il2cpp.rar

# Boehm-Demers-Weiser Garbage Collector This is version 7.4.0 of a conservative garbage collector for C and C++. You might find a more recent version [here](http://www.hpl.hp.com/personal/Hans_Boehm/gc). ## Overview This is intended to be a general purpose, garbage collecting storage allocator. The algorithms used are described in: * Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment", Software Practice & Experience, September 1988, pp. 807-820. * Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection", Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164. * Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design and Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206. * Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the 2000 International Symposium on Memory Management. Possible interactions between the collector and optimizing compilers are discussed in * Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation", The Journal of C Language Translation 4, 2 (December 1992). and * Boehm H., "Simple GC-safe Compilation", Proceedings of the ACM SIGPLAN '96 Conference on Programming Language Design and Implementation. (Some of these are also available from [here](http://www.hpl.hp.com/personal/Hans_Boehm/papers/), among other places.) Unlike the collector described in the second reference, this collector operates either with the mutator stopped during the entire collection (default) or incrementally during allocations. (The latter is supported on fewer machines.) On the most common platforms, it can be built with or without thread support. On a few platforms, it can take advantage of a multiprocessor to speed up garbage collection. Many of the ideas underlying the collector have previously been explored by others. Notably, some of the run-time systems developed at Xerox PARC in the early 1980s conservatively scanned thread stacks to locate possible pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types to a Strongly-Typed Statically Checked, Concurrent Language" Xerox PARC CSL 84-7). Doug McIlroy wrote a simpler fully conservative collector that was part of version 8 UNIX (tm), but appears to not have received widespread use. Rudimentary tools for use of the collector as a leak detector are included ([link](http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html)), as is a fairly sophisticated string package "cord" that makes use of the collector. (See doc/README.cords and H.-J. Boehm, R. Atkinson, and M. Plass, "Ropes: An Alternative to Strings", Software Practice and Experience 25, 12 (December 1995), pp. 1315-1330. This is very similar to the "rope" package in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.) Further collector documentation can be found [here](http://www.hpl.hp.com/personal/Hans_Boehm/gc). ## General Description This is a garbage collecting storage allocator that is intended to be used as a plug-in replacement for C's malloc. Since the collector does not require pointers to be tagged, it does not attempt to ensure that all inaccessible storage is reclaimed. However, in our experience, it is typically more successful at reclaiming unused memory than most C programs using explicit deallocation. Unlike manually introduced leaks, the amount of unreclaimed memory typically stays bounded. In the following, an "object" is defined to be a region of memory allocated by the routines described below. Any objects not intended to be collected must be pointed to either from other such accessible objects, or from the registers, stack, data, or statically allocated bss segments. Pointers from the stack or registers may point to anywhere inside an object. The same is true for heap pointers if the collector is compiled with `ALL_INTERIOR_POINTERS` defined, or `GC_all_interior_pointers` is otherwise set, as is now the default. Compiling without `ALL_INTERIOR_POINTERS` may reduce accidental retention of garbage objects, by requiring pointers from the heap to the beginning of an object. But this no longer appears to be a significant issue for most programs occupying a small fraction of the possible address space. There are a number of routines which modify the pointer recognition algorithm. `GC_register_displacement` allows certain interior pointers to be recognized even if `ALL_INTERIOR_POINTERS` is nor defined. `GC_malloc_ignore_off_page` allows some pointers into the middle of large objects to be disregarded, greatly reducing the probability of accidental retention of large objects. For most purposes it seems best to compile with `ALL_INTERIOR_POINTERS` and to use `GC_malloc_ignore_off_page` if you get collector warnings from allocations of very large objects. See doc/debugging.html for details. _WARNING_: pointers inside memory allocated by the standard `malloc` are not seen by the garbage collector. Thus objects pointed to only from such a region may be prematurely deallocated. It is thus suggested that the standard `malloc` be used only for memory regions, such as I/O buffers, that are guaranteed not to contain pointers to garbage collectible memory. Pointers in C language automatic, static, or register variables, are correctly recognized. (Note that `GC_malloc_uncollectable` has semantics similar to standard malloc, but allocates objects that are traced by the collector.) _WARNING_: the collector does not always know how to find pointers in data areas that are associated with dynamic libraries. This is easy to remedy IF you know how to find those data areas on your operating system (see `GC_add_roots`). Code for doing this under SunOS, IRIX 5.X and 6.X, HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default. (See doc/README.win32 for Win32 details.) On other systems pointers from dynamic library data areas may not be considered by the collector. If you're writing a program that depends on the collector scanning dynamic library data areas, it may be a good idea to include at least one call to `GC_is_visible` to ensure that those areas are visible to the collector. Note that the garbage collector does not need to be informed of shared read-only data. However if the shared library mechanism can introduce discontiguous data areas that may contain pointers, then the collector does need to be informed. Signal processing for most signals may be deferred during collection, and during uninterruptible parts of the allocation process. Like standard ANSI C mallocs, by default it is unsafe to invoke malloc (and other GC routines) from a signal handler while another malloc call may be in progress. The allocator/collector can also be configured for thread-safe operation. (Full signal safety can also be achieved, but only at the cost of two system calls per malloc, which is usually unacceptable.) _WARNING_: the collector does not guarantee to scan thread-local storage (e.g. of the kind accessed with `pthread_getspecific`). The collector does scan thread stacks, though, so generally the best solution is to ensure that any pointers stored in thread-local storage are also stored on the thread's stack for the duration of their lifetime. (This is arguably a longstanding bug, but it hasn't been fixed yet.) ## Installation and Portability As distributed, the collector operates silently In the event of problems, this can usually be changed by defining the `GC_PRINT_STATS` or `GC_PRINT_VERBOSE_STATS` environment variables. This will result in a few lines of descriptive output for each collection. (The given statistics exhibit a few peculiarities. Things don't appear to add up for a variety of reasons, most notably fragmentation losses. These are probably much more signifi