Lock/Unlock Session by the "computer using rule" in Windows.

lock.cpp

/*
Lock/Unlock Session by the "computer using rule".
Support versions >= Windows 7
https://gist.github.com/taeguk/13b607f42020981f6cf9
*/

#include <windows.h>
#include <stdio.h>
#include <time.h>
#include <Wtsapi32.h>
#include <tchar.h>
#include <locale.h>
#include <string.h>

// constants
#define DEFAULT_LIMIT_COMPUTE_SECONDS	60*40	// 40 min
#define DEFAULT_LIMIT_LOCK_SECONDS		60*10	// 10 min

#pragma comment(lib, "user32.lib")
#pragma comment(lib, "WtsApi32.lib")

bool IsSessionLocked() {
	typedef BOOL(PASCAL * WTSQuerySessionInformation)(HANDLE hServer, DWORD SessionId, WTS_INFO_CLASS WTSInfoClass, LPTSTR* ppBuffer, DWORD* pBytesReturned);
	typedef void (PASCAL * WTSFreeMemory)(PVOID pMemory);

	WTSINFOEXW * pInfo = NULL;
	WTS_INFO_CLASS wtsic = WTSSessionInfoEx;
	bool bRet = false;
	LPTSTR ppBuffer = NULL;
	DWORD dwBytesReturned = 0;
	LONG dwFlags = 0;
	WTSQuerySessionInformation pWTSQuerySessionInformation = NULL;
	WTSFreeMemory pWTSFreeMemory = NULL;

	HMODULE hLib = LoadLibrary(_T("wtsapi32.dll"));
	if (!hLib) {
		return false;
	}
	pWTSQuerySessionInformation = (WTSQuerySessionInformation)GetProcAddress(hLib, "WTSQuerySessionInformationW");
	if (!pWTSQuerySessionInformation) {
		goto EXIT;
	}

	pWTSFreeMemory = (WTSFreeMemory)GetProcAddress(hLib, "WTSFreeMemory");
	if (pWTSFreeMemory == NULL) {
		goto EXIT;
	}

	DWORD dwSessionID;
	if (!ProcessIdToSessionId(GetCurrentProcessId(), &dwSessionID)) {
		goto EXIT;
	}

	if (pWTSQuerySessionInformation(WTS_CURRENT_SERVER_HANDLE, dwSessionID, wtsic, &ppBuffer, &dwBytesReturned)) {
		if (dwBytesReturned > 0) {
			pInfo = (WTSINFOEXW*)ppBuffer;
			if (pInfo->Level == 1) {
				dwFlags = pInfo->Data.WTSInfoExLevel1.SessionFlags;
			}
			if (dwFlags == WTS_SESSIONSTATE_LOCK) {
				bRet = true;
			}
		}
		pWTSFreeMemory(ppBuffer);
		ppBuffer = NULL;
	}
EXIT:
	if (hLib != NULL) {
		FreeLibrary(hLib);
	}
	return bRet;
}

int _tmain(int argc, TCHAR *argv[])
{
	TCHAR buff[22];
	time_t baseTime, curTime;
	time_t LIMIT_COMPUTE_SECONDS = DEFAULT_LIMIT_COMPUTE_SECONDS, 
		LIMIT_LOCK_SECONDS = DEFAULT_LIMIT_LOCK_SECONDS;

	_tsetlocale(LC_ALL, _T(""));

	if (argc >= 3) {
		LIMIT_COMPUTE_SECONDS = _ttoi(argv[1]);
		LIMIT_LOCK_SECONDS = _ttoi(argv[2]);
	}

	HWND hWndConsole = GetConsoleWindow();
	ShowWindow(hWndConsole, SW_HIDE);

	baseTime = time(NULL);
	while (true) {
		curTime = time(NULL);
		if (IsSessionLocked()) {
			baseTime = curTime;
			continue;
		}
		if (curTime - baseTime > LIMIT_COMPUTE_SECONDS) {
			_tcsftime(buff, 20, _T("%Y-%m-%d %H:%M:%S"), localtime(&curTime));
			_tprintf(_T("Lock the computer! [%s]\n"), buff);
			// Lock the workstation.
			if (!LockWorkStation()) {
				_tprintf(_T("LockWorkStation failed with %d\n"), GetLastError());
				return 1;
			}
			time_t lockTime = curTime;
			curTime = time(NULL);
			while (curTime - lockTime < LIMIT_LOCK_SECONDS) {
				if (!IsSessionLocked()) {
					_tcsftime(buff, 20, _T("%Y-%m-%d %H:%M:%S"), localtime(&curTime));
					_tprintf(_T("Current your computer is locked!! [%s]\n"), buff);
					Sleep(500);
					// Lock the workstation.
					if (!LockWorkStation()) {
						_tprintf(_T("LockWorkStation failed with %d\n"), GetLastError());
						return 1;
					}
				}
				Sleep(300);
				curTime = time(NULL);
			}
			_tcsftime(buff, 20, _T("%Y-%m-%d %H:%M:%S"), localtime(&curTime));
			_tprintf(_T("Unlock the computer! [%s]\n"), buff);
			baseTime = curTime;
		}
		Sleep(700);
	}

	return 0;
}

Similar function for linux
https://gist.github.com/juice500ml/ea49ef432b5ae75b208e