In JavaScript, if you want to use a function that was introduced only in certain versions of browsers, you use Feature Detection. For example, you can ask “Hey, browser, do you have a function called `includes` on Array?” If the browser has it, you use it; and if it doesn’t, you either get along without it or load your own implementation.

It turns out that this same concept can be (and, in Firefox, is) done with Windows APIs.

Firefox for Windows is built against the Windows 10 SDK. This means the compiler knows the API calls and type definitions for all sorts of wondrous modern features like toast notifications and enumerating graphics adapters in a specific order.

However, as of writing, Firefox for Windows supports Windows 7 and up. What would happen if Firefox tried to use those fancy new Windows 10 features when running on Windows 7?

Well, at compile time (when Mozilla builds Firefox), it knows everything it needs to about the sizes and names of things used in the new features thanks to the SDK. At runtime (when a user runs Firefox), it needs to ask Windows at what address exactly all of those fancy new features live so that it can use them.

If Firefox can’t find a feature it expects to be there, it won’t start. We want Firefox to start, though, and we want to use the new features when available. So how do we both use the new feature (if it’s there) and not (if it’s not)?

Windows provides an API called GetProcAddress that allows the running program to perform some Feature Detection. It is asking Windows “Hey, so I’m looking for the address of this fancy new feature named FancyNewAPI. Do you know where that is?”. Windows will either reply “No, sorry” at which point you work around it, or “Yes, it’s over at address X” at which point to convert address X into a function pointer that takes the same number and types of arguments that the documentation said it takes and then instruct your program to jump into it and start executing.

We use this in Firefox to detect gamepad input modules, cancelable synchronous IO, display density measurements, and a whole bunch of graphics and media acceleration stuff.

And today (well, yesterday at this point) I learned about it. And now so have you.

:chutten

–edited to remove incorrect note that GetProcAddress started in WinXP– :aklotz noted that GetProcAddress has been around since ancient times, MSDN just periodically updates its “Minimum Supported Release” fields to drop older versions.