.. _glossary-c: ============================= Memory Management Glossary: C ============================= .. include:: alphabet.txt .. glossary:: C89 .. see:: :term:`C90`. C90 .. aka:: *C89*. A revision of the ANSI/ISO Standard for the :term:`C` programming language. Although more than twenty years old, it remains the only form of Standard C that is supported by all the major compilers, including Microsoft Visual C. .. mps:specific:: The public interface conforms to this standard. See :ref:`topic-interface`. .. bibref:: :ref:`ISO/IEC 9899:1990 `. C99 A revision of the ANSI/ISO Standard for C the :term:`C` programming language. .. mps:specific:: :term:`Keyword arguments` can be conveniently passed to functions using C99's compound literal syntax. See :ref:`topic-keyword`. .. bibref:: :ref:`ISO/IEC 9899:1999 `. cache (1) .. aka:: *memory cache*, *cache memory*. A processor's memory cache is a small piece of fast, but more expensive memory, usually :term:`static memory (1)`, used for copies of parts of :term:`main memory`. The cache is automatically used by the processor for fast access to any data currently :term:`resident` there. Access to the cache typically takes only a few processor clock cycles, whereas access to :term:`main memory` may take tens or even hundreds of cycles. What part of main memory is resident in a cache, and the mechanisms by which it is kept consistent, are quite varied. See :term:`cache policy`. Some systems have more than one level of cache. "Level 1 cache" is the fastest, smallest :term:`storage level`, "level 2" the next fastest, and so on. .. seealso:: :term:`storage hierarchy`, :term:`cache (2)`. cache (2) A cache is any small, fast piece of :term:`memory (1)`, used for copies of data that normally reside in a larger, slower piece of storage. The cache is used to speed up access to data :term:`resident` in the slower storage. In a typical cache, recently used data is :term:`resident` in the cache (although the details of this depend on the :term:`cache policy`). A :term:`cache (1)` is the most common example of a cache(2). .. seealso:: :term:`storage hierarchy`. cache memory .. see:: :term:`cache (1)`. cache policy Any :term:`cache (3) ` uses a *cache policy* to decide which data to store. A cache policy is an attempt to predict the future, so that the cache will provide swift responses to future requests. Cache policy may be implemented in hardware, software, or a combination of both. Some systems allow programs to influence cache policy, by giving hints or directions about future use of data. There are three main aspects of cache behavior which the cache policy can affect: 1. Fetch policy. This determines which data is fetched into the cache, usually as a result of receiving a request for data that isn't cached. 2. Eviction policy. This determines which data is discarded from the cache to provide space for newly fetched data. 3. Write policy This determines how and when modifications to cached data are synchronized with the underlying storage. .. seealso:: :term:`cache (1)`, :term:`cache (2)`, :term:`cache (3) `. .. bibref:: :ref:`Baker (1991) `, :ref:`Wilson et al. (1992) `, :ref:`Zorn (1991) `. caching (3) .. aka:: *memoization*, *tabling*. *Caching* is a heuristic that stores answers to questions asked in the past in a *cache* or a *table*, in order that they may be more quickly answered in the future. This process is also called memoization and tabling (by the :term:`Prolog` community). A "look-ahead cache" attempts to store answers to questions that will be asked soon. A :term:`cache (2)` is a common example of a cache(3). cactus stack .. aka:: *spaghetti stack*. A cactus stack is a :term:`stack` with branches. When diagrammed, its shape resembles that of a `saguaro cactus `_. In languages that support :term:`continuations`, :term:`activation records` can have :term:`indefinite extent`. One technique for implementing continuations is not to copy the activation records that are captured, but rather to create a fork in the stack below the captured :term:`stack frames`, so that new frames appear as a parallel branch. Often the process of forking is done lazily: captured frames are only duplicated if they are modified. card A card is a division of memory, all cards being of equal size (in a particular area of discourse). A card is usually bigger than a :term:`word` and smaller than a :term:`page`. Cards are used in a technique called :term:`card marking` whereby :term:`dirty bits` (which record which portions of old generations have been written into) are maintained for each card. Often the use of cards will also entail the use of a :term:`crossing map`. card marking A technique for managing :term:`pointer` :term:`stores (1)` into old :term:`generations` (which in turn is used to track :term:`inter-generational pointers`). Each generation is divided into a number of equal-sized :term:`cards`, and when a generation is written into, the particular card written to is recorded (often by using a :term:`bitmap`). Subsequently, when :term:`scanning ` an older generation in order to collect a younger generation, only the recorded cards (in the old generation) need to be scanned. .. seealso:: :term:`generational garbage collection`. .. bibref:: :ref:`Azagury et al. (1998) `, :ref:`Hosking & Hudson (1993) `, :ref:`Sobalvarro (1988) `. cell .. see:: :term:`object`. Cheney collector .. aka:: *Cheney scan*. A Cheney collector uses the :term:`tospace` of a :term:`two-space collector` as a queue of objects remaining to be :term:`scanned `, thus eliminating the need for recursion when :term:`tracing ` the :term:`graph` of :term:`objects`. .. bibref:: :ref:`Cheney (1970) `. Cheney scan .. see:: :term:`Cheney collector`. clamped state .. mps:specific:: One of the three states an :term:`arena` can be in (the others being the :term:`unclamped state` and the :term:`parked state`). In the clamped state, no object motion occurs and the staleness of :term:`location dependencies` does not change. However, a :term:`garbage collection` may be in progress. Call :c:func:`mps_arena_clamp` to put an arena into the clamped state. client arena .. mps:specific:: An :term:`arena class` that gets its :term:`memory (2)` from the :term:`client program`. See :ref:`topic-arena-client`. client object .. mps:specific:: A :term:`formatted object` that contains data from the :term:`client program`. One of three types of formatted objects, the other two being :term:`forwarding objects` and :term:`padding objects`. client program .. see:: :term:`mutator` closure A closure is a function or procedure that is saved along with the current bindings from enclosing blocks for later invocation. Some programming languages, such as :term:`ALGOL`, permit nested blocks to access the local variables of enclosing blocks. :term:`Lisp`-like languages further permit such an inner block (in particular a function or procedure) to be saved for later invocation. The act of saving such an inner block along with the current bindings of variables in the enclosing blocks that are referenced by the inner block, is called *closing over* or *capturing* those variables. The object created is termed *a closure*. A closure is invoked just like the function from which it was built, passing whatever parameters the function accepts, but when the function executes, the variables that belong to enclosing blocks will have the bindings that were in effect when the closure was created. .. relevance:: A closure is typically implemented by saving both the function and any :term:`activation records` that contain variables referenced by the function. The closure creates additional implicit :term:`references` to the bindings closed over and hence must be accounted for in any memory management scheme. The closure itself is an object that must be managed and may have either :term:`dynamic extent` or :term:`indefinite extent` depending on whether it is only used by inner blocks of the creating block or passed out of the creating block. .. seealso:: :term:`continuation`. coalesce Coalescing is the act of merging two adjacent :term:`free blocks`. Coalescing reduces :term:`external fragmentation`, but is not totally effective. Coalescing can be done as soon as blocks are freed, or it can be deferred until some time later (known as :term:`deferred coalescing`), or it might not be done at all. :ref:`Wilson et al. (1995) ` has details about fragmentation, and which coalescing strategies are effective under what circumstances. collect An :term:`object` is collected when it is :term:`reclaimed` by a :term:`garbage collector`. .. similar:: :term:`reclaim`. collection .. see:: :term:`collection cycle`. collection cycle .. aka:: *collection*. A collection cycle is a single complete execution of a :term:`tracing garbage collection` algorithm. Each collection cycle includes (not necessarily in strict order) choosing a :term:`condemned set`; :term:`scanning ` :term:`roots` and :term:`objects` that have not been condemned; :term:`tracing ` the object graph to find all condemned objects that are :term:`reachable`; and :term:`reclaiming ` those that were not reachable. In non-incremental garbage collection, the :term:`mutator` pauses at the start of a collection cycle and cannot continue until it is complete. In :term:`incremental ` and :term:`parallel ` garbage collection, a collection cycle can be interleaved with, or simultaneous to, mutator activity. collector (1) .. see:: :term:`garbage collector`. collector (2) In a :term:`garbage-collected ` system, the part that executes the garbage collection code, which discovers unused :term:`memory (1)` and :term:`reclaims` it. For purposes of describing :term:`incremental garbage collection`, the system is divided into the :term:`mutator` and the *collector*. These can be separate threads of computation, or interleaved within the same thread. .. historical:: This term is due to :ref:`Dijkstra et al. (1976) `. .. opposite:: :term:`mutator`. color