science

Unreachable memory

Unreachable memory is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Unreachable memory rather than just read about it. In short: In computer programming, unreachable memory is a block of dynamically allocated memory 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.

Key takeaways

Unreachable memory belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Unreachable memory to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Unreachable memory from memory before moving on to harder problems.

Reference excerpt

In computer programming, unreachable memory is a block of dynamically allocated memory 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 cannot 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 languages (for example, Java, C#, D, Dylan, Julia) 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 references. 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 java.lang.ref supports soft, weak and phantom references, resulting in the additional object reachability states softly reachable and phantom reachable. Unreachable memory (in languages, like C, that do not reclaim) is often associated with software aging.

External links Nicholas, Ethan (4 May 2006). "Understanding Weak References". java.net. Archived from the original on 19 August 2010. Retrieved 1 October 2010.

Worked examples

Example 1 — a first encounter with Unreachable memory

Start with the simplest possible case. Write down what Unreachable memory claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Unreachable memory before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Unreachable memory ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Unreachable memory

In research: Unreachable memory appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Unreachable memory in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Unreachable memory is common in secondary-school and first-year university syllabi. It links to neighbouring topics Automatic memory management, so understanding it makes those chapters shorter.
In everyday life: Look for Unreachable memory outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Unreachable memory in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Unreachable memory means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Unreachable memory out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Unreachable memory in simple terms?

Why does Unreachable memory matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Unreachable memory?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Unreachable memory.