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QSysInfo::machineUniqueId() empty on Windows

  • Hi
    Since Qt 5.11 there is function QSysInfo::machineUniqueId() which should return unique ID per machine (eg. processor number, mainboard number), on Linux it works, on Windows 10 it returns empty string (tested on Win 10 and Qt 5.11.1).
    Is this as it should be ? Can't get this machineUniqueId on Windows, or is this some bug?

    Best Regards

  • @Marek
    I suspect this is because from docs:

    QByteArray QSysInfo::machineUniqueId()
    See also machineHostName() and bootUniqueId().


    QByteArray QSysInfo::bootUniqueId()
    This function is currently only implemented for Linux and Apple operating systems.
    See also machineUniqueId().

    EDIT: Hmm, maybe not. From I see:

    #elif defined(Q_OS_WIN) && !defined(Q_OS_WINRT) 
        // Let's poke at the registry 
        HKEY key = NULL; 
        if (RegOpenKeyEx(HKEY_LOCAL_MACHINE, L"SOFTWARE\\Microsoft\\Cryptography", 0, KEY_READ, &key) 
                == ERROR_SUCCESS) { 
            wchar_t buffer[UuidStringLen + 1]; 
            DWORD size = sizeof(buffer); 
            bool ok = (RegQueryValueEx(key, L"MachineGuid", NULL, NULL, (LPBYTE)buffer, &size) == 
            if (ok) 
                return QStringView(buffer, (size - 1) / 2).toLatin1(); 
        return QByteArray(); 

    So, assuming not Q_OS_WINRT (whatever that is for), do you have that registry value defined, and readable to your app?

  • @JonB I will try this now, but since RegOpenKeyEx is from Win api not Qt which lib should I link to and include headers?


  • @Marek
    Not sure what you mean, but I did not mean compile anything! I meant: look at that code, and go look in your registry to see whether you even have that entry sitting there or not, because that is the code Qt will be executing for you. I can see that key value under my Win 8 (so I would expect the Qt call to work), maybe it isn't there on your machine in Win 10, or (unlikely?) your Qt app process has no permission to read it from the registry?

  • @JonB I have compiled this into app and it returns empty string, I think because I can see Q_OS_WINRT is set
    This windows is a bit slow...

  • @Marek

    Defined for Windows Runtime (Windows Store apps) on Windows 8, Windows RT, and Windows Phone 8.

    Is your Qt compiled for Win 10 "Windows Store app" ??

  • @JonB No I'm using QtIFW to create installer

  • @Marek

    • Recheck your claim "I think because I can see Q_OS_WINRT is set", because that doesn't sound likely to me?
    • Look in your registry to see what you have for the key entry referenced in the code?

  • @JonB Yeah, I have downloaded some tool for registry and I'm checking, will report

  • @Marek
    You don't need to download anything, you already have REGEDIT! :)

  • @JonB REGEDIT inside Win ? Sorry I don't work usally on Windows. I have downloaded some regmagik and I can see that Microsoft\Cryptography hasn't any keys only some branches.

  • @JonB I have searched through the registry and there is no MachineGuid in reg

  • @Marek
    Yeah, everybody knows that Windows comes with REGEDIT for examining the Registry! :)

    If you don't have HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\MachineGuid (I do), that function is not going to return anything on your machine. That's all I know.

  • @JonB anyway thanks for help, I really have Q_OS_WINRT set thats why this code returned empty string first time I have compiled

  • @Marek
    Well actually that sounds more reasonable as an explanation, because I think the registry entry really is there under all Windows.

    Now your question becomes: "why does the Qt I am using seem to have been compiled with Q_OS_WINRT defined, because I would have thought it should not?".

    I am not a Qt expert: you would need someone like @SGaist to see this, look through my understanding of what must be going on, and explain.

