How to resolve the algorithm Determine if only one instance is running step by step in the Go programming language

Published on 12 May 2024 09:40 PM
#Go

How to resolve the algorithm Determine if only one instance is running step by step in the Go programming language

Table of Contents

Problem Statement

This task is to determine if there is only one instance of an application running. If the program discovers that an instance of it is already running, then it should display a message indicating that it is already running and exit.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Determine if only one instance is running step by step in the Go programming language

Code 1:

This code checks if a single instance of the program is running by attempting to listen on a specific TCP port. If another instance is already listening on that port, it means that another instance of the program is running and the current instance terminates.

Explanation:

  • It starts by trying to listen on a specified TCP port and address using net.Listen.
  • If the net.Listen call is successful, it means that no other instance is listening on that port and the current instance is successfully started.
  • If the net.Listen call fails with an error indicating that another instance is already listening on that port, the current instance prints a message and terminates.

Code 2:

This code uses a lock file mechanism to check if a single instance of the program is running. It attempts to create a lock file at a specified location. If another instance is already running, it means that the lock file already exists, and the current instance terminates.

Explanation:

  • It attempts to create a lock file at a specified path using os.OpenFile.
  • If the file creation is successful, it means that no other instance is running, and the current instance can start.
  • If the file creation fails due to the file already existing, it means that another instance is already running, and the current instance prints a message and terminates.

Code 3:

This code also uses a lock file mechanism, but it includes additional error handling and provides more information about the running instance.

Explanation:

  • It attempts to create a lock file at a specified path using os.OpenFile.
  • If the file creation is successful, it writes the current process ID (PID) to the file and closes it. The lock file is deleted when the program exits.
  • If the file creation fails due to the file already existing, it opens the existing lock file and reads the PID of the running instance. It then prints a message with the PID of the running instance.

Source code in the go programming language

package main

import (
    "fmt"
    "net"
    "time"
)

const lNet = "tcp"
const lAddr = ":12345"

func main() {
    if _, err := net.Listen(lNet, lAddr); err != nil {
        fmt.Println("an instance was already running")
        return
    }
    fmt.Println("single instance started")
    time.Sleep(10 * time.Second)
}


package main

import (
    "fmt"
    "os"
    "time"
)

// The path to the lock file should be an absolute path starting from the root.
// (If you wish to prevent the same program running in different directories,
// that is.)
const lfn = "/tmp/rclock"

func main() {
    lf, err := os.OpenFile(lfn, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
    if err != nil {
        fmt.Println("an instance is already running")
        return
    }
    lf.Close()
    fmt.Println("single instance started")
    time.Sleep(10 * time.Second)
    os.Remove(lfn)
}


package main

import (
    "fmt"
    "os"
    "strconv"
    "strings"
    "time"
)

// The path to the lock file should be an absolute path starting from the root.
// (If you wish to prevent the same program running in different directories, that is.)
const lfn = "/tmp/rclock"

func main() {
    lf, err := os.OpenFile(lfn, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
    if err == nil {
        // good
        // 10 digit pid seems to be a standard for lock files
        fmt.Fprintf(lf, "%10d", os.Getpid())
        lf.Close()
        defer os.Remove(lfn)
    } else {
        // problem
        fmt.Println(err)
        // dig deeper
        lf, err = os.Open(lfn)
        if err != nil {
            return
        }
        defer lf.Close()
        fmt.Println("inspecting lock file...")
        b10 := make([]byte, 10)
        _, err = lf.Read(b10)
        if err != nil {
            fmt.Println(err)
            return
        }
        pid, err := strconv.Atoi(strings.TrimSpace(string(b10)))
        if err != nil {
            fmt.Println(err)
            return
        }
        fmt.Println("lock file created by pid", pid)
        return
    }
    fmt.Println(os.Getpid(), "running...")
    time.Sleep(1e10)
}

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