How do vararg functions find out the number of arguments in machine code?

Question

How can variadic functions like printf find out the number of arguments they got?

The amount of arguments obviously isn\'t passed as a (hidden) parameter (s

Answer 1

Implicitly, from the format string. Note that stdarg.h doesn't contain any macros to retrieve the total "variable" number of arguments passed. This is also one of the reasons the C calling convention requires the caller to clean the stack, even though this increases code size.