.. _glossary-o: ============================= Memory Management Glossary: O ============================= .. include:: alphabet.txt .. glossary:: object .. aka:: *cell*. In :term:`memory management`, we use the term *object* or *cell* to mean a contiguous :term:`block` of :term:`memory (2)` forming a single logical structure. Objects are the units of :term:`allocation `, :term:`deallocation `, etc. No connection to an object-oriented system is implied. .. mps:specific:: The MPS documentation generally reserves the term *object* for :term:`formatted objects`. For units of allocation in general, it uses the term :term:`block`. object format .. mps:specific:: A data structure provided by the :term:`client program` which describes the format of :term:`objects ` allocated in a :term:`pool`. The MPS uses the :term:`format methods` to find :term:`references` in an object, replace an object with :term:`padding`, replace an object with a :term:`forwarding marker`, and other essential :term:`garbage collection` tasks. See :ref:`topic-format`. object pointer In the :term:`C` programming language, a :term:`pointer` to an :term:`object`, as distinct from a :term:`function pointer`. The C programming language guarantees that you can cast any object pointer to ``void *`` and back without losing information. .. opposite:: :term:`function pointer`. off-white .. aka:: *ecru*. In a :term:`treadmill` :term:`garbage collector`, the :term:`color` off-white is used to describe :term:`objects` which are :term:`free (3)`. :ref:`Baker (1992c) ` used the term *ecru*. .. opposite:: :term:`white`, :term:`gray`, :term:`black`. .. seealso:: :term:`treadmill`, :term:`color`. old space oldspace .. see:: :term:`fromspace`. one-bit reference count The one-bit :term:`reference count ` is a heuristic mechanism that lets a program test, at low cost, whether an :term:`object` is :term:`dead`. The one-bit reference count is a special case of the :term:`limited-field reference count`. A single bit in an object, called the MRB (Multiple Reference Bit), is cleared when the object is :term:`allocated`. Whenever another :term:`reference` to the object is created, the bit is set. Thus, MRB=0 indicates that there is exactly one reference to the object, and MRB=1 indicates that there may be more than one reference to the object. The MRB can be stored in the reference rather than in the object; doing so reduces the number of memory accesses due to MRB checking and setting. When a reference is copied, the copy's MRB is set. If the MRB in the old reference is 0, it also needs to be set. Setting the MRB in the old reference requires that the program knows the location the old reference came from, and that it can prove that location has not since been overwritten with other data. The one-bit reference count is used by a compiler to augment an object lifetime analysis. When compile-time analysis predicts that a particular object may be dead (typically because the variable that references the object is dead), the compiler can generate code that will check the object's MRB at run-time. If the MRB is 0, then the object is dead. Using a one-bit reference count does have a cost: the MRB uses space that could sometimes be put to other use, and the MRB must be set every time the number of references to the object increases. The one-bit reference count is cheaper than other kinds of reference counting, however, since the space cost is only one bit and the reference count is not adjusted when references are destroyed. .. historical:: The one-bit reference count was suggested by :ref:`Friedman & Wise (1977) `. Storing the MRB in the reference was suggested by :ref:`Stoye, Clarke, and Norman (1984) `. .. bibref:: :ref:`Jones et al. (2012) `. opaque type .. mps:specific:: In the MPS interface, an *opaque type* is a pointer to an incomplete structure type. The client programs must not rely on the details of its implementation. For example, the type :c:type:`mps_arena_t` is an alias for ``struct mps_arena_s *``, but the implementation of ``struct mps_arena_s`` is not public. See :ref:`topic-interface`. .. opposite:: :term:`derived type`, :term:`transparent type`. out parameter A function parameter that points to a location for the caller to receive data from the function. .. opposite:: :term:`in parameter`. .. mps:specific:: Out parameters are given names ending with ``_o``. See :ref:`topic-interface`. out-of-band header In some :term:`memory managers`, each :term:`allocated` :term:`block` has additional information (such as the size of the block or a :term:`tag`) stored in a separate block; this is called *an out-of-band header*. .. opposite:: :term:`in-band header`. overcommit In some circumstances, although a range of :term:`virtual addresses` has been :term:`mapped` as far as the user program is concerned, the :term:`physical storage` might not be allocated until it is accessed. This is called *overcommitting*. Overcommitting shares :term:`swap space` resources more flexibly, especially when crude :term:`suballocators` are involved, but it can lead to an out-of-resource error during a :term:`memory (2)` access; few environments deal with this situation gracefully. Unix systems such as IRIX and AIX can do this on :term:`sbrk` and :term:`mmap` calls. overwriting error .. aka:: *bounds error*. An overwriting or bounds error occurs when the programmer intends his program to write to a particular :term:`block` of :term:`memory (1)`, but a program error causes the program to write outside the bounds of that block. .. seealso:: :term:`fencepost`.