monitor active window and its pid using X11 lib

monitor_active.cpp

// monitor active window and its pid using X11 lib
// compile hint: install xlib-dev and then use g++ ./xxx.cpp -lX11
#include <X11/Xlib.h>
#include <cstring>
#include <iostream>
#define MAXSTR 1000
using namespace std;

Display *display;
unsigned char *prop;

void check_status(int status, Window window)
{
    if (status == BadWindow)
    {
        printf("window id # 0x%lx does not exists!\n", window);
    }

    if (status != Success)
    {
        printf("XGetWindowProperty failed!\n");
    }
}

unsigned char *get_string_property(const char *property_name, Window window)
{
    if (window == 0)
        return 0;
    Atom actual_type, filter_atom;
    int actual_format, status;
    unsigned long nitems, bytes_after;

    filter_atom = XInternAtom(display, property_name, True);
    status = XGetWindowProperty(display, window, filter_atom, 0, MAXSTR, False, AnyPropertyType,
                                &actual_type, &actual_format, &nitems, &bytes_after, &prop);
    check_status(status, window);
    return prop;
}

unsigned long get_long_property(const char *property_name, Window window)
{
    if (window == 0)
        return 0;
    get_string_property(property_name, window);
    unsigned long long_property = static_cast<unsigned long>(prop[0] + (prop[1] << 8) + (prop[2] << 16) + (prop[3] << 24));
    return long_property;
}

unsigned long getActiveWindowPID(Window root_window)
{
    unsigned long window;
    window = get_long_property("_NET_ACTIVE_WINDOW", root_window);
    return get_long_property(("_NET_WM_PID"), window);
}

char *getActiveWindowName(Window root_window)
{
    unsigned long window;
    window = get_long_property("_NET_ACTIVE_WINDOW", root_window);
    get_string_property("_NET_WM_NAME", window);
    return reinterpret_cast<char *>(prop);
}

int main()
{
    Display *d;
    Window w;
    XEvent e;

    d = XOpenDisplay(0);
    if (!d)
    {
        cerr << "Could not open display" << endl;
        return 1;
    }
    display = d;

    w = DefaultRootWindow(d);
    XSelectInput(d, w, PropertyChangeMask);

    pid_t window_pid;

    window_pid = getActiveWindowPID(w);
    if (window_pid != 0)
    {
        cout << "Active Window Name: " << getActiveWindowName(w);
        cout << " , PID: " << window_pid << endl;
    }

    for (;;)
    {
        XNextEvent(d, &e);
        if (e.type == PropertyNotify)
        {
            if (!strcmp(XGetAtomName(d, e.xproperty.atom), "_NET_ACTIVE_WINDOW"))
            {
                window_pid = getActiveWindowPID(w);
                if (window_pid != 0)
                {
                    cout << "Active Window Name: " << getActiveWindowName(w);
                    cout << " , PID: " << window_pid << endl;
                }
            }
        }
    }

    return 0;
}