About header files

Hi,

I have some questions about header files:

Say I have a file functions.c which contains a couple of functions. I have declared some structs in this file too. The structs are defined in main.c. Now I create a header file which represents the interface of functions.c to my main program file main.c.
I put in the header file: all function prototypes with keyword extern and the declarations of the structs, which are defined in the main program.

Do I include the header file functions.h in my file functions.c to get the struct declarations or do I explicitely declare the structs in functions.c and not include the header file?

About standard libraries: functions.c uses stdio.h, stdlib.h and string.h. Thus, I include the header files. main.c also uses stdlib.c and stdio.h. Do I include them again or do I use inclusion guards to check if they are already included (e.g. in functions.c)?

If a variable is declared static, is it still legal to pass it on as a function argument to a function outside of the file? In my case I have declared a struct in my file functions.c, "exported" it via functions.h to main.c and defined a variable with it as static. main.c calls a function from functions.c using this statically defined variable. It works, but why? Isnt the scope of the variable limited to main.c?

The keyword extern in front of a function prototype in my header file says that the function is defined somewhere else, right? Do I HAVE to declare the function prototype as extern in the header file? If I remove the extern my program still compiles flawlessly.

Thanks,
Chris
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