computer science

Overlapped I/O

Overlapped I/O is a computer 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 Overlapped I/O rather than just read about it. In short: Overlapped I/O is a name used for asynchronous I/O in the Windows API. It was introduced as an extension to the API in Windows NT.

Key takeaways

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

Reference excerpt

Overlapped I/O is a name used for asynchronous I/O in the Windows API. It was introduced as an extension to the API in Windows NT. Utilizing overlapped I/O requires passing an OVERLAPPED structure to API functions that normally block, including ReadFile(), WriteFile(), and Winsock's WSASend() and WSARecv(). The requested operation is initiated by a function call which returns immediately, and is completed by the OS in the background. The caller may optionally specify a Win32 event handle to be signalled when the operation completes. Alternatively, a program may receive notification of an event via an I/O completion port, which is the preferred method of receiving notification when used in symmetric multiprocessing environments or when handling I/O on numerous files or sockets. The third and the last method to get the I/O completion notification with overlapped IO is to use ReadFileEx() and WriteFileEx(), which allow the User APC routine to be provided, which will be fired on the same thread on completion (User APC is the thing very similar to Unix/POSIX signal, with the main difference being that the signals are using signal numbers from the historically predefined enumeration, while the User APC can be any function declared as "void f(void* context)"). The so-called overlapped API presents some differences depending on the Windows version used. Asynchronous I/O is particularly useful for sockets and pipes. Unix and Linux implement the POSIX asynchronous I/O API (AIO).

References

External links MSDN Reference: ReadFile() MSDN Reference: WriteFile() MSDN Reference: OVERLAPPED Data structure I/O Completion Port

Worked examples

Example 1 — a first encounter with Overlapped I/O

Start with the simplest possible case. Write down what Overlapped I/O claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer 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 Overlapped I/O 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 Overlapped I/O 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 Overlapped I/O

In research: Overlapped I/O appears in computer 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 Overlapped I/O 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: Overlapped I/O is common in secondary-school and first-year university syllabi. It links to neighbouring topics Concurrency control, Events (computing), Input/output, so understanding it makes those chapters shorter.
In everyday life: Look for Overlapped I/O 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 Overlapped I/O in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Overlapped I/O 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 Overlapped I/O out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Overlapped I/O in simple terms?

Overlapped I/O is a name used for asynchronous I/O in the Windows API. It was introduced as an extension to the API in Windows NT.

Why does Overlapped I/O matter?

Because it connects several computer 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 Overlapped I/O?

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 Overlapped I/O.