how to wait thread created in DLLmain finish?-CodePudding

I create a new thread in Dllmain by CreateThread api, which does not involve Thread synchronization, it is only a separate thread. And the Dllmain invokes WaitForSingleObject(funcThread, INFINITE); to force the main thread to wait for funcThread to finish. Then I dynamic link this Dllmain, but the result shows the funcThread finish early. I know adding a System Pause will work, but do you have some ways to make it work with only changing dllmain file.

main.exe

#include <Windows.h>
#include <iostream>

int main() {
    HINSTANCE hModule = LoadLibrary(L"Dllmain.dll");
    return 0;
}

Dllmain.dll

#include <Windows.h>
#include <iostream>

DWORD WINAPI function(LPVOID lpParam) {
    for (int i = 0; i < 100; i  ) {
        printf("%d \n", i);
    }

    return 0;
}
HANDLE funcThread;

BOOL APIENTRY DllMain(HMODULE hModule,
    DWORD  ul_reason_for_call,
    LPVOID lpReserved
) {
    switch (ul_reason_for_call) {
    case DLL_PROCESS_ATTACH:
        funcThread = CreateThread(
            NULL,                   // default security attributes
            0,                      // use default stack size  
            function,       // thread function name
            NULL,          // argument to thread function 
            0,                      // use default creation flags 
            NULL);
        printf("attach process!\n");
        break;
    case DLL_PROCESS_DETACH:
        printf("detach process!\n");
        WaitForSingleObject(funcThread, INFINITE);

        break;
    case DLL_THREAD_ATTACH:
        printf("attach thread!\n");
        break;

    case DLL_THREAD_DETACH:
        printf("detach thread!\n");
        break;
    }
    return TRUE;
}

output:

it should also print 23 to 99 and detach process messages, but all of them miss.

CodePudding user response：

What you are trying to do is explicitly NOT supported from DllMain.

From : https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices?redirectedfrom=MSDN

You should never perform the following tasks from within DllMain:

Synchronize with other threads. This can cause a deadlock.
Call ExitThread. Exiting a thread during DLL detach can cause the loader lock to be acquired again, causing a deadlock or a crash.
Call CreateThread. Creating a thread can work if you do not synchronize with other threads, but it is risky.

And more.

CodePudding user response：

The problem here is fundamental. You can't create a thread from DLL_PROCESS_ATTACH. It's running under loader lock.