Determine if only one instance is running

This task is to determine if there is only one instance of an application running.

C++

Microsoft Windows

Works with: Windows version 2000 or later

This line needs to be near the top of the file (or in stdafx.h, if you use one.)

#include <afx.h>

You need a variable of type HANDLE with the same lifetime as your program. Perhaps as a member of your CWinApp object.

HANDLE mutex;

At the earliest possible point in your program, you need to initialize it and perform your check. "MyApp" should be a string unique to your application. See here for full details.

mutex = CreateMutex( NULL, TRUE, "MyApp" );
if ( GetLastError() == ERROR_ALREADY_EXISTS )
{
     // There's another instance running.  What do you do?
}

Finally, near the end of your program, you need to close the mutex.

CloseHandle( mutex );

Java

<java>import java.io.*;

public class SingletonApp { private static final int PORT = 10356; // random large port number private static ServerSocket s;

// static initializer { try { s = new ServerSocket(PORT, 10, InetAddress.getLocalHost()); } catch (UnknownHostException e) { // shouldn't happen for localhost } catch (IOException e) { // port taken, so app is already running System.exit(0); } } // main() and rest of application... }</java>

Python

Linux (including cygwin) and Mac OSX Leopard

Works with: Python version 2.6

Must be run from an application, not the interpreter.

import __main__, os

def isOnlyInstance():
    # Determine if there are more than the current instance of the application
    # running at the current time. If not, return 0. If so, return 1.
    if not os.system('(( $(ps -ef | grep python | grep \'[' + \
    __main__.__file__[0] + ']' + __main__.__file__[1:len(__main__.__file__)] \
    + '\' | wc -l) > 1 ))'):
        return 0
    else:
        return 1

This is not a solution - one can run the same app by copying the code to another location. A solution may be a lock file or lock directory created by the first instance and hold while the first instance is running.

Visual Basic

Works with: Visual Basic version 6

 Dim onlyInstance as Boolean
 onlyInstance = not App.PrevInstance