Welcome in the forum!
Sounds a bit like a typo, but that's a guess. Best will be to post your complete project. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
Are you sure you did not include the header in more than one source file?
Why are you placing a static function declaraction into a header?
Typically a header is used by multiple source files so they know that there is a suitably named function that they can use. (Often they have an 'extern' part of their declaration as well.)
However 'static' tells the C compiler that this funciton is not visible outside the source file.
Therefore, if you are using the header in a source file that does NOT define the function, then the compiler will complain.
First of all, thank you for welcome; I read all your posts and I find the last very interesting, in fact I did not realize this issue...now I'll declare my static function only in the source file. Thank you very much for all ;)