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<TITLE>Garbage Collector Interface</TITLE> |
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<H1>C Interface</h1> |
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On many platforms, a single-threaded garbage collector library can be built |
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to act as a plug-in malloc replacement. |
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(Build with <TT>-DREDIRECT_MALLOC=GC_malloc -DIGNORE_FREE</tt>.) |
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This is often the best way to deal with third-party libraries |
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which leak or prematurely free objects. <TT>-DREDIRECT_MALLOC</tt> is intended |
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primarily as an easy way to adapt old code, not for new development. |
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<P> |
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New code should use the interface discussed below. |
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<P> |
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Code must be linked against the GC library. On most UNIX platforms, |
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depending on how the collector is built, this will be <TT>gc.a</tt> |
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or <TT>libgc.{a,so}</tt>. |
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<P> |
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The following describes the standard C interface to the garbage collector. |
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It is not a complete definition of the interface. It describes only the |
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most commonly used functionality, approximately in decreasing order of |
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frequency of use. |
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The full interface is described in |
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<A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> |
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or <TT>gc.h</tt> in the distribution. |
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<P> |
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Clients should include <TT>gc.h</tt>. |
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<P> |
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In the case of multithreaded code, |
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<TT>gc.h</tt> should be included after the threads header file, and |
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after defining the appropriate <TT>GC_</tt><I>XXXX</i><TT>_THREADS</tt> macro. |
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(For 6.2alpha4 and later, simply defining <TT>GC_THREADS</tt> should suffice.) |
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The header file <TT>gc.h</tt> must be included |
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in files that use either GC or threads primitives, since threads primitives |
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will be redefined to cooperate with the GC on many platforms. |
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<P> |
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Thread users should also be aware that on many platforms objects reachable |
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only from thread-local variables may be prematurely reclaimed. |
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Thus objects pointed to by thread-local variables should also be pointed to |
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by a globally visible data structure. (This is viewed as a bug, but as |
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one that is exceedingly hard to fix without some libc hooks.) |
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<DL> |
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<DT> <B>void * GC_MALLOC(size_t <I>nbytes</i>)</b> |
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<DD> |
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Allocates and clears <I>nbytes</i> of storage. |
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Requires (amortized) time proportional to <I>nbytes</i>. |
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The resulting object will be automatically deallocated when unreferenced. |
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References from objects allocated with the system malloc are usually not |
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considered by the collector. (See <TT>GC_MALLOC_UNCOLLECTABLE</tt>, however.) |
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<TT>GC_MALLOC</tt> is a macro which invokes <TT>GC_malloc</tt> by default or, |
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if <TT>GC_DEBUG</tt> |
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is defined before <TT>gc.h</tt> is included, a debugging version that checks |
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occasionally for overwrite errors, and the like. |
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<DT> <B>void * GC_MALLOC_ATOMIC(size_t <I>nbytes</i>)</b> |
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<DD> |
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Allocates <I>nbytes</i> of storage. |
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Requires (amortized) time proportional to <I>nbytes</i>. |
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The resulting object will be automatically deallocated when unreferenced. |
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The client promises that the resulting object will never contain any pointers. |
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The memory is not cleared. |
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This is the preferred way to allocate strings, floating point arrays, |
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bitmaps, etc. |
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More precise information about pointer locations can be communicated to the |
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collector using the interface in |
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<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc_typedh.txt">gc_typed.h</a> in the distribution. |
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<DT> <B>void * GC_MALLOC_UNCOLLECTABLE(size_t <I>nbytes</i>)</b> |
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<DD> |
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Identical to <TT>GC_MALLOC</tt>, |
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except that the resulting object is not automatically |
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deallocated. Unlike the system-provided malloc, the collector does |
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scan the object for pointers to garbage-collectable memory, even if the |
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block itself does not appear to be reachable. (Objects allocated in this way |
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are effectively treated as roots by the collector.) |
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<DT> <B> void * GC_REALLOC(void *<I>old</i>, size_t <I>new_size</i>) </b> |
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<DD> |
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Allocate a new object of the indicated size and copy (a prefix of) the |
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old object into the new object. The old object is reused in place if |
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convenient. If the original object was allocated with |
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<TT>GC_MALLOC_ATOMIC</tt>, |
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the new object is subject to the same constraints. If it was allocated |
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as an uncollectable object, then the new object is uncollectable, and |
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the old object (if different) is deallocated. |
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<DT> <B> void GC_FREE(void *<I>dead</i>) </b> |
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<DD> |
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Explicitly deallocate an object. Typically not useful for small |
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collectable objects. |
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<DT> <B> void * GC_MALLOC_IGNORE_OFF_PAGE(size_t <I>nbytes</i>) </b> |
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<DD> |
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<DT> <B> void * GC_MALLOC_ATOMIC_IGNORE_OFF_PAGE(size_t <I>nbytes</i>) </b> |
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<DD> |
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Analogous to <TT>GC_MALLOC</tt> and <TT>GC_MALLOC_ATOMIC</tt>, |
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except that the client |
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guarantees that as long |
