers to garbage-collectable memory, even if the
|
|
block itself does not appear to be reachable. (Objects allocated in this way
|
|
are effectively treated as roots by the collector.)
|
|
<DT> <B> void * GC_REALLOC(void *<I>old</i>, size_t <I>new_size</i>) </b>
|
|
<DD>
|
|
Allocate a new object of the indicated size and copy (a prefix of) the
|
|
old object into the new object. The old object is reused in place if
|
|
convenient. If the original object was allocated with
|
|
<TT>GC_MALLOC_ATOMIC</tt>,
|
|
the new object is subject to the same constraints. If it was allocated
|
|
as an uncollectable object, then the new object is uncollectable, and
|
|
the old object (if different) is deallocated.
|
|
<DT> <B> void GC_FREE(void *<I>dead</i>) </b>
|
|
<DD>
|
|
Explicitly deallocate an object. Typically not useful for small
|
|
collectable objects.
|
|
<DT> <B> void * GC_MALLOC_IGNORE_OFF_PAGE(size_t <I>nbytes</i>) </b>
|
|
<DD>
|
|
<DT> <B> void * GC_MALLOC_ATOMIC_IGNORE_OFF_PAGE(size_t <I>nbytes</i>) </b>
|
|
<DD>
|
|
Analogous to <TT>GC_MALLOC</tt> and <TT>GC_MALLOC_ATOMIC</tt>,
|
|
except that the client
|
|
guarantees that as long
|
|
as the resulting object is of use, a pointer is maintained to someplace
|
|
inside the first 512 bytes of the object. This pointer should be declared
|
|
volatile to avoid interference from compiler optimizations.
|
|
(Other nonvolatile pointers to the object may exist as well.)
|
|
This is the
|
|
preferred way to allocate objects that are likely to be > 100KBytes in size.
|
|
It greatly reduces the risk that such objects will be accidentally retained
|
|
when they are no longer needed. Thus space usage may be significantly reduced.
|
|
<DT> <B> void GC_INIT(void) </b>
|
|
<DD>
|
|
On some platforms, it is necessary to invoke this
|
|
<I>from the main executable, not from a dynamic library,</i> before
|
|
the initial invocation of a GC routine. It is recommended that this be done
|
|
in portable code, though we try to ensure that it expands to a no-op
|
|
on as many platforms as possible.
|
|
<DT> <B> void GC_gcollect(void) </b>
|
|
<DD>
|
|
Explicitly force a garbage collection.
|
|
<DT> <B> void GC_enable_incremental(void) </b>
|
|
<DD>
|
|
Cause the garbage collector to perform a small amount of work
|
|
every few invocations of <TT>GC_MALLOC</tt> or the like, instead of performing
|
|
an entire collection at once. This is likely to increase total
|
|
running time. It will improve response on a platform that either has
|
|
suitable support in the garbage collector (Linux and most Unix
|
|
versions, win32 if the collector was suitably built) or if "stubborn"
|
|
allocation is used (see
|
|
<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a>).
|
|
On many platforms this interacts poorly with system calls
|
|
that write to the garbage collected heap.
|
|
<DT> <B> GC_warn_proc GC_set_warn_proc(GC_warn_proc <I>p</i>) </b>
|
|
<DD>
|
|
Replace the default procedure used by the collector to print warnings.
|
|
The collector
|
|
may otherwise write to sterr, most commonly because GC_malloc was used
|
|
in a situation in which GC_malloc_ignore_off_page would have been more
|
|
appropriate. See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> for details.
|
|
<DT> <B> void GC_REGISTER_FINALIZER(...) </b>
|
|
<DD>
|
|
Register a function to be called when an object becomes inaccessible.
|
|
This is often useful as a backup method for releasing system resources
|
|
(<I>e.g.</i> closing files) when the object referencing them becomes
|
|
inaccessible.
|
|
It is not an acceptable method to perform actions that must be performed
|
|
in a timely fashion.
|
|
See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> for details of the interface.
|
|
See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/finalization.html">here</a> for a more detailed discussion
|
|
of the design.
|
|
<P>
|
|
Note that an object may become inaccessible before client code is done
|
|
operating on objects referenced by its fields.
|
|
Suitable synchronization is usually required.
|
|
See <A HREF="http://portal.acm.org/citation.cfm?doid=604131.604153">here</a>
|
|
or <A HREF="http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html">here</a>
|
|
for details.
|
|
</dl>
|
|
<P>
|
|
If you are concerned with multiprocessor performance and scalability,
|
|
you should consider enabling and using thread local allocation (<I>e.g.</i>
|
|
<TT>GC_LOCAL_MALLOC</tt>, see <TT>gc_local_alloc.h</tt>. If your platform
|
|
supports it, you should build the collector with parallel marking support
|
|
(<TT>-DPARALLEL_MARK</tt>, or <TT>--enable-parallel-mark</tt>).
