.. _lang:
Memory management in various languages
**************************************
.. glossary::
ALGOL
ALGOL, designed in 1958 for scientific computing, was the
first block-structured language. It spawned a whole family of
languages, and inspired many more, including :term:`Scheme`,
:term:`Simula` and :term:`Pascal`.
The block structure of ALGOL 60 induced a :term:`stack
allocation` discipline. It had limited dynamic arrays, but no
general :term:`heap allocation`. The substantially redesigned
ALGOL 68 had both heap and stack allocation. It also had
something like the modern :term:`pointer` type, and required
:term:`garbage collection` for the heap. The new language was
complex and difficult to implement, and it was never as
successful as its predecessor.
.. bibref:: :ref:`Branquart & Lewi (1972) <BL72>`.
BASIC
BASIC is a simple and easily-learned programming language
created by T. E. Kurtz and J. G. Kemeny in 1963–4. The
motivation was to make computers easily accessible to
undergraduate students in all disciplines.
Most BASICs had quite powerful string handling operations that
required a simple :term:`garbage collector`. In many
implementations, the garbage collector could be forced to run
by running the mysterious expression ``FRE("")``.
BASIC is now old-fashioned, but survives as a scripting
language, in particular in Visual BASIC, which is an
application development environment with a BASIC-like
scripting language. These descendants invariably have
automatic memory management as well.
C
C is a systems programming language sometimes described as "a
portable assembler" because it was intended to be sufficiently
low-level to allow performance comparable to assembler or
machine code, but sufficiently high-level to allow programs to
be reused on other platforms with little or no modification.
:term:`Memory management` is typically manual (the standard
library functions for :term:`memory (2)` management in C,
:term:`malloc` and :term:`free (2)`, have become almost
synonymous with :term:`manual memory management`), although
with the Boehm-Weiser :term:`collector (1)`, it is now
possible to use :term:`garbage collection`.
The language is notorious for fostering memory management
bugs, including:
1. Accessing arrays with indexes that are out of bounds;
2. Using :term:`stack-allocated <stack allocation>` structures
beyond their :term:`lifetimes` (see :term:`use after
free`);
3. Using :term:`heap-allocated <heap allocation>` structures
after :term:`freeing <free (1)>` them (see :term:`use after
free`);
4. Neglecting to free heap-allocated objects when they are no
longer required (see :term:`memory leak`);
5. Failing to allocate memory for a :term:`pointer` before using it;
6. Allocating insufficient memory for the intended contents;
7. Loading from allocated memory before storing into it;
8. Dereferencing non-pointers as if they were pointers.
.. seealso:: :term:`automatic storage duration`, :term:`static storage duration`.
.. bibref:: :ref:`ISO/IEC 9899:1990 <C1990>`, :ref:`ISO/IEC 9899:1999 <C1999>`, :ref:`Boehm & Weiser (1988) <BW88>`, :ref:`Daconta (1993) <DACONTA93>`, :ref:`Zorn (1993) <ZORN93>`.
.. link::
`Boehm-Weiser collector <http://www.hpl.hp.com/personal/Hans_Boehm/gc/>`_,
`C standardization <http://www.open-std.org/jtc1/sc22/wg14/>`_,
`comp.lang.c Frequently Asked Questions <http://c-faq.com/>`_.
COBOL
COBOL was designed by the CODASYL committee in 1959–60 to be a
business programming language, and has been extended many
times since. A 1997 Gartner Group report estimated that 80% of
computer software (by count of source lines) was written in
COBOL.
Prior to 2002, COBOL had no :term:`heap allocation`, and did
well in its application domain without it. COBOL 2002 has
:term:`pointers` and heap allocation through ``ALLOCATE`` and
``FREE``, mainly in order to be able to use C-style
interfaces. It also supports a high level of abstraction
through object-oriented programming and :term:`garbage
collection` (including :term:`finalization`).
.. link::
`COBOL standardization <http://www.cobolstandard.info/wg4/wg4.html>`_.
Common Lisp
Common Lisp is the major dialect of the :term:`Lisp` family.
In addition to the usual Lisp features, it has an advanced
object system, data types from hash tables to complex numbers,
and a rich standard library.
