I want to create a process in fullscreen mode. I’ve tried setting STARTUPINFO when calling CreateProcess() as follows:

WCHAR cmd = L"Calc";
STARTUPINFO si;
PROCESS_INFORMATION pi;
ZeroMemory(&si, sizeof(STARTUPINFO));
ZeroMemory(&pi, sizeof(PROCESS_INFORMATION));
si.cb = sizeof(STARTUPINFO);
si.dwFlags = STARTF_USESHOWWINDOW;
si.wShowWindows = SW_MAXIMIZE;
// or si.dwFlags = STARTF_RUNFULLSCREEN;
BOOL isCreated = CreateProcess(nullptr, cmd,
    nullptr, nullptr, FALSE, 0, nullptr, nullptr, &si, &pi);

but the process didn’t start in fullscreen mode as expected.

So I tried to obtain its HWND after creating process like this:

HWND FindMainWindow(DWORD processId) {
    struct ProcHwnd {
        DWORD   process_id;
        HWND    hwnd;
    };
    ProcHwnd hProcWnd;
    auto EnumWindowCallback = [](HWND hwnd, LPARAM lParam) -> BOOL {
        ProcHwnd* data = reinterpret_cast<ProcHwnd *>(lParam);
        DWORD windowProcessID;
        GetWindowThreadProcessId(hwnd, &windowProcessID);
        if (windowProcessID == data->process_id) {
            if (IsWindowVisible(hwnd)) {
                data->hwnd = hwnd;
                return false;
            }
        }
        return true;
    };
    EnumWindows(EnumWindowCallback, reinterpret_cast<LPARAM>(&hProcWnd));
    return hProcWnd.hwnd;
}

int main() {
    WCHAR cmd = L"Calc";
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    ZeroMemory(&si, sizeof(STARTUPINFO));
    ZeroMemory(&pi, sizeof(PROCESS_INFORMATION));
    si.cb = sizeof(STARTUPINFO);
    BOOL isCreated = CreateProcess(nullptr, cmd,
        nullptr, nullptr, FALSE, 0, nullptr, nullptr, &si, &pi);
    WaitForInputIdle(pi.hProcess, INFINITE);
    HWND hWindow = FindMainWindow(pi.dwProcessId);
    if(hWindow == nullptr)
        return -1;
    ShowWindow(hWindow, SW_MAXIMIZE);
    return 0;
}

The exit code is 0. But the ShowWindow() didn’t maximize the window.