Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems.... , a memory leak is a particular type of unintentional memory consumption by a computer program
Computer program
Computer programs are Instruction for a computer. A computer requires programs to function. Moreover, a computer program does not run unless its instructions are executed by a Central processing unit; however, a program may communicate an Algorithm#Formalization of algorithms to people without running.... where the program fails to release memory when no longer needed. This condition is normally the result of a bug in a program that prevents it from freeing up memory that it no longer needs.
This term has the potential to be confusing, since memory is not physically lost from the computer. Rather, memory is allocated to a program, and that program subsequently loses the ability to access it due to program logic flaws.
A memory leak has symptoms similar to a number of other problems (see below) and generally can only be diagnosed by a programmer with access to the program source code
Source code
In computer science, source code is any collection of statements or declarations written in some human-readable computer programming language.... ; however, many people refer to any unwanted increase in memory usage as a memory leak, even if this is not strictly accurate.
Consequences A memory leak can diminish the performance of the computer by reducing the amount of available memory.
Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems.... , a memory leak is a particular type of unintentional memory consumption by a computer program
Computer program
Computer programs are Instruction for a computer. A computer requires programs to function. Moreover, a computer program does not run unless its instructions are executed by a Central processing unit; however, a program may communicate an Algorithm#Formalization of algorithms to people without running.... where the program fails to release memory when no longer needed. This condition is normally the result of a bug in a program that prevents it from freeing up memory that it no longer needs.
This term has the potential to be confusing, since memory is not physically lost from the computer. Rather, memory is allocated to a program, and that program subsequently loses the ability to access it due to program logic flaws.
A memory leak has symptoms similar to a number of other problems (see below) and generally can only be diagnosed by a programmer with access to the program source code
Source code
In computer science, source code is any collection of statements or declarations written in some human-readable computer programming language.... ; however, many people refer to any unwanted increase in memory usage as a memory leak, even if this is not strictly accurate.
Consequences
A memory leak can diminish the performance of the computer by reducing the amount of available memory. Eventually, in the worst case, too much of the available memory may become allocated and all or part of the system or device stops working correctly, the application fails, or the system slows down unacceptably due to thrashing
Thrash (computer science)
In computer science, thrash , is the term used to describe a degenerate situation on a computer where increasing resources are used to do a decreasing amount of work.... .
Memory leaks may not be serious or even detectable by normal means. In modern operating systems, normal memory used by an application is released when the application terminates. This means that a memory leak in a program that only runs for a short time is rarely serious.
Cases where leaks are much more serious include:
where the program is left running, and consumes more and more memory over time (such as background tasks, on servers, but especially in embedded devices
Embedded system
An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints.... which may be left running for many years);
where new memory is allocated frequently, such as when rendering the frames of a computer game or animated video
where the program is able to request memory (e.g. shared memory
Shared memory
In computing, shared memory is a memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies.... ) that is not released, even when the program terminates;
where the leak is happening inside the operating system
where the leak is happening in a system critical driver
An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints.... or portable device
where running on operating systems (such as AmigaOS
AmigaOS
AmigaOS is the default native operating system of the Amiga personal computer. It was developed first by Commodore International, and initially introduced in 1985 with the Amiga 1000.... ) where memory may not be automatically released on termination, and if lost can only be reclaimed by a reboot
Pseudocode is a compact and informal high-level description of a computer programming algorithm that uses the structural conventions of some programming language, but is intended for human reading rather than machine reading.... , is intended to show how a memory leak can come about, and its effects, without needing any programming knowledge.
The program in this case is part of some very simple software designed to control a lift (elevator
Elevator
An elevator or lift is a vertical transport vehicle that efficiently moves people or goods between floors of a building. They are generally powered by electric motors that either drive traction cables and counterweight systems, or pump hydraulic fluid to raise a cylindrical piston.... ). This part of the program is run whenever anyone inside the lift presses the button for a floor.
When a button is pressed:
Get some memory, which will be used to remember the floor number
Put the floor number into the memory
Are we already on the target floor?
If so, we have nothing to do: finished
Otherwise:
Wait until the lift is idle
Go to the required floor
Release the memory we used to remember the floor number
The memory leak would occur if the floor's number pressed is the same floor that the lift is on; the condition for releasing the memory would be skipped. Each time this case occurs, more memory would be leaked.
