Garbage Collector Interface

# C Interface

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=GC_malloc` is intended primarily as an easy way to adapt old code, not for new development.

New code should use the interface discussed below.

Code must be linked against the GC library. On most UNIX platforms, depending on how the collector is built, this will be `gc.a` or `libgc.{a,so}`.

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 full interface is described in gc.h or `gc.h` in the distribution.

Clients should include `gc.h`.

In the case of multi-threaded 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.) The header file `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.

Thread users should also be aware that on many platforms objects reachable only from thread-local variables may be prematurely reclaimed. Thus objects pointed to by thread-local variables should also be pointed to by a globally visible data structure. (This is viewed as a bug, but as one that is exceedingly hard to fix without some libc hooks.)

void * GC_MALLOC(size_t nbytes)
  Allocates and clears nbytes of storage. Requires (amortized) time proportional to nbytes. 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. Building the collector with `-DREDIRECT_MALLOC=GC_malloc_uncollectable is often a way around this.) `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.`
void * GC_MALLOC_ATOMIC(size_t nbytes)
  Allocates nbytes of storage. Requires (amortized) time proportional to nbytes. 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 gc_typed.h in the distribution.
void * GC_MALLOC_UNCOLLECTABLE(size_t nbytes)
  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-collectible 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.)
void * GC_REALLOC(void *old, size_t new_size)
  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 uncollectible object, then the new object is uncollectible, and the old object (if different) is deallocated.
void GC_FREE(void *dead)
  Explicitly deallocate an object. Typically not useful for small collectible objects.
void * GC_MALLOC_IGNORE_OFF_PAGE(size_t nbytes)
void * GC_MALLOC_ATOMIC_IGNORE_OFF_PAGE(size_t nbytes)
  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.
void GC_INIT(void)
  On some platforms, it is necessary to invoke this from the main executable, not from a dynamic library, before the initial invocation of a GC routine. It is recommended that this be done in portable code, though we try to ensure that it expands to a no-op on as many platforms as possible. In GC 7.0, it was required if thread-local allocation is enabled in the collector build, and `malloc` is not redirected to `GC_malloc`.
void GC_gcollect(void)
  Explicitly force a garbage collection.
void GC_enable_incremental(void)
  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 (Linux and most Unix versions, win32 if the collector was suitably built) or if "stubborn" allocation is used (see gc.h). On many platforms this interacts poorly with system calls that write to the garbage collected heap.
GC_warn_proc GC_set_warn_proc(GC_warn_proc p)
  Replace the default procedure used by the collector to print warnings. The collector may otherwise write to stderr, most commonly because GC_malloc was used in a situation in which GC_malloc_ignore_off_page would have been more appropriate. See gc.h for details.
void GC_REGISTER_FINALIZER(...)
  Register a function to be called when an object becomes inaccessible. This is often useful as a backup method for releasing system resources (e.g. 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 gc.h for details of the interface. See here for a more detailed discussion of the design.

  Note that an object may become inaccessible before client code is done operating on objects referenced by its fields. Suitable synchronization is usually required. See here or here for details.

If you are concerned with multiprocessor performance and scalability, you should consider enabling and using thread local allocation.

If your platform supports it, you should build the collector with parallel marking support (`-DPARALLEL_MARK`, or `--enable-parallel-mark`).

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 collector using the interfaces in either `gc_typed.h` or `gc_gcj.h`.

The collector distribution also includes a string package that takes advantage of the collector. For details see cord.h

# C++ Interface

The C++ interface is implemented as a thin layer on the C interface. Unfortunately, this thin layer appears to be very sensitive to variations in C++ implementations, particularly since it tries to replace the global ::new operator, something that appears to not be well-standardized. Your platform may need minor adjustments in this layer (gc_cpp.cc, gc_cpp.h, and possibly gc_allocator.h). Such changes do not require understanding of collector internals, though they may require a good understanding of your platform. (Patches enhancing portability are welcome. But it's easy to break one platform by fixing another.)

Usage of the collector from C++ is also complicated by the fact that there are many "standard" ways to allocate memory in C++. The default ::new operator, default malloc, and default STL allocators allocate memory that is not garbage collected, and is not normally "traced" by the collector. This means that any pointers in memory allocated by these default allocators will not be seen by the collector. Garbage-collectible memory referenced only by pointers stored in such default-allocated objects is likely to be reclaimed prematurely by the collector.

It is the programmers responsibility to ensure that garbage-collectible memory is referenced by pointers stored in one of

- Program variables
- Garbage-collected objects
- Uncollected but "traceable" objects

"Traceable" objects are not necessarily reclaimed by the collector, but are scanned for pointers to collectible objects. They are usually allocated by `GC_MALLOC_UNCOLLECTABLE`, as described above, and through some interfaces described below.

(On most platforms, the collector may not trace correctly from in-flight exception objects. Thus objects thrown as exceptions should only point to otherwise reachable memory. This is another bug whose proper repair requires platform hooks.)

The easiest way to ensure that collectible objects are properly referenced is to allocate only collectible objects. This requires that every allocation go through one of the following interfaces, each one of which replaces a standard C++ allocation mechanism. Note that this requires that all STL containers be explicitly instantiated with `gc_allocator`.

STL allocators

  Recent versions of the collector include a hopefully standard-conforming allocator implementation in `gc_allocator.h`. It defines

  - `traceable_allocator`
  - `gc_allocator`
  which may be used either directly to allocate memory or to instantiate container templates. The former allocates uncollectible but traced memory. The latter allocates garbage-collected memory.

  These should work with any fully standard-conforming C++ compiler.

  Users of the SGI extended STL or its derivatives (including most g++ versions) may instead be able to include `new_gc_alloc.h` before including STL header files. This is increasingly discouraged.

  This defines SGI-style allocators

  - `alloc`
  - `single_client_alloc`
  - `gc_alloc`
  - `single_client_gc_alloc`
  The first two allocate uncollectible but traced memory, while the second two allocate collectible memory. The `single_client` versions are not safe for concurrent access by multiple threads, but are faster.

  For an example, click here.

Class inheritance based interface for new-based allocation
  Users may include gc_cpp.h and then cause members of classes to be allocated in garbage collectible memory by having those classes inherit from class gc. For details see gc_cpp.h.

  Linking against libgccpp in addition to the gc library overrides ::new (and friends) to allocate traceable memory but uncollectible memory, making it safe to refer to collectible objects from the resulting memory.

C interface
  It is also possible to use the C interface from gc.h 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, as is done by libgccpp.

  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 well with the SGI versions of the STL only if the `malloc_alloc` allocator is used.