When you try to draw something on the screen, your code calls some other piece of code which calls some other code (etc.) until finally there is a "system call", which is a special instruction that the CPU can execute. These instructions can be either written in assembly or can be written in C++ if the compiler supports their "intrinsics" (which are functions that the compiler handles "specially" by converting them into special code that the CPU can understand). Their job is to tell the operating system to do something.

When a system call happens, a function gets called that calls another function (etc.) until finally the display driver is told to draw something on the screen. At that point, the display driver looks at a particular region in physical memory which is actually not memory, but rather an address range that can be written to as if it were memory. Instead, however, writing to that address range causes the graphics hardware to intercept the memory write, and draw something on the screen. Writing to this region of memory is something that could be coded in C++, since on the software side it's just a regular memory access. It's just that the hardware handles it differently. So that's a really basic explanation of how it can work.

计算机就像一个洋葱，它有很多很多层，从纯硬件的内核到最外层的应用层。每一层都将自己的一部分暴露给下一个外层，这样外层就可以使用内层的一些功能。

以Windows为例，操作系统为运行在Windows上的应用程序公开了所谓的WIN32 API。Qt库使用该API为使用Qt的应用程序提供自己的API。你使用Qt, Qt使用WIN32, WIN32使用较低级别的Windows操作系统，等等，直到它成为硬件中的电信号。

为了提供与其他答案略有不同的观点，我将这样回答。

(免责声明:我只是稍微简化了事情，我给出的情况纯粹是假设的，是作为演示概念的一种手段而写的，而不是100%真实的生活)。

从另一个角度考虑问题，假设您刚刚编写了一个简单的操作系统，具有基本的线程、窗口和内存管理功能。你想要实现一个c++库，让用户用c++编程，做一些事情，比如创建窗口，在窗口上绘图等等。问题是，如何做到这一点。

首先，由于c++编译为机器代码，您需要定义一种使用机器代码与c++进行接口的方法。这就是函数的作用所在，函数接受参数并给出返回值，因此它们提供了在不同代码段之间传输数据的标准方式。他们通过建立一种称为调用约定的东西来做到这一点。

A calling convention states where and how arguments should be placed in memory so that a function can find them when it gets executed. When a function gets called, the calling function places the arguments in memory and then asks the CPU to jump over to the other function, where it does what it does before jumping back to where it was called from. This means that the code being called can be absolutely anything and it will not change how the function is called. In this case however, the code behind the function would be relevant to the operating system and would operate on the operating system's internal state.

所以，几个月后，你已经整理好了所有的操作系统功能。你的用户可以调用函数来创建窗口并在上面绘图，他们可以创建线程和所有美妙的东西。但问题是，你的操作系统的功能将不同于Linux或Windows的功能。所以你决定给用户一个标准的界面，这样他们就可以编写可移植的代码。这就是QT的用武之地。

As you almost certainly know, QT has loads of useful classes and functions for doing the sorts of things that operating systems do, but in a way that appears independent of the underlying operating system. The way this works is that QT provides classes and functions that are uniform in the way they appear to the user, but the code behind the functions is different for each operating system. For example QT's QApplication::closeAllWindows() would actually be calling each operating system's specialised window closing function depending on the version used. In Windows it would most likely call CloseWindow(hwnd) whereas on an os using the X Window System, it would potentially call XDestroyWindow(display,window).

很明显，操作系统有很多层，所有这些层都必须通过各种各样的接口进行交互。有许多方面我还没有谈到，但要全部解释它们将需要很长时间。如果您对操作系统的内部工作原理有进一步的兴趣，我建议您查看OS dev wiki。

请记住，许多操作系统选择向C/ c++公开接口的原因是它们可以编译为机器代码，它们允许将汇编指令与它们自己的代码混合在一起，并且它们为程序员提供了很大程度的自由。

同样，这里有很多内容。我想继续解释库如何像.so和.dll文件不需要用C/ c++编写，可以用汇编或其他语言编写，但我觉得如果我再添加更多，我还不如写一整篇文章，尽管我很想这样做，但我没有一个站点来托管它。

C和c++有两个属性，允许OP所谈论的所有这些可扩展性。

C和c++可以访问内存 C和c++可以调用非C或c++语言中的汇编代码。

在内核或基本的非保护模式平台中，外围设备(如串口或磁盘驱动器)以与RAM相同的方式映射到内存映射中。内存是一系列的开关，翻转外围设备的开关(比如串口或磁盘驱动器)可以让外围设备做一些有用的事情。

在保护模式操作系统中，当用户希望从用户空间访问内核时(比如写入文件系统或在屏幕上绘制像素)，需要进行系统调用。C语言没有指令来进行系统调用，但是C语言可以调用汇编代码来触发正确的系统调用，这就是为什么C代码可以与内核对话。

为了使特定平台的编程更容易，系统调用被包装在更复杂的函数中，这些函数可以在自己的程序中执行一些有用的函数。可以直接调用系统调用(使用汇编程序)，但是使用平台提供的一个包装器函数可能更容易。

还有另一层API比系统调用有用得多。以malloc为例。这不仅会调用系统来获取大块的内存，而且还会通过对正在发生的事情进行所有簿记来管理这些内存。

Win32 api用一个公共平台小部件集包装了一些图形功能。Qt更进一步，以跨平台的方式包装了Win32(或X Windows) API。

从根本上讲，C编译器将C代码转换为机器代码，由于计算机被设计为使用机器代码，所以你应该期望C能够完成计算机所能完成的大部分工作。包装器库所做的一切都是为您做繁重的工作，这样您就不必做了。

语言(如c++ 11)是纸上的规范，通常用英语编写。看看最新的c++ 11草案(或者从ISO供应商那里购买昂贵的最终规范)。

你通常使用带有语言实现的计算机(原则上你可以在没有任何计算机的情况下运行c++程序，例如使用一群人类奴隶来解释它;那样既不道德又低效)

你的c++实现通常工作在某个操作系统之上，并与它通信(使用一些特定于实现的代码，通常在一些系统库中)。通常这种通信是通过系统调用完成的。在instance into syscalls(2)中查找Linux内核上可用的系统调用列表。

从应用程序的角度来看，系统调用是一个基本的机器指令，就像x86-64上的SYSENTER一样，具有一些约定(ABI)。

在我的Linux桌面上，Qt库位于X11客户端库之上，通过X Windows协议与X11服务器Xorg通信。

在Linux上，在可执行文件上使用ldd来查看对库的依赖项的(长)列表。在正在运行的进程上使用pmap查看在运行时“加载”了哪些文件。顺便说一句，在Linux上，你的应用程序可能只使用免费软件，你可以研究它的源代码(从Qt，到Xlib, libc，…内核)以了解更多正在发生的事情

When you try to draw something on the screen, your code calls some other piece of code which calls some other code (etc.) until finally there is a "system call", which is a special instruction that the CPU can execute. These instructions can be either written in assembly or can be written in C++ if the compiler supports their "intrinsics" (which are functions that the compiler handles "specially" by converting them into special code that the CPU can understand). Their job is to tell the operating system to do something.

When a system call happens, a function gets called that calls another function (etc.) until finally the display driver is told to draw something on the screen. At that point, the display driver looks at a particular region in physical memory which is actually not memory, but rather an address range that can be written to as if it were memory. Instead, however, writing to that address range causes the graphics hardware to intercept the memory write, and draw something on the screen. Writing to this region of memory is something that could be coded in C++, since on the software side it's just a regular memory access. It's just that the hardware handles it differently. So that's a really basic explanation of how it can work.