Just wonder if I can put that in a header file and use it.

Your syntax is not correct. Are you trying to do:

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Your right mb that's what I meant but will it work and compile right? i want both structs to be accessed from each other.

Why can't you test this yourself?

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I'm not sure even if it works if it's a legal statement and maybe something might be wrong.

There is an important fact behind it: on the platforms where you can use C, every structure-pointer has the same size (often 32 or 64 bit), so the compiler can handle declarations like `struct NotDefinedYet *ptr` simply reserving space for a structure-pointer.
(Though you won't be able to use `sizeof *ptr` or `ptr->field` until you have defined the struct.)

https://stackoverflow.com/questions/...x-and#18658591

The “C” language still does not have a strong notion of what we today consider “types.” It is a low-level language: a mere (but, very important…) step above “assembler.” It is always, at its essence, “talking to the hardware.” (Witness the Linux kernel “/arch” subdirectory.)

Contrast with: “C++.” Which, in its original incarnation, was a macro preprocessor.

For any modern “C-style” projects, I invite you to consider “C++.” Unless you are directly concerned with hardware, go ahead and take advantage of the goodness. Notions of “macro preprocessors” are long gone, and the (“gcc …”) language-stack now handles both source languages directly. (Yay!)

Of course, “C++ interfaces” to “plain-C” code (and thence to “asm”) are also well-defined by necessity.