Golang：与频道并发

Question

I wrote a short script to write files concurrently. One goroutine is supposed to write strings to a file while the others are supposed to send the messages through a channel to it. However, for some really strange reason the file is created but no message is added to it through the channel.

package main

import (
    "fmt"
    "os"
    "sync"
)

var wg sync.WaitGroup
var output = make(chan string)

func concurrent(n uint64) {
    output <- fmt.Sprint(n)
    defer wg.Done()
}

func printOutput() {
    f, err :=  os.OpenFile("output.txt", os.O_CREATE|os.O_RDWR|os.O_APPEND, 0666);
    if err != nil {
            panic(err)
    }
    defer f.Close()

    for msg := range output {
            f.WriteString(msg+"
")
    }
}

func main() {
    wg.Add(2)
    go concurrent(1)
    go concurrent(2)
    wg.Wait()
    close(output)
    printOutput()
}

The printOutput() goroutine is executed completely, if I tried to write something after the for loop it would actually get into the file. So this leads me to think that range output might be null

Answer 1

One of the reason why you get a null output is because channels are blocking for both send/receive.

According to your flow, the code snippet below will never reach wg.Done(), as sending channel is expecting a receiving end to pull the data out. This is a typical deadlock example.

func concurrent(n uint64) {
    output <- fmt.Sprint(n) // go routine is blocked until data in channel is fetched.
    defer wg.Done()
}

Let's examine the main func:

func main() {
    wg.Add(2)
    go concurrent(1)  
    go concurrent(2)
    wg.Wait()       // the main thread will be waiting indefinitely here.
    close(output)   
    printOutput()
}

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