|
|
<P>
|
|
If the collector is used in an environment in which pointer location
|
|
information for heap objects is easily available, this can be passed on
|
|
to the collector using the interfaces in either <TT>gc_typed.h</tt>
|
|
or <TT>gc_gcj.h</tt>.
|
|
<P>
|
|
The collector distribution also includes a <B>string package</b> that takes
|
|
advantage of the collector. For details see
|
|
<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/cordh.txt">cord.h</a>
|
|
|
|
<H1>C++ Interface</h1>
|
|
Usage of the collector from C++ is complicated by the fact that there
|
|
are many "standard" ways to allocate memory in C++. The default ::new
|
|
operator, default malloc, and default STL allocators allocate memory
|
|
that is not garbage collected, and is not normally "traced" by the
|
|
collector. This means that any pointers in memory allocated by these
|
|
default allocators will not be seen by the collector. Garbage-collectable
|
|
memory referenced only by pointers stored in such default-allocated
|
|
objects is likely to be reclaimed prematurely by the collector.
|
|
<P>
|
|
It is the programmers responsibility to ensure that garbage-collectable
|
|
memory is referenced by pointers stored in one of
|
|
<UL>
|
|
<LI> Program variables
|
|
<LI> Garbage-collected objects
|
|
<LI> Uncollected but "traceable" objects
|
|
</ul>
|
|
"Traceable" objects are not necessarily reclaimed by the collector,
|
|
but are scanned for pointers to collectable objects.
|
|
They are allocated by <TT>GC_MALLOC_UNCOLLECTABLE</tt>, as described
|
|
above, and through some interfaces described below.
|
|
<P>
|
|
The easiest way to ensure that collectable objects are properly referenced
|
|
is to allocate only collectable objects. This requires that every
|
|
allocation go through one of the following interfaces, each one of
|
|
which replaces a standard C++ allocation mechanism:
|
|
<DL>
|
|
<DT> <B> STL allocators </b>
|
|
<DD>
|
|
Users of the <A HREF="http://www.sgi.com/tech/stl">SGI extended STL</a>
|
|
can include <TT>new_gc_alloc.h</tt> before including
|
|
STL header files.
|
|
(<TT>gc_alloc.h</tt> corresponds to now obsolete versions of the
|
|
SGI STL.)
|
|
This defines SGI-style allocators
|
|
<UL>
|
|
<LI> alloc
|
|
<LI> single_client_alloc
|
|
<LI> gc_alloc
|
|
<LI> single_client_gc_alloc
|
|
</ul>
|
|
which may be used either directly to allocate memory or to instantiate
|
|
container templates. The first two allocate uncollectable but traced
|
|
memory, while the second two allocate collectable memory.
|
|
The single_client versions are not safe for concurrent access by
|
|
multiple threads, but are faster.
|
|
<P>
|
|
For an example, click <A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_alloc_exC.txt">here</a>.
|
|
<P>
|
|
Recent versions of the collector also include a more standard-conforming
|
|
allocator implementation in <TT>gc_allocator.h</tt>. It defines
|
|
<UL>
|
|
<LI> traceable_allocator
|
|
<LI> gc_allocator
|
|
</ul>
|
|
Again the former allocates uncollectable but traced memory.
|
|
This should work with any fully standard-conforming C++ compiler.
|
|
<DT> <B> Class inheritance based interface </b>
|
|
<DD>
|
|
Users may include gc_cpp.h and then cause members of classes to
|
|
be allocated in garbage collectable memory by having those classes
|
|
inherit from class gc.
|
|
For details see <A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc_cpph.txt">gc_cpp.h</a>.
|
|
<P>
|
|
Linking against libgccpp in addition to the gc library overrides
|
|
::new (and friends) to allocate traceable memory but uncollectable
|
|
memory, making it safe to refer to collectable objects from the resulting
|
|
memory.
|
|
<DT> <B> C interface </b>
|
|
<DD>
|
|
It is also possible to use the C interface from
|
|
<A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> directly.
|
|
On platforms which use malloc to implement ::new, it should usually be possible
|
|
to use a version of the collector that has been compiled as a malloc
|
|
replacement. It is also possible to replace ::new and other allocation
|
|
functions suitably, as is done by libgccpp.
|
|
<P>
|
|
Note that user-implemented small-block allocation often works poorly with
|
|
an underlying garbage-collected large block allocator, since the collector
|
|
has to view all objects accessible from the user's free list as reachable.
|
|
This is likely to cause problems if <TT>GC_MALLOC</tt>
|
|
is used with something like
|
|
the original HP version of STL.
|
|
This approach works well with the SGI versions of the STL only if the
|
|
<TT>malloc_alloc</tt> allocator is used.
|
|
</dl>
|
|
</body>
|
|
</html>
|