Question

I ran into this question while typing man 2 open. It says that there are two kinds of open, one with two args, and one with three! last time i checked we could not overload functions in C. How did they do this? did they write in C++?

int open(const char * pathname, int flags);
int open(const char * pathname, int flags, mode_t mode);

Answer 1

No, they just used variadic function.

int open(const char * pathname, int flags, ...);

This makes the last argument mode optional. The prototypes only show how the function should be used, not the actual interface.

Of course, unlike real overloading, the compiler cannot type-check the mode argument, so the user have to be extra careful to ensure only 2 or 3 arguments are passed, and the 3rd argument must be a mode_t.

---

BTW, if you check the man 2 open for BSD (including OS X) it shows the correct prototype as above.