How can I ask the user for elevated permissions at runtime?

Question

Some applications, started with a regular user will ask for elevated permissions when necessary (e.g. a file manager needs to write such folder), and then carry on with the operation.

How can I replicate this behavior?

Answer 1

As Tamás pointed out you need to launch a new process with elevated rights. I searched a lot in the past but I did not find any way to elevate the rights of the current process.

Lets say your primary app is App1.exe and then you call a secondary process App2.exe which requires elevated rights.

---

A. You can embed a manifest in your App2.exe but the simpler way is to create a manifest file [a text file] named App2.exe.manifest with the following contents and put it in the same directory as App2.exe. Note: !! Strangely enough, if the name of your application is not App2.exe but App2_install.exe or App2_setup.exe (i.e. if the application name contains the "install" or "setup") an UAC Dialog will appear automatically in Windows Vista / Windows 7 and will ask for elevated rights even there is no manifest file !! This is a sample of the manifest file:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
<security>
<requestedPrivileges>
<requestedExecutionLevel level="requireAdministrator" uiAccess="false" />
</requestedPrivileges>
</security>
</trustInfo>
</assembly>

---

B. You can use a code like the following in App1.exe to launch the App2.exe

QString AppToExec = qApp->applicationDirPath() + "/App2.exe";
// Put any required parameters of App2.exe to AppParams string
QString AppParams = "";
if (0 != genWin32ShellExecute(AppToExec, 
                              "",    // default verb: "open" or "exec"
                              AppParams,
                              false, // run hidden
                              true)) // wait to finish
{
    // (...) handle error
}

...and finally, this is the code of the Win32 function genWin32ShellExecute() I created to launch a process or open a document when using QT on a Win32 O/S:

Header:

#ifdef Q_OS_WIN  // Implement genWin32ShellExecute() especially for UAC
    #include "qt_windows.h"
    #include "qwindowdefs_win.h"
    #include <shellapi.h>

int genWin32ShellExecute(QString AppFullPath,
                         QString Verb,
                         QString Params,
                         bool ShowAppWindow,
                         bool WaitToFinish);
#endif

CPP:

// Execute/Open the specified Application/Document with the given command
// line Parameters
// (if WaitToFinish == true, wait for the spawn process to finish)
//
// Verb parameter values:
// ""           The degault verb for the associated AppFullPath
// "edit"       Launches an editor and opens the document for editing.
// "find"       Initiates a search starting from the specified directory.
// "open"       Launches an application. If this file is not an executable file, its associated application is launched.
// "print"      Prints the document file.
// "properties" Displays the object's properties.
//
// Ret: 0 = success
//     <0 = error
#ifdef Q_OS_WIN
int genWin32ShellExecute(QString AppFullPath,
                         QString Verb,
                         QString Params,
                         bool ShowAppWindow,
                         bool WaitToFinish)
{
    int Result = 0;

    // Setup the required structure
    SHELLEXECUTEINFO ShExecInfo;
    memset(&ShExecInfo, 0, sizeof(SHELLEXECUTEINFO));
    ShExecInfo.cbSize = sizeof(SHELLEXECUTEINFO);
    ShExecInfo.fMask = SEE_MASK_NOCLOSEPROCESS;
    ShExecInfo.hwnd = NULL;
    ShExecInfo.lpVerb = NULL;
    if (Verb.length() > 0)
        ShExecInfo.lpVerb = reinterpret_cast<const WCHAR *>(Verb.utf16());
    ShExecInfo.lpFile = NULL;
    if (AppFullPath.length() > 0)
        ShExecInfo.lpFile = reinterpret_cast<const WCHAR *>(AppFullPath.utf16());
    ShExecInfo.lpParameters = NULL;
    if (Params.length() > 0)
        ShExecInfo.lpParameters = reinterpret_cast<const WCHAR *>(Params.utf16());
    ShExecInfo.lpDirectory = NULL;
    ShExecInfo.nShow = (ShowAppWindow ? SW_SHOW : SW_HIDE);
    ShExecInfo.hInstApp = NULL;

    // Spawn the process
    if (ShellExecuteEx(&ShExecInfo) == FALSE)
    {
        Result = -1; // Failed to execute process
    } else if (WaitToFinish)
    {
        WaitForSingleObject(ShExecInfo.hProcess, INFINITE);
    }

    return Result;
}
#endif

Answer 2

See this question on elevating privileges only when required in C# and this article on User Account Control

To sum it up: one needs to launch a new process with elevated permissions. The elevation level cannot be changed at runtime. Launching with elevated permissions is done either via WinAPI or embedding a correct manifest in the executable.

Answer 3

In a nutshell: Create two executable files for windows. The regular executable, and a worker exe file that you use to perform "elevated" operations (by passing command line options).

To the second EXE file you add an application manifest file with a <requestExecutionLevel level="requireAdministrator"/> node.

When launching the worker app, make sure to use the QT function that wraps ShellExecute, NOT CreateProcess as CreateProcess simply fails to launch requireAdministrator apps, whereas ShellExecute (Being a shell function) can perform the UAC elevation prompt.

It is also possible to do this with ActiveX controls, but as you are targeting Qt that seems less appropriate.

Answer 4

You can also launch a COM object in an elevated mode. See this MSDN article for more information.