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<!DOCTYPE HTML>
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<HEAD>
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<TITLE>Garbage Collector Interface</TITLE>
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</HEAD>
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<BODY>
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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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<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.
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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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Usage of the collector from C++ is 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 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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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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Users of the <A HREF="http://www.sgi.com/tech/stl">SGI extended STL</a>
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can 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.)
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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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which may be used either directly to allocate memory or to instantiate
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container templates. 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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<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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Again the former allocates uncollectable but traced memory.
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This should work with any fully standard-conforming C++ compiler.
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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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