Empty parameter lists

C distinguishes between a function declared with an empty parameter list and a function declared with a parameter list consisting of onlyvoid. The former is an unprototyped function taking an unspecified number of arguments, while the latter is a prototyped function taking no arguments.

 // C code

    extern int  foo();          // Unspecified parameters
    extern int  bar(void);      // No parameters

    void baz()
    {
        foo(0);         // Valid C, invalid C++
        foo(1, 2);      // Valid C, invalid C++

        bar();          // Okay in both C and C++
        bar(1);         // Error in both C and C++
    } 

C++, on the other hand, makes no distinction between the two declarations and considers them both to mean a function taking no arguments.

    // C++ code

    extern int  xyz();

    extern int  xyz(void);  // Same as 'xyz()' in C++,
                            // Different and invalid in C 

For code that is intended to be compiled as either C or C++, the best solution to this problem is to always declare functions taking no parameters with an explicit void prototype. For example:

   // Compiles as both C and C++
    int bosho(void)
    {
        ...
    } 

Empty function prototypes are a deprecated feature in C99 (as they were in C89).

Refer to http://david.tribble.com/text/cdiffs.htm