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| <TITLE>Garbage Collector Interface</TITLE> |
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| <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> |
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