Golang, goroutines: panic: runtime error: invalid memory address

Question

I am new to golang and trying to understand the basic principles and write code based on gouroutines using chanels.

The other languages ​​I used didn’t have such tools, and I’m interested in getting errors like panic ...

my code is:

package main

import "fmt"
import (
    "time"
)
type Work struct {
    x,y,z int
}

func worker(in <-chan *Work, out chan<- *Work){
    for w := range in {
        w.z = w.x + w.y
        time.Sleep(time.Duration(w.z))
        out <-w
    }
}

func sendWork(in chan <- *Work){
    var wo *Work
    wo.x, wo.y, wo.z = 1,2,3
    in <- wo
    in <- wo
    in <- wo
    in <- wo
    in <- wo
}

func receiveWork(out <-chan *Work ) []*Work{
    var  slice []*Work
    for el := range out {
        slice = append(slice, el)
    }
    return slice
}

func main() {
    in, out := make(chan *Work), make(chan *Work)
    for i := 0; i<3; i++{
        go worker(in, out)
    }

    go sendWork(in)

    data := receiveWork(out)

    fmt.Printf("%v", data)
}

But on the terminal, I got the following:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x1 addr=0x0 pc=0x401130]

goroutine 8 [running]:
main.sendWork(0xc0820101e0)
        C:/temp/gocode/src/helloA/helloA.go:21 +0x20
created by main.main
        C:/temp/gocode/src/helloA/helloA.go:43 +0xe4

goroutine 1 [chan receive]:
main.receiveWork(0xc082010240, 0x0, 0x0, 0x0)
        C:/temp/gocode/src/helloA/helloA.go:31 +0x80
main.main()
        C:/temp/gocode/src/helloA/helloA.go:45 +0xf2

goroutine 5 [chan receive]:
main.worker(0xc0820101e0, 0xc082010240)
        C:/temp/gocode/src/helloA/helloA.go:12 +0x55
created by main.main
        C:/temp/gocode/src/helloA/helloA.go:40 +0xaf

goroutine 6 [runnable]:
main.worker(0xc0820101e0, 0xc082010240)
        C:/temp/gocode/src/helloA/helloA.go:11
created by main.main
        C:/temp/gocode/src/helloA/helloA.go:40 +0xaf

goroutine 7 [runnable]:
main.worker(0xc0820101e0, 0xc082010240)
        C:/temp/gocode/src/helloA/helloA.go:11
created by main.main
        C:/temp/gocode/src/helloA/helloA.go:40 +0xaf

How can I determine where the problem is, and how can I close the guuruts beautifully, and not leave them as processes ...

ps Forgive me my questions. you are welcome

Answer 1

nil:
, , . , .

:

wo.x, wo.y, wo.z = 1,2,3

, wo. ; Work. , .

nil struct nil. struct , nil.

var wo *Work

wo Work nil.

 var wo = &Work{}

wo Work Work.

:

wo := &Work{}

:

, . func worker . , .

, , sync.WaitGroup. , , .

waitgroup, goroutines .

wg.Add(3)

:

wg.Wait()

wg.Done()

, .

, , func receiveWork for range. , goroutine :

go func() {
    wg.Wait()
    close(out)
}()

:

package main

import (
    "fmt"
    "sync"
    "time"
)

type Work struct {
    x, y, z int
}

func worker(in <-chan *Work, out chan<- *Work, wg *sync.WaitGroup) {
    for w := range in {
        w.z = w.x + w.y
        time.Sleep(time.Duration(w.z))
        out <- w
    }
    wg.Done() // this worker is now done; let the WaitGroup know.
}

func sendWork(in chan<- *Work) {
    wo := &Work{x: 1, y: 2, z: 3} // more compact way of initializing the struct
    in <- wo
    in <- wo
    in <- wo
    in <- wo
    in <- wo
    close(in) // we are done sending to this channel; close it
}

func receiveWork(out <-chan *Work) []*Work {
    var slice []*Work
    for el := range out {
        slice = append(slice, el)
    }
    return slice
}

func main() {
    var wg sync.WaitGroup
    in, out := make(chan *Work), make(chan *Work)
    wg.Add(3) // number of workers
    for i := 0; i < 3; i++ {
        go worker(in, out, &wg)
    }

    go sendWork(in)

    go func() {
        wg.Wait()
        close(out)
    }()

    data := receiveWork(out)

    fmt.Printf("%v", data)
}

:

[0x104382f0 0x104382f0 0x104382f0 0x104382f0 0x104382f0]

, , , . . .

, Work, , :

for _, w := range data {
    fmt.Printf("%v", w)
}

:

&{1 2 3}&{1 2 3}&{1 2 3}&{1 2 3}&{1 2 3}

Go , , .

:

*Work , goroutines . , . Work *Work.

, , , Work , , , *Work, goroutine.

, go race detector, :

C:/Go\bin\go.exe run -race C:/gopath/src/github.com/drathier/scratchpad/go/main/main.go
[0xc0820403c0 0xc0820403c0 0xc0820403c0 0xc0820403c0 0xc0820403c0]==================
WARNING: DATA RACE
Write by goroutine 6:
  main.worker()
      C:/gopath/src/github.com/drathier/scratchpad/go/main/main.go:15 +0x8a

Previous write by goroutine 8:
  main.worker()
      C:/gopath/src/github.com/drathier/scratchpad/go/main/main.go:15 +0x8a

Goroutine 6 (running) created at:
  main.main()
      C:/gopath/src/github.com/drathier/scratchpad/go/main/main.go:45 +0x10c

Goroutine 8 (running) created at:
  main.main()
      C:/gopath/src/github.com/drathier/scratchpad/go/main/main.go:45 +0x10c
==================
Found 1 data race(s)
exit status 66

:

w.z = w.x + w.y

goroutines w.z, , w.z, , . , *Work : Work.