[SOLVED] What are the problems with creating a Singleton using an unnamed struct in C?

What are the problems with creating a Singleton using an unnamed struct in C?

I want to create a singleton struct in C and came up with this idea of using an unnamed struct for defining a global variable that we can't create another instance of it because it's struct is unnamed (therefore we have a singleton).

#ifndef __for_including_this_file_once__
#define __for_including_this_file_once__

struct { variables } singleton_strcut;

#endif // __for_including_this_file_once__

This way we can access this instance only by its name anywhere we include this .h file.

What problems might this create, if any (especially in the context of multi-threaded programming), and what workarounds can we have?

Solution

This won't work if you have multiple .c files. If more than one source file includes this header, you'll end up with a multiple definition error when you reach the linker phase.

You would need to put a declaration in the header file and a definition in exactly one source file, and to do that you would need to give the struct a name.