Injected DLL Main loop crashes the process - c++

Question

I'm currently trying to inject a DLL into a project, but everytime i use a while loop the process crashes.
This is the code:
BOOL APIENTRY DllMain( HMODULE hModule, DWORD ul_reason_for_call,LPVOID lpReserved ){
switch(ul_reason_for_call){
case DLL_PROCESS_ATTACH:
while(char c = getch()){
if(c == 'p'){
MessageBox(0,L"CAPT",L"CAPT",MB_OK);
}
}
break;
}
return TRUE;
}

Answer 1

Since you changed your original question, I'll rearrange my answer a bit...
There are significant limits on what you can safely do in a DLL entry point.
Please read carefully through the remaks on the following page:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms682583(v=vs.85).aspx
and
https://msdn.microsoft.com/en-us/library/windows/desktop/aa370448(v=vs.85).aspx
For example, it shouldn't take more than 300ms (recommended time) during the DLL_PROCESS_ATTACH. You're also not returning anything (should be a boolean).
However, I think the main reason your software hangs is because MessageBox and/or getch() probably creates a deadlock there. Regarding the MessageBox use OutputDebugString instead, refer to this answer: https://stackoverflow.com/a/10981735/5874704
Also as suggested in comments:
Don't put the while loop in DllMain. Use CreateThread in DllMain to launch a new thread. Put the while loop there
Previously you also asked about the definition of the DLLMain. This is the "bare"-function of the DllMain:
BOOL WINAPI DllMain(
HINSTANCE hDllHandle,
DWORD nReason,
LPVOID Reserved)
{
BOOL bSuccess = TRUE;
switch ( nReason )
{
case DLL_PROCESS_ATTACH:
break;
case DLL_PROCESS_DETACH:
break;
case DLL_THREAD_ATTACH:
break;
case DLL_THREAD_DETACH:
break;
}
return bSuccess;
}