Experience of slice data transfer in Golang Channel

This is a creation in Article, where the information may have evolved or changed.

Background

Suppose we want to develop a producer-consumer model of code, a routine responsible for production data, a routine responsible for consumption data, the code is as follows:

Error code

type MsgStru struct {    msg []int}var msgChan chan MsgStrufunc sendMsg() {    buf := make([]int, 10)    for i := 0; i < 10; i++ {        buf[0] = i        tmp := MsgStru{msg: buf[:1]}        fmt.Printf("send:%+v\n", tmp)        msgChan <- tmp    }    fmt.Println("finish send")}func recvMsg() {    for {        tmp, _ := <-msgChan        fmt.Printf("recv:%+v\n", tmp)    }}func main() {    msgChan = make(chan MsgStru, 10)    go sendMsg()    time.Sleep(1 * time.Second)    recvMsg()}

The expected output should be 0 to 9 in order, but actually print?

send:{msg:[0]}send:{msg:[1]}send:{msg:[2]}send:{msg:[3]}send:{msg:[4]}send:{msg:[5]}send:{msg:[6]}send:{msg:[7]}send:{msg:[8]}send:{msg:[9]}finish sendrecv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}recv:{msg:[9]}

The data received is the number 9, which is why?

Cause analysis

First look at the type representation of the data in the channel:

type MsgStru struct {    msg []int}

MSG is a slice,slice in Go is a pointer plus length, which means that the incoming channel is a pointer to buf, BUF is a global variable, so the Channel 10 msg data is a pointer to the BUF, when recvmsg ready to read the data, The data in the BUF now becomes 9, so 10 msg read from the channel is 9.

Modify Scenario 1

Change global slice to local slice

func sendMsg() {    buf := make([]int, 10)    for i := 0; i < 10; i++ {        buf[0] = i        tmpMsg := make([]int, 0)        tmpMsg = append(tmpMsg, buf[:1]...)        tmp := MsgStru{msg: tmpMsg}        fmt.Printf("send:%+v\n", tmp)        msgChan <- tmp    }    fmt.Println("finish send")}

Results

send:{msg:[0]}send:{msg:[1]}send:{msg:[2]}send:{msg:[3]}send:{msg:[4]}send:{msg:[5]}send:{msg:[6]}send:{msg:[7]}send:{msg:[8]}send:{msg:[9]}finish sendrecv:{msg:[0]}recv:{msg:[1]}recv:{msg:[2]}recv:{msg:[3]}recv:{msg:[4]}recv:{msg:[5]}recv:{msg:[6]}recv:{msg:[7]}recv:{msg:[8]}recv:{msg:[9]}

Modify Scenario 2

Change Slice to array

type MsgStru struct {    msg [2]int}var msgChan chan MsgStrufunc sendMsg() {    var buf [2]int    for i := 0; i < 10; i++ {        buf[0] = i        tmp := MsgStru{msg: buf}        fmt.Printf("send:%+v\n", tmp)        msgChan <- tmp    }    fmt.Println("finish send")}

Results

send:{msg:[0 0]}send:{msg:[1 0]}send:{msg:[2 0]}send:{msg:[3 0]}send:{msg:[4 0]}send:{msg:[5 0]}send:{msg:[6 0]}send:{msg:[7 0]}send:{msg:[8 0]}send:{msg:[9 0]}finish sendrecv:{msg:[0 0]}recv:{msg:[1 0]}recv:{msg:[2 0]}recv:{msg:[3 0]}recv:{msg:[4 0]}recv:{msg:[5 0]}recv:{msg:[6 0]}recv:{msg:[7 0]}recv:{msg:[8 0]}recv:{msg:[9 0]}

Postscript

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