Common Lisp is a :term:`garbage-collected <garbage
collection>` language, and modern implementations, such as
`LispWorks <http://www.lispworks.com/>`_ and `Allegro CL
<http://www.franz.com/products/allegro-common-lisp/>`_,
include advanced features, such as :term:`finalization` and
:term:`weakness <weak reference (1)>`.
.. link::
`Common Lisp HyperSpec <http://www.lispworks.com/documentation/HyperSpec/Front/>`_.
C#
C# is a strongly typed object-oriented language created at
Microsoft in 1999–2000. It is designed to run on the Common
Language Runtime, the virtual machine from the .NET Framework.
It also runs on the open source Mono runtime.
Memory is :term:`automatically managed <automatic memory
management>`: memory is allocated when an object is created,
and reclaimed at some point after the object becomes
:term:`unreachable`.
The language supports :term:`finalization` (classes may have
*destructor functions*, which are run just before the object
is reclaimed by the memory manager), and :term:`weak
references (1)` (via the ``WeakReference`` class).
The :term:`garbage collector` in the .NET Framework is
configurable to run in soft real time, or in batch mode.
The Mono runtime comes with two collectors: the Boehm–Weiser
:term:`conservative collector <conservative garbage
collection>`, and a :term:`generational <generational garbage
collection>` :term:`copying collector <copying garbage
collection>`.
.. link::
`Automatic memory management in C# <http://msdn.microsoft.com/en-us/library/aa691138.aspx>`_,
`WeakReference Class <http://msdn.microsoft.com/en-us/library/system.weakreference.aspx>`_,
`Memory Management and Garbage Collection in the .NET Framework <http://msdn.microsoft.com/en-us/library/hh156531.aspx>`_,
`Mono project <http://www.mono-project.com/Main_Page>`_.
C++
C++ is a (weakly) object-oriented language, extending the
systems programming language :term:`C` with a
multiple-inheritance class mechanism and simple method
dispatch.
The standard library functions for :term:`memory (2)`
management in C++ are ``new`` and ``delete``. The higher
abstraction level of C++ makes the bookkeeping required for
:term:`manual memory management` even harder. Although the
standard library provides only manual memory management, with
the Boehm-Weiser :term:`collector (1)`, it is now possible to
use :term:`garbage collection`. :term:`Smart pointers` are
another popular solution.
The language is notorious for fostering memory management
bugs, including:
1. Using :term:`stack-allocated <stack allocation>` structures
beyond their :term:`lifetimes <lifetime>` (see :term:`use
after free`);
2. Using :term:`heap-allocated <heap allocation>` structures
after :term:`freeing <free (1)>` them (see :term:`use after
free`);
3. Neglecting to free heap-allocated objects when they are no
longer required (see :term:`memory leak`);
4. Excessive copying by copy :term:`constructors (1)`;
5. Unexpected sharing due to insufficient copying by copy
constructors;
6. Allocating insufficient memory for the intended contents;
7. Accessing arrays with indexes that are out of bounds.
.. historical::
C++ was designed by Bjarne Stroustrup, as a minimal
object-oriented extension to C. It has since grown to
include some other modern programming language ideas. The
first implementations were preprocessors that produced C
code, but modern implementations are dedicated C++
compilers.
Ellis and Stroustrup write in *The Annotated C++ Reference
Manual*:
C programmers think memory management is too important to
be left to the computer. Lisp programmers think memory
management is too important to be left to the user.
.. seealso:: :term:`constructor (2)`, :term:`destructor (2)`.
.. bibref:: :ref:`Attardi & Flagella (1994) <ATTARDI94>`, :ref:`Bartlett (1989) <BARTLETT89>`, :ref:`Boehm & Weiser (1988) <BW88>`, :ref:`Edelson (1992) <EDELSON92>`, :ref:`Ellis (1993) <ELLIS93>`, :ref:`Zorn (1993) <ZORN93>`.
.. link::
`comp.lang.c++ FAQ <http://www.parashift.com/c++-faq/>`_,
`C++ standardization <http://www.open-std.org/jtc1/sc22/wg21/>`_.
