What are the three background goroutines in a Go program?

Question

It seems that Go Go always has at least 4 goroutines working at any given time. What are the three others that are not the main mountain?

http://play.golang.org/p/MQBiLmHXBK

package main import ( "fmt" "runtime" ) func main() { fmt.Println(runtime.NumGoroutine()) //4 } 

Answer 1

These are not threads, they are goroutines, and the number may vary depending on the current implementation (i.e. in go1.2 it would be printed 2).

Right now, it looks like you have 1 for main and 3 for runtime / gc.

 import "fmt" func main() { fmt.Println(func() string { panic(nil); return "" }()) } 

It shows

 goroutine 16 [running]: runtime.panic(0x0, 0x0) /usr/local/go/src/pkg/runtime/panic.c:279 +0xf5 main.func·001(0x3ea4f, 0xdc4b0) /test/threads.go:6 +0x28 main.main() /test/threads.go:6 +0x1e goroutine 17 [runnable]: runtime.MHeap_Scavenger() /usr/local/go/src/pkg/runtime/mheap.c:507 runtime.goexit() /usr/local/go/src/pkg/runtime/proc.c:1445 goroutine 18 [runnable]: bgsweep() /usr/local/go/src/pkg/runtime/mgc0.c:1976 runtime.goexit() /usr/local/go/src/pkg/runtime/proc.c:1445 goroutine 19 [runnable]: runfinq() /usr/local/go/src/pkg/runtime/mgc0.c:2606 runtime.goexit() /usr/local/go/src/pkg/runtime/proc.c:1445 

if you remove fmt and use the print boot function, you will get only 2 goroutines.

 import "runtime" func main() { print(runtime.NumGoroutine(), "\n") } // prints 2 

If you ever want to know exactly what goroutines is, print a stack trace, cause a panic, or kill a process with SIGQUIT (which prints a stack trace and exits). If you run the absolute minimum program, you can get a stack trace, you can see 2 goroutines:

 package main func main() { panic(nil) } 

Goroutines are very inexpensive, and many things will start and stop more goroutines, so trying to track their bottom line is not very useful. Note that although there are only 2 goroutines (main / runtime.panic and runtime.MHeap_Scavenger), the counter is already up to 17.

 panic: nil goroutine 16 [running]: runtime.panic(0x0, 0x0) /usr/local/go/src/pkg/runtime/panic.c:279 +0xf5 main.main() /test/threads.go:4 +0x28 goroutine 17 [runnable]: runtime.MHeap_Scavenger() /usr/local/go/src/pkg/runtime/mheap.c:507 runtime.goexit() /usr/local/go/src/pkg/runtime/proc.c:1445 exit status 2