DllMain w/ createthread and printf - c++

Question

I'm trying to build a windows dll using mingw-64 that once loaded starts printing "Hello World" indefinetly.
Here's my dll.c
#include <stdio.h>
#include <windows.h>
#include "dll.h"
#include "main.h"
HINSTANCE hThisModule;
DWORD mainThread() {
while(1) {
printf("Hello world!");
}
}
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved) {
static HANDLE hThread;
hThisModule = hinstDLL;
switch (fdwReason)
{
case DLL_PROCESS_ATTACH:
hThread = CreateThread(0, 0, mainThread, 0, 0, 0);
break;
case DLL_PROCESS_DETACH:
break;
case DLL_THREAD_ATTACH:
break;
case DLL_THREAD_DETACH:
break;
}
return TRUE;
}
void dummy() {
Hello();
}
and here's my dll.h:
#ifndef DLL_H_
#define DLL_H_
#ifdef BUILD_DLL
/* DLL export */
#define EXPORT __declspec(dllexport)
#else
/* EXE import */
#define EXPORT __declspec(dllimport)
#endif
#endif /* DLL_H_ */
so I've built a simple program that loads my DLL to see if it's working correctly, here it is: hello.cpp
#include <windows.h>
#include <iostream>
typedef int (__stdcall *f_funci)();
int main()
{
HINSTANCE hGetProcIDDLL = LoadLibrary("./wow.dll");
if (!hGetProcIDDLL) {
std::cout << "could not load the dynamic library" << std::endl;
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Now, when I compile hello.cpp into hello.exe and dll.c into wow.dll, I get nothing on my console. What's wrong?

Answer 1

As has been already mentioned, your mainThread function has wrong signature. Try something like this:
DWORD WINAPI mainThread(LPVOID lpParam)
{
UNREFERENCED_PARAMETER(lpParam);
while (1)
{
printf("Hello world!\n");
Sleep(1000);
}
return 0;
}
This works just fine for me. I modified your .exe so that you could drag and drop .dll onto it to test:
#include <windows.h>
#include <iostream>
int main(int argc, char *argv[])
{
if (argc < 2)
{
std::cout << "drag drop dll over exe" << std::endl;
std::cin.get();
return EXIT_FAILURE;
}
HINSTANCE hGetProcIDDLL = LoadLibraryA(argv[1]);
if (!hGetProcIDDLL)
{
std::cout << "could not load the dynamic library" << std::endl;
std::cin.get();
return EXIT_FAILURE;
}
std::cin.get();
return EXIT_SUCCESS;
}

Answer 2

First, I'd like to mention that it is not advisable to implement such a busy loop in your thread.
As for the issue you are experiencing, there are several potential issues here:
printf is a CRT function, however you are calling CreateThread() instead of beginthread(ex), so the CRT is not initialized properly.
Dll entry point is a notoriously problematic place. You can hardly call any kernel32 function from there, let alone CRT ones (see DllMain entry point and Dynamic Link Library Best Practices).
In most cases, it is advisable to implement separate Init and Exit functions that the client will need to call when using your library.