colour In a :term:`tri-color marking` scheme, each :term:`node` has a one of three colors: :term:`black`, :term:`white`, or :term:`gray`. In a :term:`treadmill`, nodes may also be colored :term:`off-white`. commit limit .. mps:specific:: The commit limit is a limit on the :term:`committed ` :term:`memory (2)` that the :term:`arena` will obtain from the operating system. It can be changed by calling :c:func:`mps_arena_commit_limit_set`. committed (1) .. see:: :term:`mapped`. committed (2) .. mps:specific:: A block has been *committed* if it is fully initialized and is under the management of the MPS, as opposed to a block that is merely *reserved*. See :ref:`topic-allocation-point-protocol`. compactifying .. see:: :term:`compaction`. compaction .. aka:: *compactifying*. Compaction is the process of :term:`moving ` :term:`live` :term:`objects` to eliminate :term:`dead` space between them. Some people call this *compactifying*, to distinguish it from techniques for compressing data structures. Compaction is used to avoid :term:`external fragmentation` and to increase :term:`locality of reference`. composite object In the :term:`PostScript` language, *composite objects* are the :term:`boxed` objects. Unlike a :term:`simple object`, the main data (what PostScript calls *the value*) in a composite object are stored separately, in :term:`VM (2)`. Several composite objects can share the same value. .. similar:: :term:`boxed`. .. opposite:: :term:`simple object`. comprehensive A :term:`collector (1)` is *comprehensive* if all :term:`garbage` (or, all :term:`unreachable` :term:`objects`) is :term:`reclaimed` in one :term:`collection cycle`. .. seealso:: :term:`garbage collection`. concurrent garbage collection .. see:: :term:`parallel garbage collection`. condemned set .. aka:: *threatened set*. *Condemned* :term:`objects` are those which are candidates for :term:`recycling ` within a :term:`collection cycle`. At the start of a collection cycle, the :term:`collector (1)` may choose to condemn some objects (the *condemned set* or *threatened set*) but not to condemn others (the :term:`immune set`). Objects that are not condemned are assumed to be :term:`live` and behave as :term:`roots` for the purposes of that collection cycle. Many simple :term:`tracing garbage collection` algorithms begin by condemning all objects, but :term:`generational garbage collectors ` will condemn individual :term:`generations` or combinations of generations. Often young generations are condemned but older ones are not, because objects in older generations are less likely to have become :term:`unreachable`. In collectors using :term:`tri-color marking`, at the start of a collection cycle the condemned set is exactly the set of objects that the collector colors :term:`white`. .. opposite:: :term:`immune set`. connected :term:`Objects` are connected if and only if one contains a :term:`reference` to the other. .. seealso:: :term:`graph`. cons (1) In :term:`Lisp`, ``cons`` is a primitive operation creating a list element (from English "CONStruct"). By extension, a *cons* is the element created. .. link:: `Function CONS in the Common Lisp HyperSpec `_. cons (2) .. see:: :term:`allocate`. conservative garbage collection In conservative :term:`garbage collection`, the layout of :term:`objects` and :term:`roots` is not known, instead the :term:`collector (1)` assumes that any field that looks like a :term:`pointer` *might* be a :term:`reference`. Conservative collectors can work with programs where information about the :term:`memory (2)` layout is not available, because, for example, the language doesn't support :term:`garbage collection`. A conservative collector doesn't need to know the :term:`format` of the objects, it just needs some idea of where the object boundaries are. It regards any field value that looks like a pointer to an object (or, sometimes, into the middle of one), as preventing the :term:`recycling ` of that object. It can't :term:`move ` objects, because then the references to the moved objects would need to be updated, and such :term:`ambiguous references` must not be modified, in case they weren't pointers after all. Therefore, conservative collectors are usually :term:`mark-sweep collectors `. Because references are ambiguous, some objects may be retained despite being actually :term:`unreachable`. In practice, this happens rarely, and refinements such as :term:`black-listing ` can further reduce the odds. .. opposite:: :term:`exact garbage collection`. .. seealso:: :term:`ambiguous root`, :term:`semi-conservative garbage collection`, :term:`interior pointer`. .. bibref:: :ref:`Boehm & Weiser (1988) `, :ref:`Boehm (1993) `. constant root .. mps:specific:: A :term:`root` that the :term:`client program` promises not change after it is registered, by specifying the :term:`root mode` :c:macro:`MPS_RM_CONST` when calling a registration function such as :c:func:`mps_root_create`. constructor (1) A constructor is a function