How to ensure only one process is created by CreateProcess when calling concurrently in c++?

Question

Quoted from here:

BOOL WINAPI CreateProcess(
  __in_opt     LPCTSTR lpApplicationName,
  __inout_opt  LPTSTR lpCommandLine,
  __in_opt     LPSECURITY_ATTRIBUTES lpProcessAttributes,
  __in_opt     LPSECURITY_ATTRIBUTES lpThreadAttributes,
  __in         BOOL bInheritHandles,
  __in         DWORD dwCreationFlags,
  __in_opt     LPVOID lpEnvironment,
  __in_opt     LPCTSTR lpCurrentDirectory,
  __in         LPSTARTUPINFO lpStartupInfo,
  __out        LPPROCESS_INFORMATION lpProcessInformation
);

I have two independant programe that creates exactly the same process, how can I ensure that if one of them has already created the process, the other won't create it twice?

Answer 1

You can use this function

BOOL WINAPI EnumProcesses( __out DWORD *pProcessIds, __in DWORD cb, __out DWORD *pBytesReturned );

to get a list of all the pids of all currently running processes and check if the process is running?

Answer 2

The most simple way is if you create a named object after the start of the program. For example CreateEvent, CreateMutex and so on. To verify existance of the application you can just use OpenEvent, OpenMutex and so on before creating of the object. You can choose (if desired) the name of the object with the the "Global\" prefix (see http://msdn.microsoft.com/en-us/library/aa382954.aspx) to allow only one process for all terminal server session.

UPDATED: Because how I can see there are different opinions about my suggestion I try to explain it more exactly and add the corresponding test example.

The main idea is that the application which are started create any named object is the object with the same name not yet exist. This only reserve the name in the Kernel Object Namespaces. No real usage of the object are needed. The advantaged of this way compared with creating of a file on the disk is that named objects are temporary and are owned by a application. So if the application are ended, be killed or be terminated in any other way (because of unhanded exception for example) the named object will be automatically deleted by the operation system. In the following example I don't use CloseHandle at all. How you can test the application can successfully determine whether it runs as the first instance or not.

#include <windows.h>
//#include <Sddl.h>

LPCTSTR g_pszEventName = TEXT("MyTestEvent"); // TEXT("Global\\MyTestEvent")

void DisplayFirstInstanceStartedMessage()
{
    TCHAR szText[1024];
    wsprintf (szText,
        TEXT("The first instance are started.\nThe event with the name \"%s\" is created."),
        g_pszEventName);

    MessageBox (NULL,
        szText,
        TEXT("CreateEventTest"), MB_OK);
}

void DisplayAlreadyRunningMessage ()
{
    TCHAR szText[1024];
    wsprintf (szText,
        TEXT("The first instance of the aplication is already running.\nThe event with the name \"%s\" already exist."),
        g_pszEventName);

    MessageBox (NULL,
        szText,
        TEXT("CreateEventTest"), MB_ICONWARNING | MB_OK);
}

void DisplayErrorMessage (DWORD dwErrorCode)
{
    if (dwErrorCode == ERROR_ALREADY_EXISTS)
        DisplayAlreadyRunningMessage();
    else {
        LPTSTR  pErrorString;
        if (FormatMessage (FORMAT_MESSAGE_FROM_SYSTEM |    // Always search in system message table !!!
                            FORMAT_MESSAGE_ALLOCATE_BUFFER |
                            FORMAT_MESSAGE_IGNORE_INSERTS |
                            0, NULL,                // source of message definition
                            dwErrorCode,            // message ID
    //                        0,                      // language ID
    //                        GetUserDefaultLangID(), // language ID
    //                        GetSystemDefaultLangID(),
                            MAKELANGID (LANG_NEUTRAL, SUBLANG_DEFAULT),
                            (LPTSTR)&pErrorString,   // pointer for buffer to allocate
                            0,                      // min number of chars to allocate
                            NULL)) {
            MessageBox (NULL, pErrorString, TEXT("CreateEventTest"), MB_OK);
            LocalFree (pErrorString);
        }
        else {
            TCHAR szText[1024];
            wsprintf (szText, TEXT("Error %d in the CreateEvent(..., \"%s\")"), dwErrorCode, g_pszEventName);
            MessageBox (NULL, szText, TEXT("CreateEventTest"), MB_OK);
        }
    }
}

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
    //SECURITY_ATTRIBUTES sa;
    //BOOL bSuccess;
    HANDLE hEvent = OpenEvent (EVENT_MODIFY_STATE, FALSE, g_pszEventName);// EVENT_ALL_ACCESS
    if (hEvent == NULL) {
        DWORD dwErrorCode = GetLastError();
        if (dwErrorCode != ERROR_FILE_NOT_FOUND) {
            DisplayErrorMessage(dwErrorCode);
            return 1;
        }
    }
    else {
        DisplayAlreadyRunningMessage();
        return 0;
    }

    //sa.bInheritHandle = FALSE;
    //sa.nLength = sizeof(SECURITY_ATTRIBUTES);
    //bSuccess = ConvertStringSecurityDescriptorToSecurityDescriptor (
    //    TEXT("D:(A;OICI;GA;;;WD)"),    // Allow full control 
    //    SDDL_REVISION_1,
    //    &sa.lpSecurityDescriptor,
    //    NULL);
    hEvent = CreateEvent (NULL, // &sa
        TRUE, FALSE, g_pszEventName);
    //sa.lpSecurityDescriptor = LocalFree (sa.lpSecurityDescriptor);
    if (hEvent == NULL) {
        DWORD dwErrorCode = GetLastError();
        DisplayErrorMessage(dwErrorCode);
        return 1;
    }
    else
        DisplayFirstInstanceStartedMessage();

    return 0;
    UNREFERENCED_PARAMETER (hInstance);
    UNREFERENCED_PARAMETER (hPrevInstance);
    UNREFERENCED_PARAMETER (lpCmdLine);
    UNREFERENCED_PARAMETER (nShowCmd);
}

If one want support that different users from the same desktop or from the different desktops could start only one instance of the program, one can uncomment some parts of the commented code or replace the name MyTestEvent of the event to Global\MyTestEvent.

I hope after the example my position will be clear. In such kind of the event usage no call of WaitForSingleObject() are needed.

Answer 3

You cannot do this by letting the process you start creating a named object. That's an inherent race condition, it takes time for the process to get started. Both programs need to call CreateMutex at some point before trying to create the 3rd process with an agreed-upon name. Then they need to call WaitForSingleObject() with a zero wait time to try to acquire the mutex. Whomever gets it is the one that should call CreateProcess().

More work is needed after this to deal with this 3rd process terminating.