Ошибка	Последствие
Запустить `go f()` и сразу вернуться из `main`	Программа завершится, горутина не успеет отработать — вывода не будет
Считать горутину обычным потоком ОС	Потоков ОС тысячи не создашь; горутина дешёвая, ей управляет рантайм, а не ядро
Вызвать `wg.Add(1)` внутри уже запущенной горутины	`Wait()` может увидеть нулевой счётчик и выйти раньше — гонка; `Add` делают до `go`
Забыть `wg.Done()` в одной из горутин	Счётчик не дойдёт до нуля, `Wait()` зависнет навсегда
Передавать `WaitGroup` по значению, а не указателем	Каждая горутина увидит свою копию счётчика — `Wait` не дождётся; передавайте `*sync.WaitGroup`

beginner

Goroutines

Starting a goroutine with `go`, why a goroutine is "lightweight" compared with an OS thread, and waiting for a group of goroutines with `sync.WaitGroup` and its `Add`/`Done`/`Wait` contract.

go-goroutines-intro

Practice: 2 questions →

Sections

Contents

Detailed explanation

Goroutines

Concurrency is built into Go at the language level: you start a parallel task with the single keyword go. This is the intro topic: its goal is to understand what go does, why a goroutine is "lightweight," and how to wait for a group of goroutines cleanly, without diving into how the scheduler works.

Behind the apparent simplicity sit a few rules that interviews probe at the junior level. A goroutine is a lightweight thread managed by the Go runtime, not the operating system: you can launch tens of thousands of them because the starting stack is tiny and grows on demand. But when main returns, the program exits and all remaining goroutines are cut off — so you must wait for them explicitly. The canonical way is sync.WaitGroup: Add before starting, Done in each goroutine, Wait blocks until the counter reaches zero. Exchanging data between goroutines — through channels — is a separate topic, Channels; the scheduler design and concurrency patterns live in Concurrency.

Topic map

The go keyword — go f() starts a goroutine — a lightweight runtime-managed thread; it runs concurrently, and returning from main cuts off all goroutines at once.
sync.WaitGroup — Add(n) before starting, Done() (usually via defer) in each goroutine, Wait() blocks until the counter hits zero — the canonical way to wait for a group of goroutines.

Common mistakes and traps

Mistake	Consequence
Starting `go f()` and immediately returning from `main`	The program exits before the goroutine runs — there is no output
Treating a goroutine like an ordinary OS thread	You cannot spawn thousands of OS threads; a goroutine is cheap and managed by the runtime, not the kernel
Calling `wg.Add(1)` inside an already-started goroutine	`Wait()` may see a zero counter and exit early — a race; do `Add` before `go`
Forgetting `wg.Done()` in one of the goroutines	The counter never reaches zero and `Wait()` hangs forever
Passing a `WaitGroup` by value instead of by pointer	Each goroutine sees its own copy of the counter — `Wait` never completes; pass `*sync.WaitGroup`

Why it matters for interviews

Goroutines are an almost mandatory opening for a Go junior section: interviews start with them because this is the language's "feature." Deep scheduler knowledge is not expected at this level, but the basic mechanics of starting and waiting are asked every time.

What interviewers usually check:

What go f() does and why a goroutine is "lightweight" — it is managed by the Go runtime, not the OS.
Why a program with go fmt.Println(...) in main often prints nothing (returning from main cuts off goroutines).
How to wait for several goroutines with sync.WaitGroup and why Add goes before go.
What happens if you forget Done or pass a WaitGroup by value.

Why it matters

Concurrency is Go's signature feature, and the junior section almost always opens with it. Starting a goroutine with a single `go` is easy, but that simplicity hides exactly the traps newcomers fall into: `main` returns and every goroutine is cut off before it prints anything; `WaitGroup.Add` placed after `go` lets `Wait` race ahead; a missing `Done` hangs `Wait` forever. This is the bare minimum to start a goroutine and wait for a group to finish; exchanging data over channels and the scheduler internals come later.

Ошибка	Последствие
Запустить `go f()` и сразу вернуться из `main`	Программа завершится, горутина не успеет отработать — вывода не будет
Считать горутину обычным потоком ОС	Потоков ОС тысячи не создашь; горутина дешёвая, ей управляет рантайм, а не ядро
Вызвать `wg.Add(1)` внутри уже запущенной горутины	`Wait()` может увидеть нулевой счётчик и выйти раньше — гонка; `Add` делают до `go`
Забыть `wg.Done()` в одной из горутин	Счётчик не дойдёт до нуля, `Wait()` зависнет навсегда
Передавать `WaitGroup` по значению, а не указателем	Каждая горутина увидит свою копию счётчика — `Wait` не дождётся; передавайте `*sync.WaitGroup`

beginner

Goroutines

Starting a goroutine with `go`, why a goroutine is "lightweight" compared with an OS thread, and waiting for a group of goroutines with `sync.WaitGroup` and its `Add`/`Done`/`Wait` contract.

go-goroutines-intro

Practice: 2 questions →

Sections

Contents

Detailed explanation

Goroutines

Topic map

The go keyword — go f() starts a goroutine — a lightweight runtime-managed thread; it runs concurrently, and returning from main cuts off all goroutines at once.
sync.WaitGroup — Add(n) before starting, Done() (usually via defer) in each goroutine, Wait() blocks until the counter hits zero — the canonical way to wait for a group of goroutines.

Common mistakes and traps

Mistake	Consequence
Starting `go f()` and immediately returning from `main`	The program exits before the goroutine runs — there is no output
Treating a goroutine like an ordinary OS thread	You cannot spawn thousands of OS threads; a goroutine is cheap and managed by the runtime, not the kernel
Calling `wg.Add(1)` inside an already-started goroutine	`Wait()` may see a zero counter and exit early — a race; do `Add` before `go`
Forgetting `wg.Done()` in one of the goroutines	The counter never reaches zero and `Wait()` hangs forever
Passing a `WaitGroup` by value instead of by pointer	Each goroutine sees its own copy of the counter — `Wait` never completes; pass `*sync.WaitGroup`

Why it matters for interviews

What interviewers usually check:

What go f() does and why a goroutine is "lightweight" — it is managed by the Go runtime, not the OS.
Why a program with go fmt.Println(...) in main often prints nothing (returning from main cuts off goroutines).
How to wait for several goroutines with sync.WaitGroup and why Add goes before go.
What happens if you forget Done or pass a WaitGroup by value.

Why it matters

Горутины

Горутины

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Goroutines

Goroutines

Topic map

Common mistakes and traps

Why it matters for interviews

Горутины

Горутины

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Goroutines

Goroutines

Topic map

Common mistakes and traps

Why it matters for interviews