从goroutine返回值会发生什么

Question

Could someone please give clarification values returned from the goroutine. Does the returned value from the goroutine is getting stored on stake.

example :

// function getNumber returns the "int i" and we can't use this returned value
// because this function is invoked as goroutine.
// We know that, to communicate between main and goroutine one could
// use the channel ( chan <- i), but I am interested to know about
// the use of return i in goroutine. Is it possible to get/ use this 
// returned value.
func getNumber(i int) int {
    return i   
    }

func main() {

    for i:=0; i<10; i++ {
        go printNumber(i)
        }
    time.Sleep(5)
    }

Should we try to avoid the return valued in go routine ?

Answer 1

A quick bit of looking at the assembly output shows

$ go build -gcflags -S z.go

The getNumber() function does store its results to the stack

"".getNumber t=1 size=16 value=0 args=0x10 locals=0x0
    0x0000 00000 (z.go:5)   TEXT    "".getNumber+0(SB),4,$0-16
    0x0000 00000 (z.go:6)   MOVQ    "".i+8(FP),BX
    0x0005 00005 (z.go:6)   MOVQ    BX,"".~r1+16(FP)
    0x000a 00010 (z.go:6)   RET ,

So when it is called from a goroutine, it does store its results to the stack. However this is a new stack which is destroyed when the goroutine ends so there is no possibility of retrieving the return value.

"".main t=1 size=96 value=0 args=0x0 locals=0x18
    0x0000 00000 (z.go:9)   TEXT    "".main+0(SB),$24-0
    0x0000 00000 (z.go:9)   MOVQ    (TLS),CX
    0x0009 00009 (z.go:9)   CMPQ    SP,16(CX)
    0x000d 00013 (z.go:9)   JHI ,22
    0x000f 00015 (z.go:9)   CALL    ,runtime.morestack_noctxt(SB)
    0x0014 00020 (z.go:9)   JMP ,0
    0x0016 00022 (z.go:9)   SUBQ    $24,SP
    0x001a 00026 (z.go:10)  MOVQ    $0,AX
    0x001c 00028 (z.go:10)  CMPQ    AX,$10
    0x0020 00032 (z.go:10)  JGE $0,74
    0x0022 00034 (z.go:11)  MOVQ    AX,"".i+16(SP)
    0x0027 00039 (z.go:11)  MOVQ    AX,(SP)
    0x002b 00043 (z.go:11)  MOVQ    $"".getNumber·f+0(SB),CX
    0x0032 00050 (z.go:11)  PUSHQ   CX,
    0x0033 00051 (z.go:11)  PUSHQ   $16,
    0x0035 00053 (z.go:11)  PCDATA  $0,$0
    0x0035 00053 (z.go:11)  CALL    ,runtime.newproc(SB)
    0x003a 00058 (z.go:11)  POPQ    ,CX
    0x003b 00059 (z.go:11)  POPQ    ,CX
    0x003c 00060 (z.go:10)  MOVQ    "".i+16(SP),AX
    0x0041 00065 (z.go:10)  INCQ    ,AX
    0x0044 00068 (z.go:10)  NOP ,
    0x0044 00068 (z.go:10)  CMPQ    AX,$10
    0x0048 00072 (z.go:10)  JLT $0,34
    0x004a 00074 (z.go:13)  MOVQ    $5,(SP)
    0x0052 00082 (z.go:13)  PCDATA  $0,$0
    0x0052 00082 (z.go:13)  CALL    ,time.Sleep(SB)
    0x0057 00087 (z.go:14)  ADDQ    $24,SP
    0x005b 00091 (z.go:14)  RET ,

However there is no way of retrieving these results.