Monitor for last input time change in Windows.

lastinput.cpp

/* Monitor for last input time change in Windows.

Usage: lastinput

g++ -o lastinput lastinput.cpp

* Copyright (C) 2026 Jay Satiro <raysatiro@yahoo.com>

https://gist.github.com/jay/09d422a019b778fa63bf8fab75aafd26
*/

#define _WIN32_WINNT 0x0501

#include <windows.h>
#include <stdio.h>
#include <time.h>

#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>

using namespace std;

/* system time in format: Tue May 16 03:24:31.123 PM */
string SystemTimeStr(const SYSTEMTIME *t)
{
  const char *dow[] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };
  const char *mon[] = { NULL, "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                        "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
  stringstream ss;

  unsigned t_12hr = (t->wHour > 12 ? t->wHour - 12 : t->wHour ? t->wHour : 12);
  const char *t_ampm = (t->wHour < 12 ? "AM" : "PM");

  ss.fill('0');
  ss << dow[t->wDayOfWeek] << " "
     << mon[t->wMonth] << " "
     << setw(2) << t->wDay << " "
     << setw(2) << t_12hr << ":"
     << setw(2) << t->wMinute << ":"
     << setw(2) << t->wSecond 
     // << "." << setw(3) << t->wMilliseconds
     << " "
     << t_ampm;

  return ss.str();
}

string now()
{
  SYSTEMTIME st;
  GetLocalTime(&st);
  return SystemTimeStr(&st);
}

/* The timestamp style in default mode: [Sun May 28 07:00:27.999 PM]: text */
#define TIMESTAMP \
  "[" << now() << "]: "

int main(int argc, char *argv[])
{
  LASTINPUTINFO li = { sizeof(LASTINPUTINFO), };
  DWORD prev_time_ms;

  cout << "Monitoring for last input time change..." << endl;
  
  if(GetLastInputInfo(&li)) {
    prev_time_ms = li.dwTime;
  }
  else {
    cerr << "GetLastInputInfo failed." << endl;
    return 1;
  }
  
  for(;;) {
    Sleep(1000);
    if(GetLastInputInfo(&li)) {
      if(li.dwTime != prev_time_ms) {
        DWORD diff = li.dwTime - prev_time_ms;
        DWORD seconds = diff / 1000;
        if(diff >= 1000) {
          cout << TIMESTAMP << "Input detected. "
               << "Last input was " << seconds
               << " second" << ((seconds > 1) ? "s" : "") << " ago."
               << endl;
        }
        prev_time_ms = li.dwTime;
      }
    }
  }
}