| Copyright (c) 1988, 1989 Hans-J. Boehm, Alan J. Demers |
| Copyright (c) 1991-1996 by Xerox Corporation. All rights reserved. |
| Copyright (c) 1996-1999 by Silicon Graphics. All rights reserved. |
| Copyright (c) 1999-2004 Hewlett-Packard Development Company, L.P. |
| |
| The file linux_threads.c is also |
| Copyright (c) 1998 by Fergus Henderson. All rights reserved. |
| |
| The files Makefile.am, and configure.in are |
| Copyright (c) 2001 by Red Hat Inc. All rights reserved. |
| |
| Several files supporting GNU-style builds are copyrighted by the Free |
| Software Foundation, and carry a different license from that given |
| below. |
| |
| THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED |
| OR IMPLIED. ANY USE IS AT YOUR OWN RISK. |
| |
| Permission is hereby granted to use or copy this program |
| for any purpose, provided the above notices are retained on all copies. |
| Permission to modify the code and to distribute modified code is granted, |
| provided the above notices are retained, and a notice that the code was |
| modified is included with the above copyright notice. |
| |
| A few of the files needed to use the GNU-style build procedure come with |
| slightly different licenses, though they are all similar in spirit. A few |
| are GPL'ed, but with an exception that should cover all uses in the |
| collector. (If you are concerned about such things, I recommend you look |
| at the notice in config.guess or ltmain.sh.) |
| |
| This is version 6.6 of a conservative garbage collector for C and C++. |
| |
| You might find a more recent version of this at |
| |
| 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 |
| 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 only a few 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 |
| (see 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 documantation can be found at |
| |
| 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, as is now the default. |
| |
| Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention |
| of garbage objects, by requiring pointers from the heap to to the beginning |
| of an object. But this no longer appears to be a significant |
| issue for most programs. |
| |
| 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 probablility 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 README.debugging 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 collectable 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 |
| 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. Removing -DNO_SIGNALS from Makefile |
| attempts to remedy that. But that may not be reliable with a compiler that |
| substantially reorders memory operations inside GC_malloc. |
| |
| 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 macro SILENT is defined in Makefile. |
| In the event of problems, this can be removed to obtain a moderate |
| amount 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 significant for the |
| contrived program "test.c" than for your application.) |
| |
| Note that typing "make test" will automatically build the collector |
| and then run setjmp_test and gctest. Setjmp_test will give you information |
| about configuring the collector, which is useful primarily if you have |
| a machine that's not already supported. Gctest is a somewhat superficial |
| test of collector functionality. Failure is indicated by a core dump or |
| a message to the effect that the collector is broken. Gctest takes about |
| 35 seconds to run on a SPARCstation 2. It may use up to 8 MB of memory. (The |
| multi-threaded version will use more. 64-bit versions may use more.) |
| "Make test" will also, as its last step, attempt to build and test the |
| "cord" string library. This will fail without an ANSI C compiler, but |
| the garbage collector itself should still be usable. |
| |
| The Makefile will generate a library gc.a which you should link against. |
| Typing "make cords" will add the cord library to gc.a. |
| Note that this requires an ANSI C compiler. |
| |
| It is suggested that if you need to replace a piece of the collector |
| (e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the |
| ld command line, rather than replacing the one in gc.a. (This will |
| generate numerous warnings under some versions of AIX, but it still |
| works.) |
| |
| All include files that need to be used by clients will be put in the |
| include subdirectory. (Normally this is just gc.h. "Make cords" adds |
| "cord.h" and "ec.h".) |
| |
| The collector currently is designed to run essentially unmodified on |
| machines that use a flat 32-bit or 64-bit address space. |
| That includes the vast majority of Workstations and X86 (X >= 3) PCs. |
| (The list here was deleted because it was getting too long and constantly |
| out of date.) |
| It does NOT run under plain 16-bit DOS or Windows 3.X. There are however |
| various packages (e.g. win32s, djgpp) that allow flat 32-bit address |
| applications to run under those systemsif the have at least an 80386 processor, |
| and several of those are compatible with the collector. |
| |
| In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile |
| or equivalent is supplied. Many of these have separate README.system |
| files. |
| |
