Re: For review: documentation of clone3() system call

[Florian Weimer <fweimer@redhat.com>]

* Christian Brauner:

> I've always been confused by the "..." for the glibc wrapper. The glibc
> prototype in bits/sched.h also looks like this:
>
> extern int clone (int (*__fn) (void *__arg), void *__child_stack, int __flags, void *__arg, ...) __THROW;
>
> The additionl args parent_tid, tls, and child_tid are present in _all_
> clone version in the same order. In fact the glibc wrapper here give the
> illusion that it's parent_tid, tls, child_tid. The underlying syscall
> has a different order parent_tidptr, child_tidptr, tls.
>
> Florian, can you advise what prototype we should mention for the glibc
> clone() wrapper here. I'd like it to be as simple as possible and get
> rid of the ...
> Architectural differences are explained in detail below anyway.

Our header has:

/* Clone current process.  */
extern int clone (int (*__fn) (void *__arg), void *__child_stack,
                  int __flags, void *__arg, ...) __THROW;

I have not checked all assembler implementations.  In theory there could
be one that relies on the different calling convention for variadic
functions (e.g., the existence of a parameter save area on POWER).  Or
that swaps arguments in some architecure-specific way. 8-(

I don't have much guidance on this matter, sorry.  I expect that for
clone3, we'll provide a same-stack variant as well (for fork/vfork-like
usage), which will be much closer to the kernel interface.  clone/clone2
doesn't seem very fixable to me at this point.

Thanks,
Florian