There can only be one (instance of your program)

[Josh]

This code will check to see if your program is already running, and activate the window and bring it to the front if it is. If the window is minimized, it will be restored.

String title = "My Program";
#define ERROR_ALREADY_RUNNING 1

int main(int argc, const char* argv[])
{
#ifdef _WIN32
	auto hwnd = FindWindowA(NULL, title.c_str());
	if (hwnd)
	{
		if (IsIconic(hwnd)) ShowWindow(hwnd, SW_RESTORE);
		SetForegroundWindow(hwnd);
		SetWindowPos(hwnd, NULL, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
		SetActiveWindow(hwnd);
		SetFocus(hwnd);
		return ERROR_ALREADY_RUNNING;
	}
#endif

[Josh]

Getting the process by file path might be even better:
https://docs.microsoft.com/en-us/windows/win32/psapi/enumerating-all-processes

[Josh]

https://stackoverflow.com/questions/865152/how-can-i-get-a-process-handle-by-its-name-in-c

[klepto2]

Maybe you should use the common mutex approach:

m_singleInstanceMutex = CreateMutex(NULL, TRUE, L"Some unique string for your app");
if (m_singleInstanceMutex == NULL || GetLastError() == ERROR_ALREADY_EXISTS) {
    HWND existingApp = FindWindow(0, L"Your app's window title");
    if (existingApp) SetForegroundWindow(existingApp);
    return FALSE; // Exit the app. For MFC, return false from InitInstance

 

 

[Josh]

Here is a function that will check if an app is open and if it is, it will send some data to the other application:

bool CheckIfOpen(shared_ptr<Window> source, const WString& appid, const WString& apptitle, shared_ptr<Buffer> data)
{
    const int DATA_ID = 345934;// whatever...
    auto m_singleInstanceMutex = CreateMutexW(NULL, TRUE, appid.c_str());
    if (m_singleInstanceMutex == NULL or GetLastError() == ERROR_ALREADY_EXISTS)
    {
        HWND hwnd = FindWindowW(0, apptitle.c_str());
        if (hwnd and data)
        {
            COPYDATASTRUCT copydata;
            copydata.dwData = DATA_ID;
            copydata.cbData = data->GetSize();
            copydata.lpData = data->Data();
            LPARAM srchwnd = 0L;
            if (source) srchwnd = (LPARAM) source->GetHandle();
            SendNotifyMessageW(hwnd, WM_COPYDATA, srchwnd, (LPARAM) &copydata);
        }
        return false; // Exit the app. For MFC, return false from InitInstance
    }
    return true;
}

 

[Josh]

This is the best way to do this:

struct EnumWindowInfo
{
    int messageid;
    WString procname, title;
    shared_ptr<Buffer> data;
    shared_ptr<Window> source;
    bool result;
};

BOOL CALLBACK EnumWindowsProc(_In_ HWND hwnd, _In_ LPARAM lParam)
{
    const int MAX_NAME = 2048;
    auto info = (EnumWindowInfo*)lParam;
    WString path;
    path.resize(MAX_NAME);
    std::fill(path.begin(), path.end(), 0);
    int sz = GetWindowTextW(hwnd, (LPWSTR)path.c_str(), path.size());
    if (0 == sz)
    {
        auto err = GetLastError();
        return true;
    }
    path.resize(sz);
    //Print(path); 
    if (path != info->title) return true;
    DWORD lpdwProcessId = 0;
    auto threadID = GetWindowThreadProcessId(hwnd, &lpdwProcessId);
    HANDLE proc = OpenProcess(PROCESS_ALL_ACCESS, TRUE, lpdwProcessId);
    path.resize(MAX_NAME);
    auto len = GetModuleFileNameExW(proc, NULL, (LPWSTR)path.c_str(), path.size());
    CloseHandle(proc);
    path.resize(len);
    if (StripDir(path) == info->procname)
    {
        COPYDATASTRUCT copydata;
        copydata.dwData = info->messageid;
        copydata.cbData = info->data->GetSize();
        copydata.lpData = info->data->Data();
        HWND srchwnd = 0L;
        if (info->source) srchwnd = info->source->GetHandle();
        ShowWindow(hwnd, SW_RESTORE);
        SendMessageW(hwnd, WM_COPYDATA, (LPARAM)srchwnd, (LPARAM)&copydata);
        SetForegroundWindow(hwnd);
        SetActiveWindow(hwnd);
        SetFocus(hwnd);
        info->result = true;
        return false;
    }
    return true;
}

bool SendWindowData(shared_ptr<Window> source, const WString& title, const WString& processname, const int messageid, shared_ptr<Buffer> data)
{
    EnumWindowInfo info;
    info.title = title;
    info.result = false;
    info.procname = processname;
    info.data = data;
    info.messageid = messageid;
    EnumWindows(EnumWindowsProc, (LPARAM)&info);
    return info.result;
}