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5 <p>The public interface of the Mono GC is fairly limited, and
6 its the only one that embedders should be using:
26 <p>The bridge is a mechanism for SGen to let clients override
27 the death of some unreachable objects. We use it in monodroid
28 to do garbage collection across the Mono and Java heaps.
31 means that they participate in the bridge processing step once
32 SGen considers them unreachable, i.e., dead. Bridged objects
33 must be registered for finalization.
35 <p>When SGen is done marking, it puts together a list of all
36 dead bridged objects and then does a strongly connected
37 component analysis over their object graph. That graph will
38 usually contain non-bridged objects, too.
40 <p>The output of the SCC analysis is passed to the
41 `cross_references()` callback. It is expected to set the
42 `is_alive` flag on those strongly connected components that it
43 wishes to be kept alive. The value of `is_alive` will be
44 ignored on any SCCs which lack bridges.
46 <p>In monodroid each bridged object has a corresponding Java
47 mirror object. In the bridge callback it reifies the Mono
48 object graph in the Java heap so that the full, combined
49 object graph is now instantiated on the Java side. Then it
50 triggers a Java GC, waits for it to finish, and checks which
51 of the Java mirror objects are still alive. For those it sets
52 the `is_alive` flag and returns from the callback.
54 <p>The SCC analysis is done while the world is stopped, but
55 the callback is made with the world running again. Weak links
56 to bridged objects and other objects reachable from them are
57 kept until the callback returns, at which point all links to
58 bridged objects that don't have `is_alive` set are nulled.
59 Note that weak links to non-bridged objects reachable from
60 bridged objects are not nulled. This might be considered a
61 bug.
64 enum {
65 SGEN_BRIDGE_VERSION = 5
66 };
68 typedef enum {
69 /* Instances of this class should be scanned when computing the transitive dependency among bridges. E.g. List of object*/
70 GC_BRIDGE_TRANSPARENT_CLASS,
71 /* Instances of this class should not be scanned when computing the transitive dependency among bridges. E.g. String*/
72 GC_BRIDGE_OPAQUE_CLASS,
73 /* Instances of this class should be bridged and have their dependency computed. */
74 GC_BRIDGE_TRANSPARENT_BRIDGE_CLASS,
75 /* Instances of this class should be bridged but no dependencies should not be calculated. */
76 GC_BRIDGE_OPAQUE_BRIDGE_CLASS,
77 } MonoGCBridgeObjectKind;
79 typedef struct {
80 mono_bool is_alive; /* to be set by the cross reference callback */
81 int num_objs;
82 MonoObject *objs [MONO_ZERO_LEN_ARRAY];
83 } MonoGCBridgeSCC;
85 typedef struct {
86 int src_scc_index;
87 int dst_scc_index;
88 } MonoGCBridgeXRef;
90 typedef struct {
91 int bridge_version;
92 /*
93 * Tells the runtime which classes to even consider when looking for
94 * bridged objects. If subclasses are to be considered as well, the
95 * subclass check must be done in the callback.
96 */
97 MonoGCBridgeObjectKind (*bridge_class_kind) (MonoClass *klass);
98 /*
99 * This is only called on objects for whose classes
100 * `bridge_class_kind()` returned `XXX_BRIDGE_CLASS`.
101 */
102 mono_bool (*is_bridge_object) (MonoObject *object);
103 void (*cross_references) (int num_sccs, MonoGCBridgeSCC **sccs, int num_xrefs, MonoGCBridgeXRef *xrefs);
104 } MonoGCBridgeCallbacks;
105 </div>
108 <h4><a name="api:mono_gc_wait_for_bridge_processing">mono_gc_wait_for_bridge_processing</a></h4>
112 <p>SGen is a concurrent and generational GC, features which require
113 tracking changes to the state of the heap. This is achieved through
114 write barriers. Whenever native code is changing the state of the
115 heap by storing references into another managed object, it needs to
116 do it using this write barrier API.
121 <h4><a name="api:mono_gc_wbarrier_generic_store_atomic">mono_gc_wbarrier_generic_store_atomic</a></h4>