Calculating number of variable arguments for clone function

Question

I am trying to LD_PRELOAD the linux's clone function. In my LD_PRELOADed version I need to log the input paramters before calling the original clone function. However, the problem is that clone takes variable number of arguments. It is declared like this.

int clone(int (*fn)(void *), void *child_stack, int flags, void *arg, ...
/* pid_t *pid, struct user_desc *tls, pid_t *ctid */ );

Now to pass these arguments to the original clone function, I must know the number of arguments that were passed in. How can I do that?

Answer 1

Use the va_* functions, these are the way to go when working with variable argument lists.

Here's the man page which also contains an example at the end.

Answer 2

If there is a NULL terminator, you can call va_arg while it doesn't return NULL.