Question

I have to write a function that accepts a variable number of arguments, similar to the open() function

int open (const char *filename, int flags[, mode_t mode])

1. I had a look at the va_list functionality, but how can I find out how many arguments the user has supplied? I dont want to have a parameter in the function that indicates this.

2. Being a sort of newbie, where can I look how the open() function is implemented? How does open() know whether the user has supplied the mode argument or not?

Thanks in advance.

Answer 1

1. You cannot. The caller must either supply a trailing sentinel argument (e.g. a 0), or explicitly tell the function how many arguments to expect (e.g. the format fields in printf).

2. open knows to expect the third argument because you will have set the O_CREAT flag in the flags argument.