How can I make a function with optional arguments in C?

Question

Recently I got a question when writing a program for file opening.

Let me explain my question clearly. Here I'm taking open call as an example.

To create a file:

open("file_name", O_CREAT, 0766); //passing 3 parametrs

To open a file:

open("file_name", O_RDWR); //only 2 arguments.

Then I clearly observed this point and it also works for main() too.

main(void) //worked
main(int argc, char **argv); //worked
main(int argc) //worked and it's doesn't give an error like "too few arguments". 
main() //worked 

So how we can create these optional arguments? How exactly can the compiler validate these prototypes? If possible, please write an example program.

Answer 1

The open function is declared as a variadic function. It will look something like this:

#include <stdarg.h>

int open(char const * filename, int flags, ...)
{
    va_list ap;
    va_start(ap, flags);

    if (flags & O_CREAT)
    {
        int mode = va_arg(ap, int);
        // ...
    }

    // ...

    va_end(ap);
}

The further arguments are not consumed unless you have indicated that they do in fact exist.

The same construction is used for printf.

The manual doesn't always make this explicit, since the only possible two signatures are (char const *, int) and (char const *, int, int), so there's little point in revealing that you the function actually accepts variable arguments. (You can test this by trying to compile something like open("", 1, 2, 3, 4, 5, 6).)