How to execute threads in a DLL in C - c

Question

I created a DLL that is running 3 worker threads, and the main thread is in a loop waiting for the threads to complete. The threads get created, but no execution to the threads is done.
I have tried setting MessageBox functions inside the function that gets created with CreateThread() but the box does not appear. I have also tried to debug and the return value from CreateThread() is valid so the thread gets created.
BOOL WINAPI DllMain() {
main();
return 1;
}
int main() {
HANDLE h1, h2, h3;
h1 = CreateThread(first)...
h2 = CreateThread(second)...
h3 = CreateThread(third)...
WaitForSingleObject(h3, INFINITE);
return 1;
}
first() {
MessageBoxA("print some stuff");
return;
}
I have included some pseudocode of what my layout looks like. I am unable to provide the real code due to the sensitivity of it. However this is what is happening. I use LoadLibrary in another project that loads this .DLL. The DLL gets loaded and DllMain is executed. It then calls my main function which creates 3 threads. Each thread is created. But what is inside of the thread does not execute.
EDIT:
// dllmain.cpp : Defines the entry point for the DLL application.
#include <Windows.h>
void mb() {
MessageBoxW(0, L"AAAAAAAAAAAAAAAAAAAAAAAAA", L"AAAAAAAAAAAAAAAAAAAAAAa", 1);
}
void create() {
HANDLE han;
DWORD threadId;
han = CreateThread(NULL, 0, mb, NULL, 0, &threadId);
han = CreateThread(NULL, 0, mb, NULL, 0, &threadId);
han = CreateThread(NULL, 0, mb, NULL, 0, &threadId);
}
BOOL APIENTRY DllMain() {
create();
return 1;
}

Answer 1

[MS.Docs]: DllMain entry point (emphasis is mine) states:
Calling functions that require DLLs other than Kernel32.dll may result in problems that are difficult to diagnose. For example, calling User, Shell, and COM functions can cause access violation errors, because some functions load other system components. Conversely, calling functions such as these during termination can cause access violation errors because the corresponding component may already have been unloaded or uninitialized.
[MS.Docs]: MessageBox function resides in User32.dll, so it's Undefined Behavior (meaning that in different scenarios, it might work, it might work faulty, or it might crash).
Also, as #RbMm noticed, WaitForSingleObject doesn't belong there. I'm not sure about CreateThread either (but I couldn't find any official doc to confirm / infirm it).
Just out of curiosity, could you add a printf("main called.\n"); in main, to see how many times it is called?

Answer 2

because in general case DLL can be unloaded, need add reference to DLL - for it will be not unloaded until thread, which use it code executed. this can be done by call GetModuleHandleEx - increments the module's reference count unless GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT is specified. when thread exit - we dereference DLL code by call FreeLibraryAndExitThread. wait for all threads exit in most case not need. so code, inside dll can be next
ULONG WINAPI DemoThread(void*)
{
MessageBoxW(0, L"text", L"caption", MB_OK);
// dereference dlll and exit thread
FreeLibraryAndExitThread((HMODULE)&__ImageBase, 0);
}
void someFnInDll()
{
HMODULE hmod;
// add reference to dll, because thread will be use it
if (GetModuleHandleExW(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS, (PCWSTR)&__ImageBase, &hmod))
{
if (HANDLE hThread = CreateThread(0, 0, DemoThread, 0, 0, 0))
{
CloseHandle(hThread);
}
else
{
// dereference dll if thread create fail
FreeLibrary(hmod);
}
}
}
wait inside dll entry point is wrong, because we hold here process wide critical section. if we wait for thread exit - this wait always deadlock - thread before exit (and start) try enter this critical section, but can not because we wait for him here. so we hold critical section and wait for thread(s), but threads(s) wait when we exit from this critical section. deadlock