c - Struct strange in pure C

Question

I just read a open source. In it, I see a some code, it make me unfamiliar. So it is something about the struct:

struct _NameStruct;

typedef struct _NameStruct NameStruct

And after that, they (open source) use NameStruct to contain some variable. I wonder Why struct _NameStruct not yet define some variable so how they can use it?

Answer 1

如果您仔细阅读代码，您可能会发现它们只使用指向数据类型的指针。它可能是一种不透明的数据类型，因此您可以传递指向它的指针，但实际上不必知道它是如何实现的。

这些通常用于只有库需要知道结构的实现的句柄之类的事情。