Could using ALT + F4 to close a game corrupt Save data?

Anything that interrupts the program while it is writing can corrupt the save file. If you want to avoid corrupting saves, use the game's built-in sequence to exit the program.

ALT+F4 is generally ok to shut down a game. The main issue is that doing so will tell the game that you would like it to shut down and it depends on how the programmer decided to handle when a user presses ALT+F4 when the game hasn't been saved.

This isn't really an issue for game that you save yourself by pausing and selecting "save" from the menu. But some games have "autosaving". They will automatically save your game as you're playing. If you ALT-F4 during one of those save processes you very well may corrupt your save data if the game was programmed badly.

If you add ask it to quit the game while it is saving it technically should finish saving before going through with your command to quit but there are (not often but it happens) times when a game has been programmed so badly it might use multithreading to save while also processing your quit command.

Usually games that have "autosave" will have an icon that appears in the corner of the screen somewhere to let you know it's saving. Most loading screens will tell you "Do not turn off or exit the game when you see this icon".

Here's a couple example of games telling you about their autosave icons:

TL;DR: It's generally OK to use ALT+F4 to close a game. There are some unlikely but possible times it could possibly corrupt save data but it probably won't. If you want to be safe, use the in-game menu to quit the game.

If the application crashes when it quits (which is a known issue in Undertale, Cities: Skylines, Dragon Age: Origins, and lots of other games), then you should not quit it in the middle of saving. @MasonWheeler's answer assumes the game does not crash. Crashing aborts everything, terminates all the threads, and is generally a very abrupt way of exiting a program.

While it's possible that all the writes have already been handed off to the kernel (and will be completed automatically), it's also possible that the game performs multiple writes (e.g. Minecraft actually uses multiple save files and must be using a multiple-write technique), in which case an interruption at the wrong time might leave only some of those writes complete. Furthermore, Linux (and possibly Mac OS X, I have not investigated) does not have real asynchronous writes (the aio_* functions are actually just synchronous writes running on top of a userspace thread pool), meaning it isn't really possible to "hand writes off to the kernel" in the first place on that platform.

Since you cannot know that your game doesn't have a crashing bug, you should not Alt+F4 at all unless you like to live dangerously.

According to Microsoft's Guidelines for Keyboard user Inerface, Alt + F4 should "Close the active item, or quit the active application."

On application level, that should dispacth a WM_QUIT message. For applications with unsaved or dirty "documents" open, this again should present the user with a message asking whether or not to save the changes. Hence if the programmers cared to abide to all those specifications, Alt + F4 should ask you whether you want to save the game state. However, not everybody cares for specs, so it might be safe on some games, and not safe on some.

However there is a thing that worries me; while exiting a game using alt+F4 in Windows, could that cause the save game file to be corrupted, especially during the autosave?

The short-answer

It depends on the game, and I have found that ALT+F4 is generally a safe way of closing down a game. Corruption is never intended so is better explained by bugs or poor code design.

The long-answer:

According to Microsoft's official documentation:

The user can close an application window by clicking the Close button, or by using a keyboard shortcut such as ALT+F4. Any of these actions causes the window to receive a WM_CLOSE message. The WM_CLOSE message gives you an opportunity to prompt the user before closing the window.

Therefore, using ALT+F4 is subject to how the game has been programmed to handle WM_CLOSE messages. However, there are other factors to be aware of:

• What other processes are running (e.g. auto-save)
• Code quality (i.e. potential for bugs)

From personal experience, I have never seen game corruption when using ALT+F4 mid-game. The only affects I have noticed is that you revert to your last saved checkpoint/save file (depends on the game). I actually use ALT+F4 often, preferring it to clicking around for 2 minutes trying to exit a game through menus.

You should find that any game that offers a save function will make a copy of the game world at a specific point. As long as there is an ability to load the save (normally through a Load option in the menu, but can also apply to automatically placing you at your last saved checkpoint in some games) then you should be confident that ALT+F4 mid-game is safe. In fact, you find games that offer to save when using ALT+F4.

However, ALT+F4 during the save process may carry a risk. The code would need to handle this scenario specifically. It is a case of basically allowing the auto-save functionality to finish running once a WM_CLOSE message is received.

I say basically, but it can be tricky getting the code right. Even more so if you factor in that the game is probably using threads. The threads critical to the saving of the game must be allowed to complete before closing the application. Different versions of Windows also have subtle differences in how they implement threads (as does the library that you use, even C++ Standard Library changes between versions).

When it comes to threading, so many things can go wrong. Buggy code and poor design are two factors. It is also possible that slightly different things may occur depending on when (during the process) you press ALT+F4. You may get different behaviour if you press it before ANY save file has been written.

A lot of games that offer auto-saving have a warning message telling you to not power off the machine during an auto-save. I would never consider ALT+F4 is a good idea mid-save. It may be safe, but there is no guarantee. Technically, the warning messages are talking about scenarios where a save file may be partially written because power was lost mid-process, but I like to be cautious. Losing hours that you put into a game is very frustrating. Why risk it?

One other point to consider is the destination of save "files". More and more games are now moving away from local files and onto Cloud based services and other web services. ALT+F4 during such a saving process could potentially partially write the data (and therefore create a corruption).