Re: func(va_arg_type, va_arg_type) segfault

[Bruno Haible]

> Is there a reason why this segfaults
> 
>     va_start_void(alist);
>     myfunction(va_arg_uint(alist), va_arg_ptr(alist, void*));
>     va_return_void(alist);
> 
> but this does not?
> 
>     va_start_void(alist);
>     unsigned int i = va_arg_uint(alist);
>     void *p        = va_arg_ptr(alist, void*);
>     myfunction(i, p);
>     va_return_void(alist);

In the first case, you have undefined evaluation order [1]. Which
means that the compiler can decide to evaluate va_arg_ptr(alist, void*)
before va_arg_uint(alist) or - even worse - interleave the two
expressions, by pulling an 'unsigned int' and a 'void *' from
the same memory location and advancing to the next argument
twice afterwards.

Bruno

[1] https://en.cppreference.com/w/c/language/eval_order