Cases like this wouldn't usually have any immediate effects. People do not often press the button for the floor they are already on, and in any case, the lift might have enough spare memory that this could happen a hundred or a thousand times. However, the lift will eventually run out of memory. This could take months or years, so it might never be discovered by even relatively thorough testing.
The consequences in this case would be unpleasant; at the very least, the lift would stop responding to requests to move to another floor. If the program needs memory to open the lift door, then someone may also be trapped inside, since there is no memory available for that door to open.
The memory leak would only last as long as the program was running. For example: if the lift's power were turned off the program would stop running. When power was turned on again, the program would restart and all the memory would be available again, and the slow process of leaking would start again.
Programming issues
Memory leaks are a common error in programming, especially when using languages
Programming language
A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer.... that have no built-in automatic garbage collection
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage , or memory used by Object that will never be accessed or mutated again by the Application software.... , such as C
C (programming language)
C is a general-purpose computer programming language originally developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories to implement the Unix operating system.... and C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... . Typically, a memory leak occurs because dynamically allocated
Dynamic memory allocation
In computer science, dynamic memory allocation is the allocation of computer storage storage for use in a computer program during the runtime of that program.... memory has become unreachable
Unreachable memory
In computer science, unreachable memory is a block of dynamic memory Dynamic memory allocation where the computer program that allocated the memory no longer has any reachable pointer that refers to it.... . The prevalence of memory leak bugs
Software bug
A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from behaving as intended . Most bugs arise from mistakes and errors made by people in either a program's source code or its software architecture, and a few are caused by compilers producing incorrect code.... has led to the development of a number of debugging
Debugging
Debugging is a methodical process of finding and reducing the number of computer bugs, or defects, in a computer program or a piece of electronic hardware thus making it behave as expected.... tool
Programming tool
A programming tool or software development tool is a computer program or application software that software developers use to create, debug, maintain, or otherwise support other programs and applications.... s to detect unreachable memory. IBM Rational Purify, BoundsChecker
BoundsChecker
BoundsChecker is a memory debugger used for C++ software development with Microsoft Visual C++. It is part of the . Comparable tools are Purify , Insure++ and Valgrind.... , Valgrind
Valgrind
Valgrind is a programming tool for memory debugger, memory leak detection, and performance analysis. The name Valgrind comes from the name of the main entrance to Valhalla in Norse mythology.... , Insure++
Insure++
Insure++ is a memory debugger computer program, used by software developers to detect various errors in programs written in C and C++. It is made by Parasoft, and is functionally similar to other memory debuggers, such as Purify and Valgrind.... and memwatch
Memwatch
Memwatch is a Free software programming tool for memory leak detection in C , released under the GNU General Public License.It is designed to compile and run on any system which has an ANSI C compiler.... are some of the more popular memory debugger
Memory debugger
A memory debugger is a programming tool for finding memory leaks and buffer overflows. These are due to Software bug related to the allocation and deallocation of dynamic memory.... s for C and C++ programs. Garbage collection capabilities can be added to any programming language that lacks it as a built-in feature, and libraries for doing this are available for C and C++ programs.
Languages that provide automatic memory management, like Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java .... , C#, VB.NET
Visual Basic .NET
Visual Basic , formerly called Visual Basic .NET , is an object-oriented programming computer language that can be viewed as an evolution of Microsoft Visual Basic implemented on the .NET Framework.... or LISP
Lisp programming language
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older.... , are not immune to memory leaks. For example, a program could continue to add entries to a list, but then forget to remove them when done. The memory manager would not know if the entry will be referenced again, unless the program does something to indicate it is no longer needed. Normally, this is done by removing any reference
Reference (computer science)
In computer science, a reference is an object containing information about how to locate and access the particular data item, as opposed to containing the data itself.... to the item in question. This is similar to people placing items on a pile or in a drawer, and then forgetting about them.
A memory manager is a part of a computer program which accepts requests from the program to allocate and deallocate chunks of computer memory.... can recover memory that has become unreachable and therefore logically useless, it cannot free memory that is still reachable and therefore potentially still useful. Modern memory managers therefore provide techniques for programmers to semantically mark memory with varying levels of usefulness, which correspond to varying levels of reachability. The memory manager
Memory manager
A memory manager is a part of a computer program which accepts requests from the program to allocate and deallocate chunks of computer memory.... does not free an object that is strongly reachable. An object is strongly reachable if it is reachable either directly by a strong reference or indirectly by a chain of strong references. (A strong reference is a reference that, unlike a weak reference
Weak reference
In computer programming, a weak reference is a Reference that does not protect the referent object from collection by a garbage collection . An object referenced only by weak references is considered Unreachable memory and so may be collected at any time.... , prevents an object from being garbage collected.) To prevent this type of memory leak, the developer is responsible for cleaning up references after use, typically by setting the reference to null once it is no longer needed and, if necessary, by unregistering any event listeners that maintain strong references to the object.