| Dynamic libraries are completely supported only under SunOS/Solaris, |
| (and even that support is not functional on the last Sun 3 release), |
| Linux, FreeBSD, NetBSD, IRIX 5&6, HP/UX, Win32 (not Win32S) and OSF/1 |
| on DEC AXP machines plus perhaps a few others listed near the top |
| of dyn_load.c. On other machines we recommend that you do one of |
| the following: |
| |
| 1) Add dynamic library support (and send us the code). |
| 2) Use static versions of the libraries. |
| 3) Arrange for dynamic libraries to use the standard malloc. |
| This is still dangerous if the library stores a pointer to a |
| garbage collected object. But nearly all standard interfaces |
| prohibit this, because they deal correctly with pointers |
| to stack allocated objects. (Strtok is an exception. Don't |
| use it.) |
| |
| In all cases we assume that pointer alignment is consistent with that |
| enforced by the standard C compilers. If you use a nonstandard compiler |
| you may have to adjust the alignment parameters defined in gc_priv.h. |
| Note that this may also be an issue with packed records/structs, if those |
| enforce less alignment for pointers. |
| |
| A port to a machine that is not byte addressed, or does not use 32 bit |
| or 64 bit addresses will require a major effort. A port to plain MSDOS |
| or win16 is hard. |
| |
| For machines not already mentioned, or for nonstandard compilers, the |
| following are likely to require change: |
| |
| 1. The parameters in gcconfig.h. |
| The parameters that will usually require adjustment are |
| STACKBOTTOM, ALIGNMENT and DATASTART. Setjmp_test |
| prints its guesses of the first two. |
| DATASTART should be an expression for computing the |
| address of the beginning of the data segment. This can often be |
| &etext. But some memory management units require that there be |
| some unmapped space between the text and the data segment. Thus |
| it may be more complicated. On UNIX systems, this is rarely |
| documented. But the adb "$m" command may be helpful. (Note |
| that DATASTART will usually be a function of &etext. Thus a |
| single experiment is usually insufficient.) |
| STACKBOTTOM is used to initialize GC_stackbottom, which |
| should be a sufficient approximation to the coldest stack address. |
| On some machines, it is difficult to obtain such a value that is |
| valid across a variety of MMUs, OS releases, etc. A number of |
| alternatives exist for using the collector in spite of this. See the |
| discussion in gcconfig.h immediately preceding the various |
| definitions of STACKBOTTOM. |
| |
| 2. mach_dep.c. |
| The most important routine here is one to mark from registers. |
| The distributed file includes a generic hack (based on setjmp) that |
| happens to work on many machines, and may work on yours. Try |
| compiling and running setjmp_t.c to see whether it has a chance of |
| working. (This is not correct C, so don't blame your compiler if it |
| doesn't work. Based on limited experience, register window machines |
| are likely to cause trouble. If your version of setjmp claims that |
| all accessible variables, including registers, have the value they |
| had at the time of the longjmp, it also will not work. Vanilla 4.2 BSD |
| on Vaxen makes such a claim. SunOS does not.) |
| If your compiler does not allow in-line assembly code, or if you prefer |
| not to use such a facility, mach_dep.c may be replaced by a .s file |
| (as we did for the MIPS machine and the PC/RT). |
| At this point enough architectures are supported by mach_dep.c |
| that you will rarely need to do more than adjust for assembler |
| syntax. |
| |
| 3. os_dep.c (and gc_priv.h). |
| Several kinds of operating system dependent routines reside here. |
| Many are optional. Several are invoked only through corresponding |
| macros in gc_priv.h, which may also be redefined as appropriate. |
| The routine GC_register_data_segments is crucial. It registers static |
| data areas that must be traversed by the collector. (User calls to |
| GC_add_roots may sometimes be used for similar effect.) |
| Routines to obtain memory from the OS also reside here. |
| Alternatively this can be done entirely by the macro GET_MEM |
| defined in gc_priv.h. Routines to disable and reenable signals |
| also reside here if they are need by the macros DISABLE_SIGNALS |
| and ENABLE_SIGNALS defined in gc_priv.h. |
| In a multithreaded environment, the macros LOCK and UNLOCK |
| in gc_priv.h will need to be suitably redefined. |
| The incremental collector requires page dirty information, which |
| is acquired through routines defined in os_dep.c. Unless directed |
| otherwise by gcconfig.h, these are implemented as stubs that simply |
| treat all pages as dirty. (This of course makes the incremental |
| collector much less useful.) |
| |
| 4. dyn_load.c |
| This provides a routine that allows the collector to scan data |
| segments associated with dynamic libraries. Often it is not |
| necessary to provide this routine unless user-written dynamic |
| libraries are used. |
| |
| For a different version of UN*X or different machines using the |
| Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture, |
| it should frequently suffice to change definitions in gcconfig.h. |
| |
