is_main_thread.cpp

#ifdef _MSC_VER
#define NOMINMAX
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <tlhelp32.h>
#else
#include <pthread.h>
#endif
#include <memory>

int is_main_thread()
{
#ifdef _MSC_VER
	DWORD pid = GetCurrentProcessId();
	FILETIME now;
	SYSTEMTIME st;
	GetSystemTime(&st);
	if (!SystemTimeToFileTime(&st, &now)) {
		return -1;
	}
	FILETIME oldest = now;
	DWORD oldest_tid = 0;

	std::shared_ptr<void> h(CreateToolhelp32Snapshot(TH32CS_SNAPTHREAD, 0), CloseHandle);
	if (h.get() == INVALID_HANDLE_VALUE) {
		return -1;
	}
	THREADENTRY32 te;
	te.dwSize = sizeof(te);
	if (!Thread32First(h.get(), &te)) {
		return -1;
	}
	do {
		if (te.dwSize < FIELD_OFFSET(THREADENTRY32, th32OwnerProcessID) + sizeof(te.th32OwnerProcessID)) {
			continue;
		}
		if (te.th32OwnerProcessID != pid) {
			continue;
		}
		std::shared_ptr<void> th(OpenThread(THREAD_QUERY_LIMITED_INFORMATION, FALSE, te.th32ThreadID), CloseHandle);
		if (th.get() == INVALID_HANDLE_VALUE) {
			return -1;
		}
		FILETIME creation, e, k, u;
		if (!GetThreadTimes(th.get(), &creation, &e, &k, &u)) {
			return -1;
		}
		if (oldest_tid == 0) {
			oldest = creation;
			oldest_tid = te.th32ThreadID;
		} else if (CompareFileTime(&oldest, &creation) >= 0) {
			oldest = creation;
			oldest_tid = te.th32ThreadID;
		}
	} while((te.dwSize = sizeof(te)) && Thread32Next(h.get(), &te));

	if (oldest_tid == 0) {
		return -1;
	}

	if (oldest_tid == GetCurrentThreadId()) {
		return 1;
	} else {
		return 0;
	}
#else
	if (pthread_main_np() != 0) {
		return 1;
	} else {
		return 0;
	}
#endif
}