In general, automatic memory management is more robust and convenient for developers, as they don't need to implement freeing routines or worry about the sequence in which cleanup is performed or be concerned about whether or not an object is still referenced. It is easier for a programmer to know when a reference is no longer needed than to know when an object is no longer referenced. However, automatic memory management can impose a performance overhead, and it does not eliminate all of the programming errors that cause memory leaks.
Resource Acquisition Is Initialization, often referred to by the acronym RAII, is a popular design pattern in several Object-oriented_programming_language like C++, D and Ada .... , is an approach to the problem commonly taken in C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... , D, and Ada
Ada (programming language)
Ada is a structured programming, statically typed, Imperative programming, and Object-oriented programming high-level language computer programming programming language, extended from Pascal and other languages.... . It involves associating scoped objects with the acquired resources, and automatically releasing the resources once the objects are out of scope. Compare the following C and C++ examples:
/* C version */
include
void f
// C++ version
include
void f
The C version, as implemented in the example, requires explicit deallocation; the array is allocated from the heap
Dynamic memory allocation
In computer science, dynamic memory allocation is the allocation of computer storage storage for use in a computer program during the runtime of that program.... , and continues to exist until explicitly freed. It should be noted, however, that this is only an example. C has the concept of automatic storage duration, and it would be perfectly possible, for example, to declare the array as a local array of integers, which would be deallocated automatically when leaving the function.
The C++ version requires no explicit deallocation; it will always occur automatically as soon as the object array goes out of scope. This avoids the overhead of garbage collection
Garbage collection
Garbage collection may refer to:* Waste collection, one part of the municipal waste management cycle.* Garbage collection , the detection and pruning of unused or inaccessible data structures.... schemes, and can even be applied to resources other than memory such as file handles, which mark-and-sweep garbage collection does not handle as gracefully. However, using RAII correctly is not as easy as garbage collection and it has its own pitfalls. For instance, in C++, if one is not careful, it is possible to create dangling pointer
Dangling pointer
Dangling pointers and wild pointers in computer programming are data pointer that do not point to a valid object of the appropriate type. Dangling pointers arise when an object is deleted or deallocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the deallocated memory.... s (or references) by returning data by reference, only to have that data be deleted when its containing object goes out of scope.
D uses a combination of RAII and garbage collection, employing automatic destruction when it is clear that an object cannot be accessed outside its original scope, and garbage collection otherwise.
Reference counting and cyclic references
More modern garbage collection schemes are often based on a notion of reachability - if you don't have a usable reference to the memory in question, it can be collected. Other garbage collection schemes can be based on reference counting
Reference counting
In computer science, reference counting is a technique of storing the number of references, pointers, or handles to a resource such as an object or block of memory.... , where an object is responsible for keeping track of how many references are pointing to it. If the number goes down to zero, the object is expected to release itself and allow its memory to be reclaimed. The flaw with this model is that it doesn't cope with cyclic references, and this is why nowadays we are prepared to accept the burden of the more costly mark and sweep type of systems.
The following code illustrates the canonical reference-counting memory leak.
Dim A, B
Set A = CreateObject("Some.Thing")
Set B = CreateObject("Some.Thing")
' At this point, the two objects each have one reference,
Set A.member = B
Set B.member = A
' Now they each have two references.
Set A = Nothing
' You could still get out of it...
Set B = Nothing
' You now have a memory leak.
In practice, this trivial example would be spotted straight away and fixed. In most real examples, the cycle of references spans more than two objects, and is more difficult to detect.
A well-known example of this kind of leak came to prominence with the rise of AJAX programming techniques in web browsers. Javascript code which associated a DOM element with an event handler and failed to remove the reference before exiting, would leak memory. (AJAX web pages keep a given DOM alive for a lot longer than traditional web pages, so this leak was much more apparent.)
Effects
If a program has a memory leak and its memory usage is steadily increasing, there will not usually be an immediate symptom. Every physical system has a finite amount of memory, and if the memory leak is not contained (for example, by restarting the program with the leak) it will sooner or later start to cause problems.
