Goroutines
Starting a Goroutine
Prefixing a function call with the go keyword launches it as a goroutine, a lightweight, independently scheduled unit of concurrent execution managed by the Go runtime rather than the operating system directly.
Example: Starting a Goroutine
package main
import (
"fmt"
"time"
)
func sayHello() {
fmt.Println("hello from a goroutine")
}
func main() {
go sayHello()
time.Sleep(50 * time.Millisecond) // give the goroutine time to run
fmt.Println("main finished")
}Login to try C/C++/Java/PHP code in the editor
main() Doesn't Wait
The program exits as soon as main() returns, regardless of whether any goroutines it launched have finished -- this is why real code needs explicit synchronization, not just a hope that things finish 'fast enough'.
Note: Without some form of synchronization, a program can exit before its goroutines finish -- always coordinate with channels or WaitGroup instead of Sleep in real code.
Example: main() Doesn't Wait
package main
import "fmt"
func main() {
done := make(chan bool)
go func() {
fmt.Println("goroutine work happening")
done <- true
}()
<-done // waits until the goroutine signals completion
fmt.Println("main continues only after goroutine finishes")
}Login to try C/C++/Java/PHP code in the editor
Many Lightweight Goroutines
Because goroutines start with a small, growable stack (just a few KB) rather than a full OS thread's overhead, a Go program can comfortably run thousands of them concurrently.
Example: Many Lightweight Goroutines
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func(n int) {
defer wg.Done()
fmt.Println("worker", n, "done")
}(i)
}
wg.Wait()
}Login to try C/C++/Java/PHP code in the editor
- Launching a goroutine and letting main() return before it finishes, silently killing it mid-work with no error.
- Assuming 'go func()' runs immediately and synchronously -- it's scheduled to run concurrently, with no guaranteed start order.
- Forgetting that goroutines sharing data without synchronization can race, corrupting shared state unpredictably.
- The go keyword launches a function call as a new, concurrently running goroutine.
- Goroutines are extremely lightweight compared to OS threads -- thousands can run at once.
- The main goroutine doesn't wait for other goroutines automatically; the program exits when main() returns.
- Coordinating goroutines typically requires channels or sync primitives like WaitGroup.
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