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+<!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 -DREDIRECT_MALLOC=GC_malloc
+-DIGNORE_FREE.) This is often the best way to deal with third-party libraries
+which leak or prematurely free objects. -DREDIRECT_MALLOC 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,
+this will be gc.a.
+<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 description assumes an ANSI C compiler.
+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 gc.h.
+<P>
+In the case of multithreaded code,
+gc.h should be included after the threads header file, and
+after defining the appropriate GC_XXXX_THREADS macro.
+(For 6.2alpha4 and later, simply defining GC_THREADS should suffice.)
+Gc.h 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 GC_MALLOC_UNCOLLECTABLE, however.)
+GC_MALLOC is a macro which invokes GC_malloc by default or, if GC_DEBUG
+is defined before gc.h 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 GC_MALLOC, 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 *old, size_t new_size) </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 GC_malloc_atomic,
+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.
+(Use GC_REALLOC with GC_MALLOC, etc.)
+<DT> <B> void GC_FREE(void *dead) </b>
+<DD>
+Explicitly deallocate an object. Typically not useful for small
+collectable objects. (Use GC_FREE with GC_MALLOC, etc.)
+<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 GC_MALLOC and GC_MALLOC_ATOMIC, 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_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 GC_malloc 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 (Irix and most other 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 p) </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 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>
+GC_LOCAL_MALLOC, see <TT>gc_local_alloc.h</tt>. If your platform
+supports it, you should build the collector with parallel marking support
+(-DPARALLEL_MARK, or --enable-parallel-mark).
+<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 colllector 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>
+There are three distinct ways to use the collector from C++:
+<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 implemention 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 certain classes to
+be allocated in garbage collectable memory by inheriting 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>.
+<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.
+<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 GC_malloc is used with something like
+the original HP version of STL.
+This approach works with the SGI versions of the STL only if the
+<TT>malloc_alloc</tt> allocator is used.
+</dl>
+</body>
+</html>