Handle (computing)
Encyclopedia
In computer programming
, a handle is a particular kind of smart pointer
. Handles are used when application software
references blocks of memory or objects managed by another system, such as a database
or an operating system
.
While a pointer literally contains the address
of the item to which it refers, a handle is an abstraction
in the form of a reference
controlled by a separate system; its opacity allows the referent to be relocated in memory by the system without invalidating the handle, which is impossible with pointers. The extra layer of indirection
also increases the control the managing system has over operations performed on the referent. Typically the handle is an index or a pointer into a global array of tombstone
s.
Handles were a popular solution to memory management
in operating systems of the 1980s, such as Mac OS
and Windows
. Unix
file descriptor
s are essentially handles. Like other desktop environment
s, the Windows API
heavily uses handles to represent objects in the system and to provide a communication pathway between the operating system and user space
. For example, a window on the desktop
is represented by a handle of type
Doubly indirect handles have fallen out of favour in recent times, as increases in available memory and improved virtual memory
algorithms have made the use of the simpler pointer more attractive. However, many operating systems still apply the term to pointers to opaque, "private" data structure
s—opaque pointer
s—or to indexes into internal arrays passed from one process
to its client
.
A handle leak
is a type of software bug
that occurs when a computer program asks for a handle to a resource but does not free the handle when it is no longer used.
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, a handle is a particular kind of smart pointer
Smart pointer
In computer science, a smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. These additional features are intended to reduce bugs caused by the misuse of pointers while retaining efficiency...
. Handles are used when application software
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
references blocks of memory or objects managed by another system, such as a database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
or an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
.
While a pointer literally contains the address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
of the item to which it refers, a handle is an abstraction
Abstraction (computer science)
In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...
in the form of a reference
Reference (computer science)
In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
controlled by a separate system; its opacity allows the referent to be relocated in memory by the system without invalidating the handle, which is impossible with pointers. The extra layer of indirection
Indirection
In computer programming, indirection is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value through its memory address. For example, accessing a variable through the use of a...
also increases the control the managing system has over operations performed on the referent. Typically the handle is an index or a pointer into a global array of tombstone
Tombstone (programming)
Tombstones are a mechanism to detect dangling pointers that can appear in certain computer programming languages, e. g. C, C++ and assembly languages, and to act as a containment to their dangerous effects....
s.
Handles were a popular solution to memory management
Memory management
Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed. This is critical to the computer system.Several...
in operating systems of the 1980s, such as Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...
and Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
. Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
file descriptor
File descriptor
In computer programming, a file descriptor is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems...
s are essentially handles. Like other desktop environment
Desktop environment
In graphical computing, a desktop environment commonly refers to a style of graphical user interface derived from the desktop metaphor that is seen on most modern personal computers. These GUIs help the user in easily accessing, configuring, and modifying many important and frequently accessed...
s, the Windows API
Windows API
The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces available in the Microsoft Windows operating systems. It was formerly called the Win32 API; however, the name "Windows API" more accurately reflects its roots in 16-bit Windows and its support on...
heavily uses handles to represent objects in the system and to provide a communication pathway between the operating system and user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
. For example, a window on the desktop
Desktop metaphor
The desktop metaphor is an interface metaphor which is a set of unifying concepts used by graphical user interfaces to help users more easily interact with the computer. The desktop metaphor treats the monitor of a computer as if it is the user's desktop, upon which objects such as documents and...
is represented by a handle of type
HWND (handle, window).Doubly indirect handles have fallen out of favour in recent times, as increases in available memory and improved virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
algorithms have made the use of the simpler pointer more attractive. However, many operating systems still apply the term to pointers to opaque, "private" data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
s—opaque pointer
Opaque pointer
In computer programming, an opaque pointer is a special case of an opaque data type, a datatype that is declared to be a pointer to a record or data structure of some unspecified type....
s—or to indexes into internal arrays passed from one process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
to its client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
.
A handle leak
Handle leak
A handle leak is a type of software bug that occurs when a computer program asks for a handle to a resource but does not free the handle when it is no longer used. If this occurs frequently or repeatedly over an extended period of time, a large number of handles may be marked in-use and thus...
is a type of software bug
Software bug
A software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...
that occurs when a computer program asks for a handle to a resource but does not free the handle when it is no longer used.