我一直在阅读有关链接器如何工作以及围绕此过程的所有内容，以便在此（链接）问题中解释我的问题（很短，对参考感到抱歉，但它是相关的）。
问题： 如果在标题中我有一个变量声明（使用 extern），并且我想在多个源文件中使用这个变量（#包括每个课程中的头文件），我必须为此提供一个定义某处变量。问题是，如果我在某些源文件的主函数中提供定义，例如，其他文件仍然没有“看到”这个定义（这会导致链接错误）。如果我在其中一个全局范围内定义这个外部变量，它就可以正常工作，并且每个人都可以看到它。
为什么？其他文件如何访问另一个文件的全局范围？全局范围变量不是危险的吗？将变量定义放在其他什么范围内可以解决这样的链接错误？有人在标题中使用这样的外部变量声明吗？出于什么目的？
编辑：确切的示例情况在我一开始发布的链接中
A few clarifications:
What you're calling a "file" is more properly called a translation unit. It doesn't matter how many files are involved. Indeed, the same file could be re-compiled (say with different #defines, etc.)
Armed with that term, we can now rephrase your question: "How can a translation unit access the global scope of another translation unit"?
Quoting from the wikipedia article on Linkage:
If the name has external linkage, the entity that name denotes may be referred to from another translation unit using a distinct declaration for that same name, and from other scopes within the same translation unit using distinct declarations.
In other words: by making sure that name as external linkage, it's in the global scope, and that's global for the program, not the translation unit.
Your remaining questions:
Aren't global scope variables dangerous?
I would never use the word "dangerous" to describe them, as I would certain standard library functions that have security flaws, etc. But I would say that can very often lead to very poorly designed code, and in the case of global variables specifically, a lot of headaches in multithreaded code. A good rule: avoid them unless you have a good reason not to.
How to avoid the linkage error (in some other scope)
No other magic declaration here: it's either visible outside the translation unit (external) or not (static). What you're probably asking for is : "What's the right way to let other modules access the variable?" And the answer is: write an accessor function (or even better, a class that manages that data with methods).
Who uses this stuff and for what purpose?
Always a hotly debated topic. Many people advise avoiding completely, but you'd be hard pressed not to find an extern in a project of sufficient size. The best reasons usually come down to the kind of things purists ignore: compatibility with legacy systems, performance, easy access for debuggers and other introspecting tools… or, of course, just cause it was easy, it was understood, and it just worked.
当您放置一个变量时in the main function，您定义了一个在该函数中可见的局部变量。
您必须在一个源文件的文件范围内定义变量。