| |
| THE C INTERFACE TO THE ALLOCATOR |
| |
| The following routines are intended to be directly called by the user. |
| Note that usually only GC_malloc is necessary. GC_clear_roots and GC_add_roots |
| calls may be required if the collector has to trace from nonstandard places |
| (e.g. from dynamic library data areas on a machine on which the |
| collector doesn't already understand them.) On some machines, it may |
| be desirable to set GC_stacktop to a good approximation of the stack base. |
| (This enhances code portability on HP PA machines, since there is no |
| good way for the collector to compute this value.) Client code may include |
| "gc.h", which defines all of the following, plus many others. |
| |
| 1) GC_malloc(nbytes) |
| - allocate an object of size nbytes. Unlike malloc, the object is |
| cleared before being returned to the user. Gc_malloc will |
| invoke the garbage collector when it determines this to be appropriate. |
| GC_malloc may return 0 if it is unable to acquire sufficient |
| space from the operating system. This is the most probable |
| consequence of running out of space. Other possible consequences |
| are that a function call will fail due to lack of stack space, |
| or that the collector will fail in other ways because it cannot |
| maintain its internal data structures, or that a crucial system |
| process will fail and take down the machine. Most of these |
| possibilities are independent of the malloc implementation. |
| |
| 2) GC_malloc_atomic(nbytes) |
| - allocate an object of size nbytes that is guaranteed not to contain any |
| pointers. The returned object is not guaranteed to be cleared. |
| (Can always be replaced by GC_malloc, but results in faster collection |
| times. The collector will probably run faster if large character |
| arrays, etc. are allocated with GC_malloc_atomic than if they are |
| statically allocated.) |
| |
| 3) GC_realloc(object, new_size) |
| - change the size of object to be new_size. Returns a pointer to the |
| new object, which may, or may not, be the same as the pointer to |
| the old object. The new object is taken to be atomic iff the old one |
| was. If the new object is composite and larger than the original object, |
| then the newly added bytes are cleared (we hope). This is very likely |
| to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h. |
| Even then, it is likely to recycle the old object only if the object |
| is grown in small additive increments (which, we claim, is generally bad |
| coding practice.) |
| |
| 4) GC_free(object) |
| - explicitly deallocate an object returned by GC_malloc or |
| GC_malloc_atomic. Not necessary, but can be used to minimize |
| collections if performance is critical. Probably a performance |
| loss for very small objects (<= 8 bytes). |
| |
| 5) GC_expand_hp(bytes) |
| - Explicitly increase the heap size. (This is normally done automatically |
| if a garbage collection failed to GC_reclaim enough memory. Explicit |
| calls to GC_expand_hp may prevent unnecessarily frequent collections at |
| program startup.) |
| |
| 6) GC_malloc_ignore_off_page(bytes) |
| - identical to GC_malloc, but the client promises to keep a pointer to |
| the somewhere within the first 256 bytes of the object while it is |
| live. (This pointer should nortmally be declared volatile to prevent |
| interference from compiler optimizations.) This is the recommended |
| way to allocate anything that is likely to be larger than 100Kbytes |
| or so. (GC_malloc may result in failure to reclaim such objects.) |
| |
| 7) GC_set_warn_proc(proc) |
| - Can be used to redirect warnings from the collector. Such warnings |
| should be rare, and should not be ignored during code development. |
| |
| 8) GC_enable_incremental() |
| - Enables generational and incremental collection. Useful for large |
| heaps on machines that provide access to page dirty information. |
| Some dirty bit implementations may interfere with debugging |
| (by catching address faults) and place restrictions on heap arguments |
| to system calls (since write faults inside a system call may not be |
| handled well). |
| |
| 9) Several routines to allow for registration of finalization code. |
| User supplied finalization code may be invoked when an object becomes |
| unreachable. To call (*f)(obj, x) when obj becomes inaccessible, use |
| GC_register_finalizer(obj, f, x, 0, 0); |
| For more sophisticated uses, and for finalization ordering issues, |
| see gc.h. |
| |
| The global variable GC_free_space_divisor may be adjusted up from its |
| default value of 4 to use less space and more collection time, or down for |
| the opposite effect. Setting it to 1 or 0 will effectively disable collections |
| and cause all allocations to simply grow the heap. |
| |
| The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect |
| the amount of memory allocated by the above routines that should not be |
| considered as a candidate for collection. Careless use may, of course, result |
| in excessive memory consumption. |
| |
| Some additional tuning is possible through the parameters defined |
| near the top of gc_priv.h. |
| |
| If only GC_malloc is intended to be used, it might be appropriate to define: |
| |
