# 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 `libgc.a` or `libgc.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. This somewhat duplicates the information in `gc.man` file. The full
interface is described in `gc.h` file.

Clients should include `gc.h` file (i.e., not `gc_config_macros.h`,
`gc_pthread_redirects.h`, `gc_version.h` files). In the case of multi-threaded
code, `gc.h` file should be included after the threads header file, and after
defining `GC_THREADS` macro. `gc.h` file 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 area, e.g. thread's stack. (This behavior is viewed as a bug, but
as one that is exceedingly hard to fix without some `libc` hooks.)

`void * GC_MALLOC(size_t _bytes_)` - Allocates and clears _bytes_
of storage. Requires (amortized) time proportional to _bytes_. 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` file is included, a debugging variant that checks
occasionally for overwrite errors, and the like.

`void * GC_MALLOC_ATOMIC(size_t _bytes_)` - Allocates _bytes_
of storage. Requires (amortized) time proportional to _bytes_. 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` file.

`void * GC_MALLOC_UNCOLLECTABLE(size_t _bytes_)` - 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_object_, size_t _new_bytes_)` - Allocates
a new object of the indicated size and copy the old object's content 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 * _object_)` - Explicitly deallocates an _object_.
Typically not useful for small collectible objects.

`void * GC_MALLOC_IGNORE_OFF_PAGE(size_t _bytes_)` and
`void * GC_MALLOC_ATOMIC_IGNORE_OFF_PAGE(size_t _bytes_)` - 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 heap block (`hblk`) 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. Another way is `GC_set_all_interior_pointers(0)`
called at program start (this, however, is generally not suitable for C++ code
because of multiple inheritance).

`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.

`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
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` file 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` file 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
["Destructors, Finalizers, and Synchronization"](https://dl.acm.org/doi/10.1145/604131.604153)
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 also 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_gcj.h` or `gc_typed.h` file.

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_badalc.cc`,
`gc_cpp.cc`, `gc_cpp.h`, and possibly `gc_allocator.h` files). 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` file. It defines `gc_allocator`
and `traceable_allocator` which may be used either directly to allocate memory
or to instantiate container templates. The former allocates garbage-collected
memory. The latter allocates uncollectible but traced memory.

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

### Class inheritance based interface for new-based allocation

Users may include `gc_cpp.h` file and then cause members of classes to be
allocated in the garbage collectible memory by having those classes inherit
from class `gc`. For details see `gc_cpp.h` file.

Linking against `gccpp` 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.

If the user includes `gc_cpp.h` file but `::new` should not be overridden,
then `gctba` (in addition to the `gc`) library should be linked with to
provide the definition of `GC_throw_bad_alloc` C++ function used by operator
`new` of class `gc`. Alternatively, the client may define
`GC_NEW_ABORTS_ON_OOM` macro before include `gc_cpp.h` file (this instructs
`::new` to issue an abort instead of throwing an exception), or may define
`GC_INCLUDE_NEW` one before include `gc_cpp.h` file (however, this might not
compile or work as expected on some platforms).

## C interface

It is also possible to use the C interface from `gc.h` file directly.
On platforms which use `malloc` to implement `::new`, it should usually be
possible to use a variant 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 `gccpp`.

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 STL `malloc_alloc` allocator is used.
