How is the static function/variable protected

Question

I want to know how a static variable or function is protected to be used only for the file it is defined in. I know that such variables and functions are declared in data section (heap area to be precise), but is it tagged with the file name ? Suppose I make a fool of the compiler by assigning such a static function (defined in foo.c) to a global function pointer, and call that function pointer in some other file (bar.c). Obviously my code wont give any compilation warning, but incidentally, it gives segmentation fault. Obviously, it is a protection fault, but I am interested in knowing how it is implemented inside the system.

Thanks. MS

Answer 1

The linker takes care of restricting the scope of mapping the function name to the function.

There is no protection for static functions called by function pointer - it's not that uncommon an idiom. For example, the recommended way of implementing GObject methods is to expose a pointer to a static function (see the virtual public methods section in this GObject how-to)

Answer 2

It is 'protected' simply by not having its symbol/location made known to the linker. So you cannot write code in another module that explicitly references the static object by its symbol name, because the linker has no such symbol. There is no run-time protection.

If you pass an address to a static object to some other module at runtime, then you will then be able to access it through such a pointer. That is not "making a fool of the compiler" (or linker in fact), such action may be entirely legitimate.

The fact that you got a seg-fault is probably for an entirely different reason (an invalid pointer for example). The compiler may choose to in-line the code in which case a pointer to it would not be be possible, but if you explicitly take the address of an object, the compiler should instantiate it, so this seems unlikely.

Answer 3

you "may" get a problem if foo.c and bar.c are compiled into different dynamic loaded libraries.

Answer 4

The purpose of static is not to 'protect' the variable/function but to protect the namespace and protect the rest of your program from having its behavior messed up by symbols with conflicting names. It also allows a good bit more optimization in that the compiler knows it doesn't have to facilitate access to the symbol name by outside modules.