# C/C++ Interface

On many platforms, a single-threaded garbage collector library can be built
to act as a plug-in `malloc` replacement. (Build it 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` file.

Clients should include `gc.h` (i.e., not `gc_config_macros.h`,
`gc_pthread_redirects.h`, `gc_version.h`). In the case of multi-threaded code,
`gc.h` should be included after the threads header file, and after defining
`GC_THREADS` macro. 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`.

**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_)** - Allocates 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 deallocates an object.
Typically not useful for small collectible objects.

**void * `GC_MALLOC_IGNORE_OFF_PAGE`(size_t _nbytes_)** and
**void * `GC_MALLOC_ATOMIC_IGNORE_OFF_PAGE`(size_t _nbytes_)** - Analogous
to `GC_MALLOC` and `GC_MALLOC_ATOMIC`, respectively, 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
more than 100 KB 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()`** - 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 v7.0, 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 forces a garbage collection.

**void `GC_enable_incremental`(void)** - Causes 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). On many platforms this
interacts poorly with system calls that write to the garbage collected heap.

**void `GC_set_warn_proc`(GC_warn_proc)** - Replaces 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`(...)** - Registers 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 also [here](finalization.md) 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](http://portal.acm.org/citation.cfm?doid=604131.604153)
or [here](http://www.hpl.hp.com/techreports/2002/HPL-2002-335.html) 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`); configure has it on by default.

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` file.

## 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 is 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`
and `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](http://www.sgi.com/tech/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

  * `traceable_alloc`
  * `single_client_traceable_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.

See sample code [here](http://www.hboehm.info/gc/gc_alloc_exC.txt).

### 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` file.

Linking against `libgccpp` in addition to the `gc` library overrides `::new`
(and friends) to allocate traceable 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.
