Under ANSI C (not C++)
int f1();
is very different from
int f1(void);

declaration 1 declares function f1 without any prototypes, so can be called
with any arguments.
Declaration 2 declares function f2 with a prototype having no parameters,
so can only be called with no arguments.
Declaration 2 is still valid C++ code, so that is not a problem.

There are many instances of this within the code, which generates extra
warnings.