Conservative garbage collection, Operating System

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Conservative GC can be used for languages such as C and C++, which were not explicitly designed for garbage collection. This is a non-copying technique. A conservative garbage collector is one that, instead of knowing the exact location of each object, discovers the set of live objects by scanning a region of memory, looking for areas that may be objects. Because C and C++ allow casting, anything that can hold a pointer could conceivably be a pointer, such as an unsigned long which is cast as a pointer, or a ?oating-point type which is cast as a pointer, etc. Conservative GC more or less uses the "duck test", paraphrased here as "if it looks like a pointer and acts like a pointer, it's probably a pointer". Starting from the roots, we can ?nd all objects which look like pointers, and the objects to which they would point if they were in fact pointers, and mark those objects as live. By doing this, we can provide garbage collection to languages which otherwise would not support it.

The possible objects found may or may not actually be objects, but we are ensured that all live objects referred to from that particular area are found by the garbage collector. Since we must discover the areas that might be objects, we also have to know how to identify pointers; usually, if something looks like a pointer, we assume it is a pointer. Then, conservative GC traces through those "pointers", marking everything that is still live.

Some of the drawbacks of this method are: 1) areas of memory that look like pointers to objects, but aren't, cause garbage objects to be retained as long as the fake pointer exists; this increases memory usage of the garbage collector, and can cause other limited resources to be exhausted; and 2) the methods of ?nding exactly which areas to trace aren't always portable.


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