DLLs written in C++ QT6 cannot be linked by C programs

[pedisChen]

Cause: I wrote a DLL file that uses QT6's QTCPSocket component. This file can be linked and used by QLibrary without any problems, but it cannot be linked and used by LoadLibrary in a C program, which will result in an error. I want to know why, thanks.

example code like this:

C program that links DLL by LoadLibrary

#include <windows.h>
#include <stdio.h>
int main() {
    HMODULE hModule = LoadLibrary("myDLL.dll");
    if (hModule) {
        printf("ok!");
        FreeLibrary(hModule);
    } else {
        DWORD error = GetLastError();
        printf("Could not load the DLL. Error: %lu\n", error);
    }
    return 0;
}

and Then, the err message like this

PS D:\Code\CProgram>  & 'c:\Users\12037\.vscode\extensions\ms-vscode.cpptools-1.20.5-win32-x64\debugAdapters\bin\WindowsDebugLauncher.exe' '--stdin=Microsoft-MIEngine-In-z0eabn4e.l2k' '--stdout=Microsoft-MIEngine-Out-a2ifctuv.t5z' '--stderr=Microsoft-MIEngine-Error-5vdihyxk.yrk' '--pid=Microsoft-MIEngine-Pid-i1z3tzaa.qen' '--dbgExe=C:\TDM-GCC-64\bin\gdb.exe' '--interpreter=mi' 
Could not load the DLL. Error: 126

Supplement:I have packaged all relevant DLLs using the windeployqt6.exe.

[Pl45m4]

@Christian-Ehrlicher said in DLLs written in C++ QT6 cannot be linked by C programs:

Also I don't see how this dll should work at all - where is your running Q(Core)Application?

I think it's related to this topic and the dll contains the complete Qt GUI program including the QApplication instance.
(No idea if this works like this... never linked a Qt GUI app to a C program)

@pedisChen try an absolute path to your DLL, if the only issue is that it can't locate it...
Check your working directory (where your C program is executed in) and how this is relative to the directory where your DLL is.

[Christian Ehrlicher]

Your dll is not found according the error number as you can see here: https://learn.microsoft.com/en-us/windows/win32/debug/system-error-codes--0-499-

[pedisChen]

@Christian-Ehrlicher thank you for your reply, so how can I solve this problem, I have already used windeployqt6.exe to ship all DLL dependency files to my c program project, I don't know which dll files I need to add.

[Christian Ehrlicher]

The error message is 'dll not found' - not some dependencies not found. Make sure the dll you want to load is in the current work dir.
Also I don't see how this dll should work at all - where is your running Q(Core)Application?

[Pl45m4]

@Christian-Ehrlicher said in DLLs written in C++ QT6 cannot be linked by C programs:

Also I don't see how this dll should work at all - where is your running Q(Core)Application?

I think it's related to this topic and the dll contains the complete Qt GUI program including the QApplication instance.
(No idea if this works like this... never linked a Qt GUI app to a C program)

@pedisChen try an absolute path to your DLL, if the only issue is that it can't locate it...
Check your working directory (where your C program is executed in) and how this is relative to the directory where your DLL is.

[pedisChen]

@Pl45m4 It is workable. thank you!