| #define malloc(n) GC_malloc(n) |
| #define calloc(m,n) GC_malloc((m)*(n)) |
| |
| For small pieces of VERY allocation intensive code, gc_inl.h |
| includes some allocation macros that may be used in place of GC_malloc |
| and friends. |
| |
| All externally visible names in the garbage collector start with "GC_". |
| To avoid name conflicts, client code should avoid this prefix, except when |
| accessing garbage collector routines or variables. |
| |
| There are provisions for allocation with explicit type information. |
| This is rarely necessary. Details can be found in gc_typed.h. |
| |
| THE C++ INTERFACE TO THE ALLOCATOR: |
| |
| The Ellis-Hull C++ interface to the collector is included in |
| the collector distribution. If you intend to use this, type |
| "make c++" after the initial build of the collector is complete. |
| See gc_cpp.h for the definition of the interface. This interface |
| tries to approximate the Ellis-Detlefs C++ garbage collection |
| proposal without compiler changes. |
| |
| Cautions: |
| 1. Arrays allocated without new placement syntax are |
| allocated as uncollectable objects. They are traced by the |
| collector, but will not be reclaimed. |
| |
| 2. Failure to use "make c++" in combination with (1) will |
| result in arrays allocated using the default new operator. |
| This is likely to result in disaster without linker warnings. |
| |
| 3. If your compiler supports an overloaded new[] operator, |
| then gc_cpp.cc and gc_cpp.h should be suitably modified. |
| |
| 4. Many current C++ compilers have deficiencies that |
| break some of the functionality. See the comments in gc_cpp.h |
| for suggested workarounds. |
| |
| USE AS LEAK DETECTOR: |
| |
| The collector may be used to track down leaks in C programs that are |
| intended to run with malloc/free (e.g. code with extreme real-time or |
| portability constraints). To do so define FIND_LEAK in Makefile |
| This will cause the collector to invoke the report_leak |
| routine defined near the top of reclaim.c whenever an inaccessible |
| object is found that has not been explicitly freed. Such objects will |
| also be automatically reclaimed. |
| Productive use of this facility normally involves redefining report_leak |
| to do something more intelligent. This typically requires annotating |
| objects with additional information (e.g. creation time stack trace) that |
| identifies their origin. Such code is typically not very portable, and is |
| not included here, except on SPARC machines. |
| If all objects are allocated with GC_DEBUG_MALLOC (see next section), |
| then the default version of report_leak will report the source file |
| and line number at which the leaked object was allocated. This may |
| sometimes be sufficient. (On SPARC/SUNOS4 machines, it will also report |
| a cryptic stack trace. This can often be turned into a sympolic stack |
| trace by invoking program "foo" with "callprocs foo". Callprocs is |
| a short shell script that invokes adb to expand program counter values |
| to symbolic addresses. It was largely supplied by Scott Schwartz.) |
| Note that the debugging facilities described in the next section can |
| sometimes be slightly LESS effective in leak finding mode, since in |
| leak finding mode, GC_debug_free actually results in reuse of the object. |
| (Otherwise the object is simply marked invalid.) Also note that the test |
| program is not designed to run meaningfully in FIND_LEAK mode. |
| Use "make gc.a" to build the collector. |
| |
| DEBUGGING FACILITIES: |
| |
| The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc, |
| and GC_debug_free provide an alternate interface to the collector, which |
| provides some help with memory overwrite errors, and the like. |
| Objects allocated in this way are annotated with additional |
| information. Some of this information is checked during garbage |
| collections, and detected inconsistencies are reported to stderr. |
| |
| Simple cases of writing past the end of an allocated object should |
| be caught if the object is explicitly deallocated, or if the |
| collector is invoked while the object is live. The first deallocation |
| of an object will clear the debugging info associated with an |
| object, so accidentally repeated calls to GC_debug_free will report the |
| deallocation of an object without debugging information. Out of |
| memory errors will be reported to stderr, in addition to returning |
| NIL. |
| |
| GC_debug_malloc checking during garbage collection is enabled |
| with the first call to GC_debug_malloc. This will result in some |
| slowdown during collections. If frequent heap checks are desired, |
| this can be achieved by explicitly invoking GC_gcollect, e.g. from |
| the debugger. |
| |
| GC_debug_malloc allocated objects should not be passed to GC_realloc |
| or GC_free, and conversely. It is however acceptable to allocate only |
| some objects with GC_debug_malloc, and to use GC_malloc for other objects, |
| provided the two pools are kept distinct. In this case, there is a very |
| low probablility that GC_malloc allocated objects may be misidentified as |
| having been overwritten. This should happen with probability at most |
| one in 2**32. This probability is zero if GC_debug_malloc is never called. |