  • @JonB does this matter if on my win 10 there is o MachineGuid key ?
    I'm checking something, I'm using MinGW 32 bit, try to compile with msvc2017 64bit

  • @JonB OK, success, Thanks for your help ;)
    so windows can provide "virtual" registry for 32 bit applications and this registry does not have MachineGuid. I have compiled with msvc2017 64Bit and it works. I can call it a day ;)


  • @Marek
    Yes it can. It's all to do with the Wow6432Node key:

    Yes, for 32-bit software 64-bit OS will substitute HKLM\SOFTWARE\ path with HKLM\SOFTWARE\Wow6432Node\. So you were trying to read HKLM\SOFTWARE\Wow6432Node\Microsoft\Cryptography\MachineGuid which doesn't exist.

    You did not say in your question that any 32-bitted-ness was involved at your side, I assumed all 64-bit, else I would have said about needing to change where you look in the registry :)

  • Banned

    This post is deleted!

  • @JonB I did not supposed it matters, frankly I prefer to use MinGW (old habits maybe) and I didn't see Qt 5.11 for MinGW 64bit.
    @CoreyAnderson how this HP printer support is related to Qt or even Windows 10, I don't get it, you mean bug in Windows that it does not provide key for 32bit apps ?

  • @Marek

    Ignore @CoreyAnderson 's post, he is just advertising his hyperlink! It's a scam.

    You can use a 64-bit compiler with no issue. If you want to use a 32-bit compiler, you will have to do some work.

  • @JonB but I was downloading Qt 5.11.1 throught Maintenance tool and there was MinGW 32Bit not 64Bit

  • @Marek
    I don't know what to say other than you have the choice of what compiler and bitted-ness are used to compile Qt, and you decide which target platforms you wish to support. In this case you have found a situation where there is a 32-bit/64-bit Windows Registry issue if you mix bitted-nesses.

  • Moderators

    I sense a bit of miscommunication in the last few messages; @Marek and @JonB are talking about slightly different things.

    @Marek's observation is correct: The Maintenance Tool does not provide a pre-built 64-bit version of Qt for MinGW.

    @JonB's point is also correct: We can choose from a variety of 32-bit and 64-bit versions. (But if we want 64-bit MinGW, then we need to build Qt ourselves -- we can't just download it through the MaintenanceTool)

  • @JKSH
    Ah, I don't even know what that "Maintenance Tool" is! I actually thought it might be for building Qt yourself, now I understand it gives pre-compiled versions. And you're saying it does not offer 64-bit for MinGW.

    You still have two choices for your issue:

    • If you use a 64-bit Qt there is no problem. That could be pre-compiled or compiled by you, and it could be MinGW, MSVC or whatever. I do not know whether you find using either a pre-supplied MSVC 64-bit (does Maint Tool have that?) or compiling for yourself a 64-bit MinGW acceptable or a pain in the ass.

    • If you use a 32-bit Qt --- perhaps the conveniently pre-compiled MinGW one --- there is a problem, because of where that looks in the Registry. You can probably address this by doing something in your app code, but we haven't investigated this yet as you haven't said which Qt 32/64-bit route you wish to pursue.

  • @JonB Thanks for your help.
    MSVC 64Bit is provided by Maintenance Tool.
    Compiling Qt on Windows with 64-Bit MinGW might be a pain ;) I'm doing this quite often on embedded platforms but then I need much less modules. What is more, I'm writing code that someone else will use in his projects, so it needs to be compatible with commercial Qt license.
    I will stick to MSVC 64Bit.


  • @Marek
    So it sounds like you will use 64-bit, this problem will go away and we don't need to investigate any further.
    I presume all your targets will be 64-bit, none 32-bit(?).

    Now that we know how QSysInfo::machineUniqueId() relies on finding that "MachineGuid" in the correct place in the Registry where it looks, and there's some possibility it may not find it, you might be best documenting this somewhere for both developers & end-users, and/or providing a "fallback" mechanism if it returns an empty string.

  • @JonB I'm reading mac addresses of network cards when machineUniqueId returns empty string. This can be problematic especially when someone uses detachable usb->wifi adapter but I don't see other option with Qt.

  • @Marek
    Well Qt is only a library providing "abstracted, convenience" functions where possible, it's not magic, so yes sometimes you have to supply your own alternatives as best you can. If you can't find anything, chances are nor can Qt.

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