Error when trying to load DLL with QLibrary

Linhares

I'm using Qt5 and I want to work with a DLL file that I know is fully functional and which I can even use with other programs on my computer with no problems.

However, when I try to load it in my program using QLibrary, I get an error message.

Here's the .cpp file:

#include <QLibrary>
#include <QDebug>

myObject::myObject()
{
    QLibrary myLibrary("./myDll");
    myLibrary.load();
    if(myLibrary.isLoaded())
        qDebug() << "It worked!";
    else
    {
        qDebug() << myLibrary.errorString();
    }
}

When I run this code, I get this error message:

"Cannot load library .\myDll: Unknown error 0x000000c1."

I've run Dependency Walker and I get a lot of yellow flags. However, I can't understand how I've been using this DLL with other (3rd party) programs on the same computer without any problems.

I contacted the developer of the DLL and he told me this library is a plain C (not C++) 32-bit library, and that perhaps I should adjust some of the compiler options. If this is the case, which options should I change?

jsulm

@Linhares said in Error when trying to load DLL with QLibrary:

QLibrary myLibrary("./myDll");

You are using a relative path: are you sure the lib is in the current working directory?

JonB

@Linhares said in Error when trying to load DLL with QLibrary:

"Cannot load library .\myDll: Unknown error 0x000000c1."

(Additional to @jsulm.) Most (in fact I think it is mandatory?) Windows DLL files end in .DLL ... ?

EDIT
Given the error Cannot load library .\myDll: Unknown error 0x000000c1 that might it is found, somehow, but is "not acceptable". Google dll Unknown error 0x000000c1, seems to only come from Qt programs, e.g. see Cannot load library *.dll Unknown error 0x000000c1

The error list you need is here. The symbolic name for that error is ERROR_BAD_EXE_FORMAT, and the error message is <filesoandso> is not a valid Win32 application. The DLL you're trying to open is either corrupt, or - most likely - it is for a different architecture. If you're compiling for 32 bits, use 32-bit DLLs. If you're compiling for 64 bits, use 64-bit DLLs.

If the developer says it is 32-bit are you compiling your Qt program 32-bit or 64-bit? :)

Linhares

@jsulm Yes, the lib is in the folder. From the error message, I understand the program can find the lib, but it can't load it correctly.

jsulm

@Linhares Take a look at this: https://stackoverflow.com/questions/61080829/cannot-load-library-dll-unknown-error-0x000000c1
Is it possible that this DLL is built for a different architecture? Like your app is x86_64, but the lib x86.

Linhares

@JonB this is the kit I've been using:

My understanding is that it's 32-bit compiler.

@jsulm how do I check my app's architecture?

jsulm

@Linhares If that is the Kit you are using to build your app then you're building a 32bit app.

Linhares

@jsulm Ok, so I'm using the correct architecture, right?

mchinand

Can you try to build and link to that dll instead using QLibrary? You may get a more informative link error than what you are getting when loading with QLibrary.

JonB

@Linhares
Although you have said "Yes, the lib is in the folder", and presumably you claim the relative path ./myDll finds it successfully, you still have not said anything about my

Most (in fact I think it is mandatory?) Windows DLL files end in .DLL ... ?

? [Oh, are we supposed to guess that you are in fact not using literal ./myDll but rather ./something.dll??] You could always do a QFileInfo on whatever you pass to make sure....

Otherwise @mchinand's suggestion of quickly making your application actually try to link with thedll.lib (you do have that?) by calling a function in it from your Qt program is a good idea to get a better error message,

Linhares

It turned out that I noticed I was being fooled by Qt Creator. Although the settings showed MSVC 32-bit as default, it was not installed. Hence, I was running a 64-bit compiler and thinking I was using a 32-bit one.
Thank you guys for the help with this!