| |
| GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two |
| additional trailing arguments, a string and an integer. These are not |
| interpreted by the allocator. They are stored in the object (the string is |
| not copied). If an error involving the object is detected, they are printed. |
| |
| The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and |
| GC_REGISTER_FINALIZER are also provided. These require the same arguments |
| as the corresponding (nondebugging) routines. If gc.h is included |
| with GC_DEBUG defined, they call the debugging versions of these |
| functions, passing the current file name and line number as the two |
| extra arguments, where appropriate. If gc.h is included without GC_DEBUG |
| defined, then all these macros will instead be defined to their nondebugging |
| equivalents. (GC_REGISTER_FINALIZER is necessary, since pointers to |
| objects with debugging information are really pointers to a displacement |
| of 16 bytes form the object beginning, and some translation is necessary |
| when finalization routines are invoked. For details, about what's stored |
| in the header, see the definition of the type oh in debug_malloc.c) |
| |
| INCREMENTAL/GENERATIONAL COLLECTION: |
| |
| The collector normally interrupts client code for the duration of |
| a garbage collection mark phase. This may be unacceptable if interactive |
| response is needed for programs with large heaps. The collector |
| can also run in a "generational" mode, in which it usually attempts to |
| collect only objects allocated since the last garbage collection. |
| Furthermore, in this mode, garbage collections run mostly incrementally, |
| with a small amount of work performed in response to each of a large number of |
| GC_malloc requests. |
| |
| This mode is enabled by a call to GC_enable_incremental(). |
| |
| Incremental and generational collection is effective in reducing |
| pause times only if the collector has some way to tell which objects |
| or pages have been recently modified. The collector uses two sources |
| of information: |
| |
| 1. Information provided by the VM system. This may be provided in |
| one of several forms. Under Solaris 2.X (and potentially under other |
| similar systems) information on dirty pages can be read from the |
| /proc file system. Under other systems (currently SunOS4.X) it is |
| possible to write-protect the heap, and catch the resulting faults. |
| On these systems we require that system calls writing to the heap |
| (other than read) be handled specially by client code. |
| See os_dep.c for details. |
| |
| 2. Information supplied by the programmer. We define "stubborn" |
| objects to be objects that are rarely changed. Such an object |
| can be allocated (and enabled for writing) with GC_malloc_stubborn. |
| Once it has been initialized, the collector should be informed with |
| a call to GC_end_stubborn_change. Subsequent writes that store |
| pointers into the object must be preceded by a call to |
| GC_change_stubborn. |
| |
| This mechanism performs best for objects that are written only for |
| initialization, and such that only one stubborn object is writable |
| at once. It is typically not worth using for short-lived |
| objects. Stubborn objects are treated less efficiently than pointerfree |
| (atomic) objects. |
| |
| A rough rule of thumb is that, in the absence of VM information, garbage |
| collection pauses are proportional to the amount of pointerful storage |
| plus the amount of modified "stubborn" storage that is reachable during |
| the collection. |
| |
| Initial allocation of stubborn objects takes longer than allocation |
| of other objects, since other data structures need to be maintained. |
| |
| We recommend against random use of stubborn objects in client |
| code, since bugs caused by inappropriate writes to stubborn objects |
| are likely to be very infrequently observed and hard to trace. |
| However, their use may be appropriate in a few carefully written |
| library routines that do not make the objects themselves available |
| for writing by client code. |
| |
| |
| BUGS: |
| |
| Any memory that does not have a recognizable pointer to it will be |
| reclaimed. Exclusive-or'ing forward and backward links in a list |
| doesn't cut it. |
| Some C optimizers may lose the last undisguised pointer to a memory |
| object as a consequence of clever optimizations. This has almost |
| never been observed in practice. Send mail to boehm@acm.org |
| for suggestions on how to fix your compiler. |
| This is not a real-time collector. In the standard configuration, |
| percentage of time required for collection should be constant across |
| heap sizes. But collection pauses will increase for larger heaps. |
| (On SPARCstation 2s collection times will be on the order of 300 msecs |
| per MB of accessible memory that needs to be scanned. Your mileage |
| may vary.) The incremental/generational collection facility helps, |
| but is portable only if "stubborn" allocation is used. |
| Please address bug reports to boehm@acm.org. If you are |
| contemplating a major addition, you might also send mail to ask whether |
| it's already been done (or whether we tried and discarded it). |
| |