Understanding this part of this Header file

Question

#ifndef STDARG_H_INCLUDED
#define STDARG_H_INCLUDED

typedef __builtin_va_list va_list;

#define va_start(v,l)   __builtin_va_start(v,l) // I don't understand this
#define va_arg(v,l)     __builtin_va_arg(v,l)
#define va_end(v)       __builtin_va_end(v)

#endif

It seems that the author defines some kind of a macro function, but I don't understand how this function works?

Answer 1

#define va_start(v,l)   __builtin_va_start(v,l)

simply means va_start(v,l) will be replaced by __builtin_va_start(v,l), and this __builtin_va_start() is obviously a builtin function or macro, it is implemented by the compiler or libc itself.