C/C++ 运行时从何而来？

Question

I recently read that C/C++ runtime in windows has 2 parts. First is VC Runtime and other is Universal C Runtime. As per my reading, Universal C runtime is part of the OS.

Question 1: So does it comes with OS installation by default? Or has the Universal C Runtime to do something with Windows SDK ie Windows SDK install provides UCRT?

Question 2: If the UCRT is part of OS, then what supplies the VC Runtime?

Also, I read that platform toolset provides compiler, linker and standard libraries.

Question 3: So platform toolset has nothing to do with C/C++ runtime (which is split in 2 parts)? Can we say that platform toolset and C/C++ runtime don't come as single unit on windows OS?

Please clarify.

Answer 1

Universal C runtime comes with the OS on Windows 10 (it's installed when you install the OS). On older Windows versions (Vista through 8.1), it is distributed through Windows Update. It can also be locally deployed with your app.

If your app supports an OS older than Windows 10, you need to think how you're going to distribute it. You can either include a copy of it with your app (this is the most common solution) or you can require the Windows Update for it to be installed.

You can read more about it here: https://docs.microsoft.com/en-us/cpp/windows/universal-crt-deployment?view=vs-2019

VC Runtime can be supplied in two ways: through Visual C++ redistributable packages installer, or deployed locally with your app. If you ship an installer with your app, you can have your installer also install VC Runtime packages. If you ship software that doesn't get installed, you will have to drop VC Runtime DLLs next to your application executable. You can read more about deploying with VC Runtime here: https://docs.microsoft.com/en-us/cpp/windows/redistributing-visual-cpp-files?view=vs-2019

Also, I read that platform toolset provides compiler, linker and standard libraries.

This is not entirely correct. It provides standard C++ libraries and runtime (also known as VC Runtime). Standard C libraries and runtime come from Universal C Runtime.

Answer 2

"Universal C Runtime" is, at best, peculiar terminology.

The C compiler comes with a standard library that you link into your code. The standard library provides all of the functions required by the C standard. It's up to the library implementor to figure out how to implement them.

The operating system provides services that are useful to programs: memory management, file system, display screen, keyboard input, mouse, whatever. It has its own interface, and code in the standard library uses it as appropriate.

Different operating systems have different ways of providing these services, so the standard library has to be written differently for different operating systems. But calls to the operating system are not necessarily written in C, which is why "Universal C Runtime" isn't really appropriate. For example, back in the olden days of MSDOS, calling the operating system meant setting some values in processor registers and then generating a software interrupt. That's generally not C code, although some runtime libraries provided a code layer so that you could do that without having to write your own assembly code.