Dylan
Dylan is a modern programming language invented by Apple
around 1993 and developed by `Harlequin
<http://en.wikipedia.org/wiki/Harlequin_(software_company)>`_
and other partners. The language is a distillation of the best
ideas in dynamic and object-oriented programming. Its
ancestors include :term:`Lisp`, :term:`Smalltalk`, and
:term:`C++`. Dylan is aimed at building modular component
software and delivering safe, compact applications. It also
facilitates the rapid development and incremental refinement
of prototype programs.
Dylan provides :term:`automatic memory management`. The
generic allocation function is called ``make``. Most
implementations provide :term:`finalization` and :term:`weak
<weak reference (1)>` hash tables, although interfaces for
these features have not yet been standardized. An object may
be registered for finalization via the function
``finalize-when-unreachable``, in which case there will be a
call to the ``finalize`` function once the :term:`garbage
collector` has determined that the object is
:term:`unreachable`. Weak hash tables may have either weak
keys or values, depending on a parameter supplied at
allocation time. A hash table entry will be deleted once the
garbage collector has determined that there are no
:term:`strong references` to the key or value of the entry,
for weak key or value tables, respectively.
.. link::
`Open Dylan <http://opendylan.org/>`_.
Emacs Lisp
Emacs Lisp or elisp is a dialect of :term:`Lisp` used in the
Emacs family of text editors, of which the most widely-used is
`GNU Emacs <http://www.gnu.org/software/emacs/emacs.html>`_.
Like most Lisps, Emacs Lisp requires :term:`garbage
collection`. GNU Emacs has a simple :term:`mark-sweep`
collector. It has been speculated that the
non-:term:`incremental <incremental garbage collection>`
nature of the Emacs collector, combined with the fact that,
prior to version 19.31 (May 1996), it printed a message
whenever it collected, gave garbage collection a bad name in
programming circles.
Erik Naggum reported at the time:
I have run some tests at the U of Oslo with about 100
users who generally agreed that Emacs had become faster in
the latest Emacs pretest. All I had done was to remove the
"Garbage collecting" message which people perceive as
slowing Emacs down and tell them that it had been sped up.
It is, somehow, permissible for a program to take a lot of
time doing any other task than administrative duties like
garbage collection.
Emacs was originally written in Teco, not in Lisp, but it
still had a garbage collector, though this was heuristic and
conservative in nature. Teco-based Emacs was capable of
running for weeks at a time in a 256 kB :term:`address space`.
.. link::
`GNU Emacs Lisp Reference Manual <http://www.gnu.org/software/emacs/manual/elisp.html>`_,
`Entry on Garbage Collection <http://www.gnu.org/software/emacs/manual/html_node/elisp/Garbage-Collection.html>`_.
Fortran
Fortran, created in 1957, was one of the first languages
qualifying as a high-level language. It is popular among
scientists and has substantial support in the form of
numerical libraries.
Early versions of Fortran required the size of arrays to be
known at compilation time, and the earliest Fortran compilers
accordingly used only :term:`static allocation` (however, the
1966 standard gave compiler writers freedom to use other
allocation mechanisms).
The Fortran 90 standard added recursion and automatic arrays
with :term:`stack allocation` semantics (though many compilers
in fact allocate them on the :term:`heap`). It also added
:term:`dynamic allocation` using ``ALLOCATE`` with manual
deallocation using ``DEALLOCATE``. Fortran 95 made it explicit
that allocated arrays have :term:`dynamic extent` and are
automatically deallocated when they go out of scope.
.. link::
`Fortran standardization <http://www.j3-fortran.org/>`_.
Java
A modern object-oriented language with a rich collection of
useful features. The Java language started as an attempt by
the Java group at Sun Microsystems to overcome software
engineering problems introduced by :term:`C++`. Key reasons
for the language's success were the security model and the
portable execution environment, the Java Virtual Machine
(JVM), which created a lot of interest for it as a platform
for distributed computing on open networks.
Java is :term:`garbage-collected <garbage collection>`, as
this facilitates object-oriented programming and is essential
for security (which :term:`use after free` would break). It
had :term:`finalization` from version 1.0 and three kinds of
:term:`weakness <weak reference (1)>` from version 1.2
(confusingly, part of the Java 2 Platform).
