computer science

Select (Unix)

Select (Unix) 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 Select (Unix) rather than just read about it. In short: select is a system call and application programming interface (API) in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels. An enhanced facility with support for signal masking is pselect.

Key takeaways

Select (Unix) 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 Select (Unix) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Select (Unix) from memory before moving on to harder problems.

Reference excerpt

select is a system call and application programming interface (API) in Unix-like and POSIX-compliant operating systems for examining the status of file descriptors of open input/output channels. An enhanced facility with support for signal masking is pselect. The select system call is similar to the newer poll facility introduced in UNIX System V and later operating systems. However, with the C10k problem, both select and poll have been superseded by the likes of kqueue, /dev/poll, epoll and I/O completion ports. One common use of select outside of its stated use of waiting on filehandles is to implement a portable sub-second sleep. This can be achieved by passing NULL for all three fd_set arguments, and the duration of the desired sleep as the timeout argument. In the C programming language, the select system call is declared in the header file sys/select.h or unistd.h, and has the following syntax:

fd_set type arguments may be manipulated with four utility macros: FD_SET(), FD_CLR(), FD_ZERO(), and FD_ISSET(). Select returns the total number of bits set in readfds, writefds and errorfds, or zero if the timeout expired, and -1 on error. The sets of file descriptor used in select are finite in size, depending on the operating system. The newer system call poll provides a more flexible solution.

Example

See also Berkeley sockets Polling epoll kqueue Input/output completion port (IOCP)

References

External links select – System Interfaces Reference, The Single UNIX Specification, Version 5 from The Open Group man-pages for select(2) in FreeBSD, NetBSD, OpenBSD and DragonFly BSD

Worked examples

Example 1 — a first encounter with Select (Unix)

Start with the simplest possible case. Write down what Select (Unix) 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 Select (Unix) 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 Select (Unix) 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 Select (Unix)

In research: Select (Unix) 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 Select (Unix) 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: Select (Unix) is common in secondary-school and first-year university syllabi. It links to neighbouring topics C POSIX library, Events (computing), System calls, so understanding it makes those chapters shorter.
In everyday life: Look for Select (Unix) 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 Select (Unix) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Select (Unix) 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 Select (Unix) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Select (Unix) in simple terms?

Why does Select (Unix) 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 Select (Unix)?

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 Select (Unix).