This is not possible.
It cannot be done.
It is beyond the grasp of mortals.
Well, unless the C code uses embedded assembly. But even there it's a little iffy.
Note that a decompiler does not "convert assembly to C". A decompiler attempts to approximate a program's original source code. The mapping is not necessarily perfect, and not possible for arbitrary assembly (again, unless you use inline assembly in the resulting C).
