[Bug fortran/109861] Optimization is marking uninitialized C_PTR being passed to a C function, causes segfault.

["anlauf at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109861

--- Comment #4 from anlauf at gcc dot gnu.org ---
(In reply to Scot Breitenfeld from comment #3)
> I guess the issue is whether marking TYPE(C_PTR) as CLOBBER is correct. I
> looked through the 2018 standard and could not locate anything that
> addresses this use case. Are you interpreting the possibility that a
> TYPE(C_PTR) should not be declared INTENT(OUT)?

Maybe I am missing your intention, but I interpret your code that you
want to pass a (C) pointer to variable attr_rdata0 to return your result.
But that needs to be intent(in).  Your subroutine is not really supposed
to return a pointer but a result in the location the pointer dereferences.

Feel free to correct me.

> I can instead change the subroutine to declare buf as
> 
> INTEGER(C_INT), INTENT(OUT), TARGET :: buf
> 
> and f_ptr = C_LOC(buf) and there is no issue.

I cannot confirm this with my gcc installations, and there is no reason
that this should make a difference.

> So it seems to depend on the
> TYPE of the argument being passed.

There are cases where no clobber is currently generated.  For example,
if the dummy variable is a Fortran pointer, which has a completely
different semantics from TYPE(C_PTR).

Still I don't understand why you don't use INTENT(IN) for the pointer.
In that case you could do things in the main like:

  CALL H5Aread_async_f(C_LOC(attr_rdata0))

which appears to represent what I am guessing, and which gets rejected for
INTENT /= IN with a possibly more helpful error message.