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as the resulting object is of use, a pointer is maintained to someplace |
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inside the first 512 bytes of the object. This pointer should be declared |
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volatile to avoid interference from compiler optimizations. |
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(Other nonvolatile pointers to the object may exist as well.) |
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This is the |
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preferred way to allocate objects that are likely to be > 100KBytes in size. |
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It greatly reduces the risk that such objects will be accidentally retained |
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when they are no longer needed. Thus space usage may be significantly reduced. |
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<DT> <B> void GC_INIT(void) </b> |
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<DD> |
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On some platforms, it is necessary to invoke this |
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<I>from the main executable, not from a dynamic library,</i> before |
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the initial invocation of a GC routine. It is recommended that this be done |
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in portable code, though we try to ensure that it expands to a no-op |
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on as many platforms as possible. As of GC 7.0, it is required if |
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thread-local allocation is enabled in the collector build, and <TT>malloc</tt> |
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is not redirected to <TT>GC_malloc</tt>. |
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<DT> <B> void GC_gcollect(void) </b> |
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<DD> |
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Explicitly force a garbage collection. |
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<DT> <B> void GC_enable_incremental(void) </b> |
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<DD> |
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Cause the garbage collector to perform a small amount of work |
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every few invocations of <TT>GC_MALLOC</tt> or the like, instead of performing |
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an entire collection at once. This is likely to increase total |
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running time. It will improve response on a platform that either has |
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suitable support in the garbage collector (Linux and most Unix |
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versions, win32 if the collector was suitably built) or if "stubborn" |
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allocation is used (see |
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<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a>). |
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On many platforms this interacts poorly with system calls |
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that write to the garbage collected heap. |
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<DT> <B> GC_warn_proc GC_set_warn_proc(GC_warn_proc <I>p</i>) </b> |
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<DD> |
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Replace the default procedure used by the collector to print warnings. |
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The collector |
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may otherwise write to sterr, most commonly because GC_malloc was used |
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in a situation in which GC_malloc_ignore_off_page would have been more |
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appropriate. See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> for details. |
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<DT> <B> void GC_REGISTER_FINALIZER(...) </b> |
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<DD> |
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Register a function to be called when an object becomes inaccessible. |
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This is often useful as a backup method for releasing system resources |
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(<I>e.g.</i> closing files) when the object referencing them becomes |
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inaccessible. |
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It is not an acceptable method to perform actions that must be performed |
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in a timely fashion. |
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See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> for details of the interface. |
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See <A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/finalization.html">here</a> for a more detailed discussion |
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of the design. |
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<P> |
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Note that an object may become inaccessible before client code is done |
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operating on objects referenced by its fields. |
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Suitable synchronization is usually required. |
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See <A HREF="http://portal.acm.org/citation.cfm?doid=604131.604153">here</a> |
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or <A HREF="http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html">here</a> |
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for details. |
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</dl> |
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<P> |
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If you are concerned with multiprocessor performance and scalability, |
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you should consider enabling and using thread local allocation (<I>e.g.</i> |
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<TT>GC_LOCAL_MALLOC</tt>, see <TT>gc_local_alloc.h</tt>. If your platform |
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supports it, you should build the collector with parallel marking support |
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(<TT>-DPARALLEL_MARK</tt>, or <TT>--enable-parallel-mark</tt>). |
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<P> |
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If the collector is used in an environment in which pointer location |
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information for heap objects is easily available, this can be passed on |
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to the collector using the interfaces in either <TT>gc_typed.h</tt> |
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or <TT>gc_gcj.h</tt>. |
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<P> |
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The collector distribution also includes a <B>string package</b> that takes |
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advantage of the collector. For details see |
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<A HREF="http://www.hpl.hp.com/personal/Hans_Boehm/gc/gc_source/cordh.txt">cord.h</a> |
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<H1>C++ Interface</h1> |
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The C++ interface is implemented as a thin layer on the C interface. |
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Unfortunately, this thin layer appears to be very sensitive to variations |
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in C++ implementations, particularly since it tries to replace the global |
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::new operator, something that appears to not be well-standardized. |
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Your platform may need minor adjustments in this layer (gc_cpp.cc, gc_cpp.h, |
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and possibly gc_allocator.h). Such changes do not require understanding |
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of collector internals, though they may require a good understanding of |
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your platform. (Patches enhancing portability are welcome. |
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But it's easy to break one platform by fixing another.) |
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<P> |
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Usage of the collector from C++ is also complicated by the fact that there |
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are many "standard" ways to allocate memory in C++. The default ::new |
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operator, default malloc, and default STL allocators allocate memory |