Early JVMs had simple collectors that didn't scale well for
large programs, but the current crop is catching up to the
state of the art.
.. seealso:: :term:`reference object`, :term:`strong reference`, :term:`soft reference`, :term:`weak reference (2)`, :term:`phantom reference`, :term:`strongly reachable`, :term:`softly reachable`, :term:`weakly reachable`, :term:`phantom reachable`.
JavaScript
JavaScript is a scripting language used by web browsers. The
loose type system resembles other scripting languages,
although the syntax follows :term:`C`. There's a
prototype-based object system. Note that JavaScript is not
related to :term:`Java` in any way except name. There's a
standard by `ECMA <http://www.ecma-international.org>`_, known
as ECMAScript.
Despite the :term:`C++`\-like syntax (with ``new`` and
``delete`` operators), JavaScript is :term:`garbage-collected
<garbage collection>`.
.. link::
`Standard ECMA-262: ECMAScript Language Specification <http://www.ecma-international.org/publications/standards/Ecma-262.htm>`_.
Lisp
Lisp is a family of computer languages combining functional
and procedural features with automatic memory management.
Lisp was invented by John McCarthy around 1958 for the
manipulation of symbolic expressions. As part of the original
implementation of Lisp, he invented :term:`garbage
collection`. He noted:
This process, because it is entirely automatic, is more
convenient for the programmer than a system in which he
has to keep track of lists and erase unwanted lists.
Modern Lisp implementations, such as `LispWorks
<http://www.lispworks.com/>`_ and `Allegro CL
<http://www.franz.com/products/allegro-common-lisp/>`_, have
advanced :term:`garbage collectors`.
Lisp is now used for all kinds of symbolic programming and
other advanced software development. Major dialects today are
:term:`Emacs Lisp`, :term:`Common Lisp` and :term:`Scheme`.
Most modern dialects and related languages, such as
:term:`Dylan`, are object-oriented.
.. seealso:: :term:`cons (1)`.
.. bibref:: :ref:`Baker (1978) <BAKER78>`, :ref:`Edwards <EDWARDS>`, :ref:`McCarthy & Minsky (1959) <MM59>`, :ref:`McCarthy (1960) <MCCARTHY60>`, :ref:`McCarthy (1979) <MCCARTHY79>`, :ref:`Moon (1984) <MOON84>`, :ref:`Moon (1990) <MOON87>`, :ref:`Moon (1991) <MOON91>`, :ref:`Sobalvarro (1988) <SOBALVARRO88>`, :ref:`Zorn (1988) <ZORN88>`.
.. link::
`Common Lisp HyperSpec <http://www.lispworks.com/documentation/HyperSpec/Front/>`_.
Lisp Machine
Of particular interest in the history of memory management are
the *Lisp Machines*, early workstation computers built around
a custom processor designed to improve the execution speed of
Lisp by implementing primitive Lisp operations in microcode.
The Lisp Machine :term:`garbage collector` is a generalization
of the algorithm described in :ref:`Baker (1978) <BAKER78>`
and used a technique similar to that described in :ref:`Ungar
(1984) <UNGAR84>`, but utilizing hardware to improve
performance.
A description of the garbage collector of one particular model
is in :ref:`Moon (1984) <MOON84>`. The features important for
its performance were:
1. Hardware support for data typing using :term:`tags`;
2. Reference-based :term:`read barriers` for
:term:`incremental <incremental garbage collection>`
collecting;
3. :term:`Write barriers` for :term:`remembered sets` and
:term:`generational <generational garbage collection>`
collecting;
4. A tight integration with the :term:`virtual memory`
system.
The remembered sets were based on a :term:`BIBOP` division of
the virtual :term:`address space`. The Lisp Machine
:term:`page table`, unlike virtually all modern virtual memory
systems, was a flat, hash-based table (sometimes called an
:term:`inverted page table`), and thus insensitive to
sparsely-populated virtual address spaces associated with
BIBOP schemes.
These custom processors eventually lost out to rapidly
advancing stock hardware. Many of the techniques pioneered on
Lisp Machines are used in today's implementations, at a cost
of a few more cycles.
