Implementing WM_QUERYENDSESSION - c++

Question

I've just started programming in C++ and I have a lot of fun fiddeling around. But right now I have hit a brick wall for me: the WM_QUERYENDSESSION. I have no idea how to implement it. Let me explain how my code should work.
I have a txt document where I write down the time in days, hours, minutes and seconds. My program reads this txt and converts them into seconds. If the seconds are negative, a message box will appear. If they are positive, my program will get the current system time. When the user decides to shut down the PC, my program should get the current system time again and calculate the elapsed-seconds from start to end. The program now calculates the difference between the seconds from my txt document and the elapsed-seconds, converting this back to my days, hours, minutes, seconds format and re-write them back into my txt document.
I tested my code with a manual stop, just worked fine. I just have to implement the WM_QUERYENDSESSION. I thought about a while-loop with a variable that's true in the beginning and in this loop my program should check if the lParam (I think) is 0. If it is, set the loop variable to false, run the rest of the program and close it.
I hope, some of you can help me and explain how it works and what something does.
Note: Some codelines are going to be deleted. They were just to see if he read the text file the way i wanted it to for example.
I already searched for examples for that but i never understood exactly how to do it. The Microsoft website didn't help me aswell
#include <iostream>
#include <ctime>
#include <string>
#include <cmath>
#include <fstream>
#include <chrono>
#include <windows.h>
using namespace std;
double k; //Get leftover time from txt doc//
int s; //Seconds//
int m; //Minutes//
int h; //Hours//
int d; //Days//
int t; //Var containing new leftover time//
bool z = true; //Loop variable//
int main()
{
ifstream Check ("Test.txt");
Check >> d; //Writing days from txt doc to d//
Check >> h; //Writing hours from txt doc to h//
Check >> m; //Writing minutes from txt doc to m//
Check >> s; //Writing seconds from txt doc to s//
cout << d << "\n" << h << "\n" << m << "\n" << s << "\n";
//Display leftover time; will be deleted//
k = (s + 60*m + 3600*h + 24*3600*d);
//Converting days, hours, minutes and seconds into seconds//
cout << k << "\n";
//Displaying k; will be deleted//
if (k <= 0) //Show messagebox when k <= 0//
{
MessageBox(0,
"Textmessage",
"Texttitle",
MB_OK | MB_ICONEXCLAMATION); //Support MessageBox//
}
auto start = std::chrono::system_clock::now();
//Get current time starting the program//
while (z = true)
{
if () {
}
};
auto end = std::chrono::system_clock::now();
//Get current time ending the program//
std::chrono::duration<double> elapsed_seconds = end-start;
//Calculate difference between start and end//
t = (k-round(elapsed_seconds.count()));
//Calculate time difference in seconds//
//Converting seconds into days, hours, minutes, seconds//
d = t / (24*3600);
h = (t / 3600) % 24;
m = (t / 60) % 60;
s = t % 60;
//End of converting//
ofstream Write ("Test.txt");
Write << d << "\n" << h << "\n" << m << "\n" << s;
//Write all values back to txt doc//
}

Answer 1

Since you don't have a window, or a window handle, or a message pump for that window, you should use the console functions. In particular, you should register a handler with SetConsoleCtrlHandler to receive CTRL_CLOSE_EVENT.

Answer 2

WM_QUERYENDSESSION is a "Windows Message", part of Win32 API.
To receive and process such messages you need to put a message loop in your program.
This loop should be put after all your initialization code, because it will not end until the user exits your program.
Usually, the loop just dispatches the message to GUI windows, and each window has a function attached that handles relevant messages.
This is what is shown in the example in the link.
Your program is different, because you do not have any windows, and only run in background.
So your loop should look something like this:
// Start the message loop.
MSG msg;
BOOL bRet;
while( (bRet = GetMessage( &msg, NULL, WM_QUERYENDSESSION, WM_QUERYENDSESSION )) != 0)
{
if (bRet == -1)
{
// handle the error and possibly exit
}
else
{
DispatchMessage(&msg); //just in case
//if you want to check that user is logging off:
if (msg.lParam & ENDSESSION_LOGOFF > 0) {
//do the time thing you want, then exit the program
}
}
}
Your program will only look for one message and handle it directly in the loop.
I left the dispatch function in because I could not find any documentation about what would happen if a message is retrieved and not dispatched.
You can try removing it, but it may cause issues when the user tries to shutdown the computer, because the OS needs all apps to return non-zero value (TRUE) for this message to close cleanly.
EDIT: The lParam of the message is a bit mask, so should not be compared to possible values directly (it is in the doc, but easy to miss).
Also, since your application does not have a permanent window on screen, it may be a better choice to use a different method to detect shutdown, such as this:
BOOL WINAPI ShutdownHandler(DWORD dwCtrlType) {
//you can delete the second condition if you only care about shutdown
if (dwCtrlType == CTRL_SHUTDOWN_EVENT || dwCtrlType == CTRL_LOGOFF_EVENT) {
//do the date and time stuff
return TRUE;
}
return FALSE;
}
int WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nShowCmd) {
//you code to read the file
BOOL res = SetConsoleCtrlHandler(ShutdownHandler, TRUE);
if (res) {
while (true) { Sleep(10000); } //note your app will never exit like this
//maybe add some more logic to allow user to shut it down
} else {
//handle the error, you will not get shutdown notifications in this case!
}
}
Another option is to turn your app in to a service and then use RegisterServiceCtrlHandlerEx to get shutdown notifications.