Prevent DLL from being injected twice - c++

Question

Im making a DLL in c++ that is injected into another program.
When injecting more than once the program crashes so I want the DLL to first check if it was already injected and if so it will do some code like showing a message box or just quitting

Answer 1

This can be done using EnumProcessModules
BOOL isInjected() {
const char ProcessName = "Process.exe";
std::string DllName = "MyDll.dll";
DWORD dwProcessId = NULL;
HANDLE hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
if (hSnapshot == INVALID_HANDLE_VALUE)
return false;
PROCESSENTRY32 pe{};
pe.dwSize = sizeof(PROCESSENTRY32);
if (Process32First(hSnapshot, &pe))
{
do
{
if (_tcsicmp(pe.szExeFile, _T(ProcessName)) == 0)
{
dwProcessId = pe.th32ProcessID;
break;
}
} while (Process32Next(hSnapshot, &pe));
}
CloseHandle(hSnapshot);
HANDLE hProcess = OpenProcess(PROCESS_QUERY_INFORMATION | PROCESS_VM_READ, FALSE, dwProcessId);
if (hProcess == NULL)
{
return false;
}
HMODULE hMods[1024];
DWORD cbNeeded;
if (EnumProcessModules(hProcess, hMods, sizeof(hMods), &cbNeeded))
{
int amount = 0;
for (unsigned int i = 0; i < (cbNeeded / sizeof(HMODULE)); i++)
{
TCHAR szModName[MAX_PATH];
if (GetModuleFileNameEx(hProcess, hMods[i], szModName, sizeof(szModName) / sizeof(TCHAR)))
{
std::string str1 = szModName;
if (str1.find(DllName) != std::string::npos) amount++;
}
}
if (amount >= 2) return true;
}
return false;
}