Lua
Lua is a dynamically typed language created by Roberto
Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes
in 1993. The language supports object-oriented and functional
styles of programming, and is designed to be easily embedded
in a larger programming system as an extension or scripting
language.
Lua uses :term:`automatic memory management` and comes with a
:term:`non-moving <non-moving garbage collector>`
:term:`incremental <incremental garbage collection>`
:term:`garbage collector` supporting soft real time
applications. This uses a software :term:`barrier (1)` in
order to be highly portable.
The language supports :term:`weak references (1)` in the form
of weak (hash) tables, which have the unusual feature that
their keys and values can be dynamically switched from being
:term:`strong references` to weak references, and vice versa
(by assigning to the ``__mode`` field of the table's
metatable). It also supports :term:`finalization` (by
assigning the ``__gc`` field of the object's metatable).
.. link::
`Lua <http://lua.org>`_,
`Garbage Collection <http://www.lua.org/manual/5.1/manual.html#2.10>`_.
ML
ML is a family of strongly-typed functional languages, of
which the principal members are Standard ML and Caml.
Like other functional languages, ML provides :term:`automatic
memory management`. Modern ML implementations usually have
advanced :term:`garbage collectors`. The combination of clean
functional semantics and strong typing allows advanced
techniques, such as :term:`region inference`.
The Standard ML of New Jersey (SML/NJ) system, which
implements a slight variant of Standard ML, has been important
to memory management research for three reasons. Firstly, the
source code is publicly available and widely ported, allowing
experimentation with both the :term:`collector (2)` and
:term:`mutator`. Secondly, the compiler generates code that
does not use a :term:`control stack`, but :term:`allocates`
function :term:`activation records` on the :term:`heap`
instead. This means that the allocation rate is very high (up
to one byte per instruction), and also that the collector has
a very small :term:`root set`. Thirdly, it uses a simple
:term:`copying collector <copying garbage collection>` that is
easy to modify.
.. seealso:: :term:`immutable`.
.. bibref:: :ref:`Cooper et al. (1992) <CNS92>`, :ref:`Doligez (1993) <DOLIGEZ93>`, :ref:`Tofte & Talpin (1997) <TT97>`.
.. link::
`comp.lang.ml FAQ <http://www.faqs.org/faqs/meta-lang-faq/>`_.
Modula-3
An object-oriented descendant of :term:`Pascal`.
Modula-3 is mostly :term:`garbage-collected <garbage
collection>`, although it is possible to use :term:`manual
memory management` in certain modules.
.. link::
`modula3.org <http://www.modula3.org/>`_,
`Modula-3 language definition <http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-52.pdf>`_.
Pascal
An imperative language characterized by block structure and a
relatively strong (for its time) static type system. Pascal
was designed by Niklaus Wirth around 1970.
Pascal was popular as a teaching language due to its small
size, but it lacked many features needed for applications
programming. Now it's been largely supplanted by its more
feature-rich descendants Modula-2, :term:`Modula-3`, and
Oberon, mainly surviving in the popular Delphi development
tool.
Pascal uses :term:`manual memory management` (with the
operators ``NEW`` and ``DISPOSE``). The descendants mentioned
all offer :term:`automatic memory management`.
.. link::
`Borland Delphi Home Page <http://www.borland.com/delphi/>`_,
`Pascal standardization <http://www.open-std.org/JTC1/sc22/docs/oldwgs/wg2.html>`_.
Perl
Perl is a complex but powerful language that is an eclectic
mixture of scripting languages and programming languages.
Perl programmers can work with strings, arrays, and
associative arrays without having to worry about :term:`manual
memory management`. Perl is well-suited to complex text file
manipulation, such as report generation, file format
conversion, and web server CGI scripts. It is also useful for
rapid prototyping, but large Perl scripts are often
unmaintainable.
Perl's :term:`memory management` is well-hidden, but is based
on :term:`reference counts <reference counting>` and
:term:`garbage collection`. It also has *mortal* variables,
whose :term:`lifetimes` are limited to the current context. It
is possible to :term:`free (1)` the :term:`memory (2)`
assigned to variables (including arrays) explicitly, by
``undef``\-ing the only reference to them.
.. link::
`The Perl Programming Language <http://www.perl.org/>`_.