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that is not garbage collected, and is not normally "traced" by the |
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collector. This means that any pointers in memory allocated by these |
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default allocators will not be seen by the collector. Garbage-collectable |
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memory referenced only by pointers stored in such default-allocated |
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objects is likely to be reclaimed prematurely by the collector. |
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<P> |
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It is the programmers responsibility to ensure that garbage-collectable |
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memory is referenced by pointers stored in one of |
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<UL> |
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<LI> Program variables |
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<LI> Garbage-collected objects |
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<LI> Uncollected but "traceable" objects |
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</ul> |
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"Traceable" objects are not necessarily reclaimed by the collector, |
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but are scanned for pointers to collectable objects. |
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They are usually allocated by <TT>GC_MALLOC_UNCOLLECTABLE</tt>, as described |
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above, and through some interfaces described below. |
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<P> |
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(On most platforms, the collector may not trace correctly from in-flight |
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exception objects. Thus objects thrown as exceptions should only |
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point to otherwise reachable memory. This is another bug whose |
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proper repair requires platform hooks.) |
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<P> |
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The easiest way to ensure that collectable objects are properly referenced |
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is to allocate only collectable objects. This requires that every |
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allocation go through one of the following interfaces, each one of |
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which replaces a standard C++ allocation mechanism: |
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<DL> |
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<DT> <B> STL allocators </b> |
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<DD> |
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<P> |
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Recent versions of the collector also include a more standard-conforming |
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allocator implementation in <TT>gc_allocator.h</tt>. It defines |
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<UL> |
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<LI> traceable_allocator |
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<LI> gc_allocator |
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</ul> |
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which may be used either directly to allocate memory or to instantiate |
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container templates. |
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The former allocates uncollectable but traced memory. |
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The latter allocates garbage-collected memory. |
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<P> |
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These should work with any fully standard-conforming C++ compiler. |
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<P> |
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Users of the <A HREF="http://www.sgi.com/tech/stl">SGI extended STL</a> |
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or its derivatives (including most g++ versions) |
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can alternatively include <TT>new_gc_alloc.h</tt> before including |
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STL header files. |
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(<TT>gc_alloc.h</tt> corresponds to now obsolete versions of the |
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SGI STL.) This interface is no longer recommended, but it has existed |
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for much longer. |
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<P> |
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This defines SGI-style allocators |
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<UL> |
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<LI> alloc |
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<LI> single_client_alloc |
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<LI> gc_alloc |
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<LI> single_client_gc_alloc |
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</ul> |
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The first two allocate uncollectable but traced |
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memory, while the second two allocate collectable memory. |
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The single_client versions are not safe for concurrent access by |
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multiple threads, but are faster. |
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<P> |
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For an example, click <A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_alloc_exC.txt">here</a>. |
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<DT> <B> Class inheritance based interface </b> |
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<DD> |
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Users may include gc_cpp.h and then cause members of classes to |
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be allocated in garbage collectable memory by having those classes |
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inherit from class gc. |
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For details see <A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gc_cpph.txt">gc_cpp.h</a>. |
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<P> |
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Linking against libgccpp in addition to the gc library overrides |
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::new (and friends) to allocate traceable memory but uncollectable |
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memory, making it safe to refer to collectable objects from the resulting |
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memory. |
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<DT> <B> C interface </b> |
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<DD> |
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It is also possible to use the C interface from |
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<A HREF="http://hpl.hp.com/personal/Hans_Boehm/gc/gc_source/gch.txt">gc.h</a> directly. |
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On platforms which use malloc to implement ::new, it should usually be possible |
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to use a version of the collector that has been compiled as a malloc |
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replacement. It is also possible to replace ::new and other allocation |
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functions suitably, as is done by libgccpp. |
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<P> |
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Note that user-implemented small-block allocation often works poorly with |
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an underlying garbage-collected large block allocator, since the collector |
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has to view all objects accessible from the user's free list as reachable. |
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This is likely to cause problems if <TT>GC_MALLOC</tt> |
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is used with something like |
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the original HP version of STL. |
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This approach works well with the SGI versions of the STL only if the |
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<TT>malloc_alloc</tt> allocator is used. |
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</dl> |
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</body> |
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</html> |
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