or method that :term:`allocates` and initializes an :term:`object`. .. opposite:: :term:`destructor (1)`. constructor (2) In :term:`C++`, a *constructor* is a member function that is used to initialize a newly-:term:`allocated` object. The actual allocation of :term:`memory (2)` is performed by ``operator new`` or the compiler (for :term:`static ` and :term:`stack allocation`), and the new :term:`block` is then passed to the appropriate constructor. .. seealso:: :term:`destructor (2)`. continuation A continuation is the data required to restore an execution context after invocation of another context, typically as a subroutine. .. relevance:: If continuations can be represented as first-class objects, as in :term:`Scheme`, the execution contexts can no longer be stored on a :term:`stack`, instead, (at least some) :term:`activation records` have to be :term:`heap-allocated `. .. seealso:: :term:`closure`. control stack .. aka:: *activation stack*, *execution stack*. A :term:`stack` that stores :term:`activation records`, particularly subroutine return information, is known as a *control stack*. Typically the control stack is supported and used by the hardware architecture and the operating system, limiting the types and sizes of :term:`objects` that can be stored on it. Often, only one type of object, a :term:`stack frame`, is permitted, and the layout of that is defined by the hardware architecture. .. relevance:: Theoretically, a control stack is simply an array of activation records, and hence just another object managed by the :term:`memory manager`. In practice, the control stack is central to the performance of the hardware architecture and may require special treatment. In particular, it may not be accessible as ordinary :term:`memory (2)`, or it may have its own :term:`cache (2)` with specific updating requirements. .. similar:: :term:`stack`. .. seealso:: :term:`data stack`. cool .. mps:specific:: A :term:`variety` in which most MPS functions :term:`assert ` that their data structures are valid, even functions on the :term:`critical path`. See :ref:`guide-build`. Compare :term:`hot` and :term:`rash`. copy method .. mps:specific:: A copy method is one of the methods in an :term:`object format`. Formerly, the MPS called this method to copy a :term:`formatted object` during :term:`moving garbage collection `. Now it just copies the bytes and the copy method is ignored. copying garbage collection .. aka:: *scavenging garbage collection*. Copying garbage collection is a kind of :term:`tracing garbage collection` that operates by :term:`relocating ` :term:`reachable` :term:`objects` (this is sometimes called *scavenging*) and then :term:`reclaiming ` objects that are left behind, which must be :term:`unreachable` and therefore :term:`dead`. A copying garbage collection relies on being able to find and correct all :term:`references` to copied objects. .. figure:: ../diagrams/copying.svg :align: center :alt: Diagram: Copying garbage collection. Copying garbage collection. .. similar:: :term:`moving `. .. seealso:: :term:`broken heart`, :term:`forwarding pointer`, :term:`two-space collector`. core A historical synonym for :term:`main memory`, deriving from the *cores* or ferrite rings which were once the main technology used to implement it. .. similar:: :term:`main memory`. creation space In :term:`generational garbage collection`, when :term:`generations` are divided into :term:`buckets`, the creation space is where new :term:`objects` are created in each generation. This term is sometimes used as a synonym for :term:`nursery space`. .. opposite:: :term:`aging space`. .. seealso:: :term:`generational garbage collection`. critical path .. mps:specific:: The sequence of operations on which the MPS spends the majority of its time, consisting of :term:`scanning `, :term:`fixing `, :term:`marking` and :term:`copying `. See :ref:`topic-critical`. crossing map Where :term:`memory (2)` has already been divided into some fixed-sized unit (for example, :term:`pages` or :term:`cards`), a crossing map records where :term:`objects` lie across the boundaries of the fixed-sized units. In other words, which fixed-sized units do not start with the beginning of an object. A system which implements :term:`remembered sets` by :term:`page marking` or :term:`card marking` needs to scan all the :term:`pointers` in the page or card. If the system can not :term:`scan` partial objects (or requires information in the object :term:`header ` in order to scan a partial object), a crossing map is necessary to find the beginning of the first object in the unit. .. relevance:: In a sense, a crossing map is an optimization of :term:`tagged architecture`. It represents the minimum information necessary to determine how to interpret any word of memory. cyclic data structure A data structure is cyclic if some of its :term:`references` form a loop; that is, there's an :term:`object` that can be reached by following references from itself.