summaryrefslogtreecommitdiff
path: root/doc/gcinterface.html
blob: 7b336ec811ba6f040cfe46d155629c4561a7d1da (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
<!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>