Most modern consumer desktop operating systems have both main memory which is physically housed in RAM microchips, and secondary storage such as a hard drive. Memory allocation is dynamic - each process gets as much memory as it requests. Active pages
Paging
In computer operating systems that have their main memory divided into page , paging is a transfer of pages between main memory and an auxiliary store, such as hard disk drive.... are transferred into main memory for fast access; inactive pages are pushed out to secondary storage to make room, as needed. When a single process starts consuming a large amount of memory, it usually occupies more and more of main memory, pushing other programs out to secondary storage - usually significantly slowing performance of the system. Even if the leaking program is terminated, it may take some time for other programs to swap back into main memory, and for performance to return to normal.
When all the memory on a system is exhausted (whether there is virtual memory or only main memory, such as on an embedded system) any attempt to allocate more memory will fail. This usually causes the program attempting to allocate the memory to terminate itself, or to generate a segmentation fault
Segmentation fault
A segmentation fault is a particular error condition that can occur during the operation of computer software. A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed .... . Some programs are designed to recover from this situation (possibly by falling back on pre-reserved memory). The first program to experience the out-of-memory may or may not be the program that has the memory leak.
In computing, multitasking is a method by which multiple tasks, also known as Computer process, share common processing resources such as a Central processing unit.... operating systems have special mechanisms to deal with an out-of-memory condition, such as killing processes at random (which may affect "innocent" processes), or killing the largest process in memory (which presuambly is the one causing the problem). Some operating systems have a per-process memory limit, to prevent any one program from hogging all of the memory on the system. The disadvantage to this arrangement is that the operating system sometimes must be re-configured to allow proper operation of programs that legitimately require large amounts of memory, such as those dealing with graphics, video, or scientific calculations.
If the memory leak is in the kernel, the operating system itself will likely fail. Computers without sophisticated memory management, such as embedded systems, may also completely fail from a persistent memory leak.
The term web server can mean one of two things:# A computer program that is responsible for accepting Hypertext Transfer Protocol requests from clients , and Server them HTTP responses along with optional data contents, which usually are web pages such as Hypertext Markup Language documents and linked objects .... s or router
Router
A router is a Computer network device whose software and hardware are usually tailored to the tasks of routing and forwarding information. For example, on the Internet, information is directed to various paths by routers.... s are prone to denial-of-service attacks if an attacker discovers a sequence of operations which can trigger a leak. Such a sequence is known as an exploit
Exploit (computer security)
An exploit is a piece of software, a chunk of data, or sequence of commands that take advantage of a software bug, glitch or vulnerability in order to cause unintended or unanticipated behavior to occur on computer software, hardware, or something electronic .... .
Other memory consumers
Note that constantly increasing memory usage is not necessarily evidence of a memory leak. Some applications will store ever increasing amounts of information in memory (e.g. as a cache
Cache
In computer science, a cache is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch or to compute, compared to the cost of reading the cache.... ). If the cache can grow so large as to cause problems, this may be a programming or design error, but is not a memory leak as the information remains nominally in use. In other cases, programs may require an unreasonably large amount of memory because the programmer has assumed memory is always sufficient for a particular task; for example, a graphics file processor might start by reading the entire contents of an image file and storing it all into memory, something that is not viable where a very large image exceeds available memory.
To put it another way, a memory leak arises from a particular kind of programming error, and without access to the program code, someone seeing symptoms can only guess that there might be a memory leak. It would be better to use terms such as "constantly increasing memory use" where no such inside knowledge exists.
Simple C example
The following C function deliberately leaks memory by losing the pointer to the allocated memory. Since the program loops forever calling the defective function, malloc will eventually fail (returning NULL) when no more memory is available to the program. The address of each allocation is stored in a local variable that only exists inside the function; this address is lost when the function returns, so it is impossible to free any of the previously allocated blocks.
In computer security and computer programming, a buffer overflow, or buffer overrun, is an Anomaly in software condition where a process attempts to store data beyond the boundaries of a fixed-length buffer ....
Memory management is the act of managing computer memory. In its simpler forms, this involves providing ways to allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed....
A memory debugger is a programming tool for finding memory leaks and buffer overflows. These are due to Software bug related to the allocation and deallocation of dynamic memory....
Harri Porten is a software. Porten, a KDE developer and former Trolltech employee, is the CEO of Froglogic, a consultancy company related to Qt development....
Mac OS X is a line of computer operating systems developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems.... "
