---
date: 20200901
id: 56b6e0d5-090d-4859-9f06-c54a1a116515
title: Goroutines
---

# Basics

A `goroutine` is a lightweight thread managed by the Go runtime.

``` go
package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}
```

# Channels

Channels are a typed conduit through whichyou can send and receive
values with the channel operator \`\<-\`. By default, sends and receives
block until the other side is ready. This allows goroutines to
synchronize without explicit locks or condition variables. Channels
should generally be used for [passing ownership of
data](https://github.com/golang/go/wiki/MutexOrChannel).

``` go
package main

import "fmt"

func sum(s []int, c chan int) {
    sum := 0
    for _, v := range s {
        sum += v
    }
    c <- sum // send sum to c
}

func main() {
    s := []int{7, 2, 8, -9, 4, 0}

    c := make(chan int)
    go sum(s[:len(s)/2], c)
    go sum(s[len(s)/2:], c)
    x, y := <-c, <-c // receive from c

    fmt.Println(x, y, x+y)
}
```

Channels can also be **buffered**. Provide the buffer length as the
second argument to \`make\` to initialize a buffered channel:

``` go
package main

import "fmt"

func main() {
    ch := make(chan int, 2)
    ch <- 1
    ch <- 2
    fmt.Println(<-ch)
    fmt.Println(<-ch)
}
```

Sends to a buffered channel block only when the buffer is full. Receives
block when the buffer is empty.

# Range and close

A sender can close a channel to indicate that no more values will be
sent. Receivers can test whether a channel has been closed by assigning
a second parameter to the receive expression.

Only the sender should close a channel, never the receiver. Sending on a
closed channel will cause a panic. Channels aren't like files; you don't
usually need to close them. Closing is only necessary when the receiver
must be told there are no more values coming, such as to terminate a
range loop.

``` go
package main

import (
    "fmt"
)

func fibonacci(n int, c chan int) {
    x, y := 0, 1
    for i := 0; i < n; i++ {
        c <- x
        x, y = y, x+y
    }
    close(c)
}

func main() {
    c := make(chan int, 10)
    go fibonacci(cap(c), c)
    for i := range c {
        fmt.Println(i)
    }
}
```

# Select

The select statement lets a goroutine wait on multiple communication
operations.

A select blocks until one of its cases can run, then it executes that
case. It chooses one at random if multiple are ready.

``` go
package main

import "fmt"

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println(<-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}
```

\`default\` case in \`select\` is run if no other case is ready, as one
would expect

``` go
package main

import (
    "fmt"
    "time"
)

func main() {
    tick := time.Tick(100 * time.Millisecond)
    boom := time.After(500 * time.Millisecond)
    for {
        select {
        case <-tick:
            fmt.Println("tick.")
        case <-boom:
            fmt.Println("BOOM!")
            return
        default:
            fmt.Println("    .")
            time.Sleep(50 * time.Millisecond)
        }
    }
}
```

## Timeout

Often you want to set a timeout value for `select` so it won't run
forver. [time.After](https://golang.org/pkg/time/#After) is a good way
of doing this:

``` go
package main

import (
    "fmt"
    "time"
)

var c chan int

func handle(int) {}

func main() {
    select {
    case m := <-c:
        handle(m)
    case <-time.After(10 * time.Second):
        fmt.Println("timed out")
    }
}
```

# sync.Mutex

TO make sure only one goroutine at a time can access a variable we can
use \`sync.Mutex\`

``` go
package main

import (
    "fmt"
    "sync"
    "time"
)

// SafeCounter is safe to use concurrently.
type SafeCounter struct {
    mu sync.Mutex
    v  map[string]int
}

// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
    c.mu.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    c.v[key]++
    c.mu.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
    c.mu.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    defer c.mu.Unlock()
    return c.v[key]
}

func main() {
    c := SafeCounter{v: make(map[string]int)}
    for i := 0; i < 1000; i++ {
        go c.Inc("somekey")
    }

    time.Sleep(time.Second)
    fmt.Println(c.Value("somekey"))
}
```