How to Resolve \'Fatal Error: All Goroutines Are Asleep - Deadlock!\' in Go When Using Unbuffered Channels?

"Go: Fatal Error: All Goroutines Are Asleep - Deadlock"

Problem:

You have a text file with a single line of words and you're attempting to store each word in a channel. Subsequently, you want to retrieve these words from the channel and print them one at a time. The following code snippet represents your current approach:

package main

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    f, _ := os.Open("D:\input1.txt")
    scanner := bufio.NewScanner(f)
    file1chan := make(chan string)

    for scanner.Scan() {
        line := scanner.Text()
        parts := strings.Fields(line)

        for i := range parts {
            file1chan <- parts[i]
        }
    }
    print(file1chan)
}

func print(in <-chan string) {
    for str := range in {
        fmt.Printf("%s\n", str)
    }
}

However, upon running this code, you encounter the error message: "fatal error: all goroutines are asleep - deadlock!"

Solution:

The error arises because your file1chan is an unbuffered channel. When you attempt to send a value down this channel, it blocks indefinitely, waiting for a receiver. To resolve this deadlock, you can either start a new goroutine responsible for sending values into the channel or declare the channel as buffered. Two approaches are outlined below:

Using a New Goroutine:

package main

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    f, _ := os.Open("D:\input1.txt")
    scanner := bufio.NewScanner(f)
    file1chan := make(chan string)

    // Start a new goroutine to send strings down file1chan
    go func() {
        for scanner.Scan() {
            line := scanner.Text()
            parts := strings.Fields(line)

            for i := range parts {
                file1chan <- parts[i]
            }
        }
        close(file1chan) // Close the channel when done sending.
    }()

    print(file1chan) // Read strings from file1chan
}

func print(in <-chan string) {
    for str := range in {
        fmt.Printf("%s\n", str)
    }
}

Using a Buffered Channel:

For handling a single string, you can define a buffered channel of size 1:

package main

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    f, _ := os.Open("D:\input1.txt")
    scanner := bufio.NewScanner(f)
    file1chan := make(chan string, 1) // Buffer size of one

    for scanner.Scan() {
        line := scanner.Text()
        parts := strings.Fields(line)

        for i := range parts {
            file1chan <- parts[i]
        }
    }

    close(file1chan) // Close the channel when done sending.
    print(file1chan)
}

func print(in <-chan string) {
    for str := range in { // Read all values until channel gets closed
        fmt.Printf("%s\n", str)
    }
}

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