meet it

Put `go` in front of a call and it runs alongside `main`.

So far your programs did one thing at a time, top to bottom. Write the keyword go in front of a function call and Go starts it as a : a separate strand of work running alongside everything else. Juggling several jobs in overlapping time like this is called .

main is a goroutine too, and when it returns the whole program stops, even if other goroutines are halfway through. So main has to wait. A is a counter for that: Add(1) before starting, Done() when the work is finished, Wait() to pause until the count is zero.

Hand the goroutine a pointer, &wg, so both sides share one counter instead of two separate copies.

package main
import (
	"fmt"
	"sync"
)
func bake(wg *sync.WaitGroup) {
	fmt.Println("Baking the bread...")
	wg.Done()
}
func main() {
	var wg sync.WaitGroup
	wg.Add(1)
	go bake(&wg)
	wg.Wait()
	fmt.Println("Dinner is ready!")
}

Without wg.Wait(), main could finish before the bread is baked, and you might never see that line.

guess first

The kettle boils in a goroutine. What does `main` see?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

package main
import (
	"fmt"
	"sync"
)
func boil(kettle *string, wg *sync.WaitGroup) {
	*kettle = "boiled"
	wg.Done()
}
func main() {
	var wg sync.WaitGroup
	kettle := "cold"
	wg.Add(1)
	go boil(&kettle, &wg)
	wg.Wait()
	fmt.Println("The kettle is", kettle)
}

your turn

Your turn. Pack a bag in a goroutine.

Fill the three blanks: how many goroutines main waits for, something to pack, and the method that makes main wait. Then run it.

Your code

output

press run to see what happens

fix it

This trip never sets off. Can you fix it?

Broken code to fix

output

press run to see what happens

Read the error: all goroutines are asleep. main is waiting for something that never comes.
Count the goroutines started with go, then compare that with the number given to wg.Add.
There is one goroutine, so change wg.Add(2) to wg.Add(1).

nailed it

your code can do two things at once. :D

go in front of a call starts a goroutine.
When main returns, every other goroutine stops with it.
Add, Done and Wait on a WaitGroup make main wait.

new words in your collection

goroutine
concurrency
WaitGroup

This lesson isn’t saved. A free account remembers every lesson you finish.

Put go in front of a call and it runs alongside main.

The kettle boils in a goroutine. What does main see?

Your turn. Pack a bag in a goroutine.

This trip never sets off. Can you fix it?

your code can do two things at once. :D

Put `go` in front of a call and it runs alongside `main`.

The kettle boils in a goroutine. What does `main` see?