PostScript
The PostScript language is an interpretive language with
powerful graphics features, widely used as a page description
language for printers and typesetters.
The Level 1 PostScript language has a simple
:term:`stack`\-like memory management model, using ``save``
and ``restore`` operators to :term:`recycle` memory. The Level
2 PostScript language adds :term:`garbage collection` to this
model.
.. seealso:: :term:`VM (2)`, :term:`composite object`, :term:`simple object`.
.. link::
`Harlequin RIP <http://en.wikipedia.org/wiki/Harlequin_RIP>`_.
Prolog
A logic programming language invented by Alain Colmerauer
around 1970, Prolog is popular in the AI and symbolic
computation community. It is special because it deals directly
with relationships and inference rather than functions or
commands.
Storage is usually managed using a :term:`garbage collector`,
but the complex control flow places special requirements on
the collector.
.. link::
`Prolog Standardization <http://people.sju.edu/~jhodgson/wg17/>`_,
`Prolog Memory Management - Garbage Collection <http://www.informatik.uni-trier.de/%7Eley/db/prolog/gc.html>`_.
Python
Python is a "duck-typed" object-oriented language created in
the early 1990s by Guido van Rossum.
There are several implementations running on a variety of
virtual machines: the original "CPython" implementation runs
on its own virtual machine; IronPython runs on the Common
Language Runtime; Jython on the Java Virtual Machine.
CPython manages memory using a mixture of :term:`reference
counting` and :term:`non-moving <non-moving memory manager>`
:term:`mark-and-sweep` :term:`garbage collection`. Reference
counting ensures prompt deletion of objects when their
reference count falls to zero, while the garbage collector
reclaims :term:`cyclic data structures`.
The language supports :term:`finalization` (classes may have a
``__del__`` method, which is run just before the object is
destroyed), and :term:`weak references (1)` (via the
``weakref`` module).
.. link::
`Python <http://python.org/>`_,
`Garbage Collector interface <http://docs.python.org/3/library/gc.html>`_,
`__del__ method <http://docs.python.org/3/reference/datamodel.html#object.__del__>`_,
`weakref module <http://docs.python.org/3/library/weakref.html>`_.
Scheme
A small functional language blending influences from
:term:`Lisp` and :term:`Algol`.
Key features of Scheme include symbol and list operations,
:term:`heap allocation` and :term:`garbage collection`,
lexical scoping with first-class function objects (implying
:term:`closures`), reliable tail-call elimination (allowing
iterative procedures to be described tail-recursively), the
ability to dynamically obtain the current :term:`continuation`
as a first-class object, and a language description that
includes a formal semantics.
Scheme has been gaining popularity as an extension language;
Project GNU's extension package of choice, `Guile
<http://www.gnu.org/software/guile/>`_, is a Scheme
interpreter. :term:`Garbage collection` is an important part
of the ease of use that is expected from an extension
language.
.. link::
`Scheme Standards documents <http://www.cs.indiana.edu/scheme-repository/doc.standards.html>`_,
`Scheme Requests for Implementation <http://srfi.schemers.org/>`_.
Simula
Simula was designed as a language for simulation, but it
expanded into a full general-purpose programming language and
the first object-oriented language.
Simula I, designed in 1962–64 by Kristen Nygaard and Ole-Johan
Dahl, was based on :term:`ALGOL` 60, but the :term:`stack
allocation` discipline was replaced by a two-dimensional
:term:`free list`.
It was Simula 67 that pioneered classes and inheritance to
express behavior. This domain-oriented design was supported by
:term:`garbage collection`.
.. bibref:: :ref:`Dahl (1963) <DAHL63>`.
Smalltalk
Smalltalk is an object-oriented language with single
inheritance and message-passing.
:term:`Automatic memory management` is an essential part of
the Smalltalk philosophy. Many important techniques were first
developed or implemented for Smalltalk.
.. bibref:: :ref:`Deutsch & Bobrow (1976) <DB76>`, :ref:`Ungar (1984) <UNGAR84>`, :ref:`Ungar (1988) <UNGAR88>`.
.. link::
`Smalltalk standardization <http://www.smalltalk.org/versions/ANSIStandardSmalltalk.html>`_.