Unreachable memory
Encyclopedia
In computer science
, unreachable memory is a block of memory allocated dynamically where the program
that allocated the memory no longer has any reachable pointer that refers to it. Similarly, an unreachable object is a dynamically allocated object
that has no reachable reference
to it. Informally, unreachable memory is dynamic memory that the program can not reach directly, nor get to by starting at an object it can reach directly, and then following a chain of pointer references.
In dynamic memory allocation implementations that employ a garbage collector
, objects are reclaimed after they become unreachable. The garbage collector is able to determine if an object is reachable; any object that is determined to no longer be reachable can be deallocated. Many programming language
s (for example, Java
, C#, D, Dylan
) use automatic garbage collection.
In contrast, when memory becomes unreachable in dynamic memory allocation implementations that require explicit deallocation, the memory can no longer be explicitly deallocated. Unreachable memory in systems that use manual memory management results in a memory leak
.
Some garbage collectors implement weak reference
s. If an object is reachable only through either weak references or chains of references that include a weak reference, then the object is said to be weakly reachable. The garbage collector can treat a weakly reachable object graph
as unreachable and deallocate it. (Conversely, references that prevent an object from being garbage collected are called strong references; a weakly reachable object is unreachable by any chain consisting only of strong references.) Some garbage-collected object-oriented languages, such as Java
and Python
, feature weak references. The Java package
Unreachable memory is often associated with software aging
.
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, unreachable memory is a block of memory allocated dynamically where the program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
that allocated the memory no longer has any reachable pointer that refers to it. Similarly, an unreachable object is a dynamically allocated object
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
that has no reachable 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...
to it. Informally, unreachable memory is dynamic memory that the program can not reach directly, nor get to by starting at an object it can reach directly, and then following a chain of pointer references.
In dynamic memory allocation implementations that employ a garbage collector
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 occupied by objects that are no longer in use by the program...
, objects are reclaimed after they become unreachable. The garbage collector is able to determine if an object is reachable; any object that is determined to no longer be reachable can be deallocated. Many programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s (for example, 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 platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, C#, D, Dylan
Dylan programming language
Dylan is a multi-paradigm programming language that includes support for functional and object-oriented programming, and is dynamic and reflective while providing a programming model designed to support efficient machine code generation, including fine-grained control over dynamic and static...
) use automatic garbage collection.
In contrast, when memory becomes unreachable in dynamic memory allocation implementations that require explicit deallocation, the memory can no longer be explicitly deallocated. Unreachable memory in systems that use manual memory management results in a memory leak
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
.
Some garbage collectors implement weak reference
Weak reference
In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector . An object referenced only by weak references is considered unreachable and so may be collected at any time...
s. If an object is reachable only through either weak references or chains of references that include a weak reference, then the object is said to be weakly reachable. The garbage collector can treat a weakly reachable object graph
Object graph
An Object graph is a view of an object system at a particular point in time. Whereas a normal data model such as a UML Class diagram details the relationships between classes, the object graph relates their instances. Object diagrams are subsets of the overall object graph.Object-oriented...
as unreachable and deallocate it. (Conversely, references that prevent an object from being garbage collected are called strong references; a weakly reachable object is unreachable by any chain consisting only of strong references.) Some garbage-collected object-oriented languages, such as 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 platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
and Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, feature weak references. The Java package
Java package
A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time...
java.lang.ref supports soft, weak and phantom references, resulting in the additional object reachability states softly reachable and phantom reachable.Unreachable memory is often associated with software aging
Software aging
In software engineering, software aging refers to progressive performance degradation or a sudden hang/crash of a software system due to exhaustion of operating system resources, fragmentation and accumulation of errors. A proactive fault management method to deal with the software aging...
.