Re: [wwwdata] Update Fortran section of gcc-4.6/changes.html

[Tobias Burnus <burnus@net-b.de>]

Gerald Pfeifer wrote:
> +<li>Null pointers (including<code>NULL()</code>) and not
> +       allocated variables can be used as actual argument to optional
> +       non-pointer, non-allocatable dummy arguments, denoting an absent
> +       argument.</li>

> +<li>Non-pointer variables with<code>TARGET</code>  attribute can

Quote from the introduction of the Fortran 2008 standard (cf. 
http://gcc.gnu.org/wiki/GFortranStandards):

"Fortran 2008 contains several extensions to Fortran 2003; some of these 
are listed below." [...]
"Programs and procedures: [...] An actual argument with the TARGET
attribute can correspond to a dummy pointer. A null pointer or 
unallocated allocatable can be used to
denote an absent nonallocatable nonpointer optional argument."

Regarding the latter:
a) Cf. Steve's email about the word "dummy" vs. "actual argument".

b) Fortran allows to have optional arguments. Internally, gfortran (and 
most other Fortran compilers) passes a NULL pointer if the (actual) 
argument is absent. However, doing so as user is only allowed since 
Fortran 2008. (For arrays also a code change was required.)

Daniel Franke wrote:
> since I missed the commit and this is news to me - maybe one could add a word
> why this_is_  news and actually noteworthy?

Regarding the noteworthiness: It is listed in the Fortran 2008 standard 
as being a new feature - thus listing it helps users knowing the F2008 
list  (while others are made aware of this feature); additionally, I 
think it is a quite useful feature as one now can pass, e.g., a 
not-allocated variable as absent argument. Before it only worked for 
scalar variables, relying on a particular implementation - now it also 
works for arrays.

However, one could also move it into the "Minor changes" item at 
http://gcc.gnu.org/gcc-4.6/changes.html#Fortran


> IIRC, there was something that absent dummy arguments where marked by 0L,
> hence one could not pass a NULL pointer as it would be seen as "absent" not as
> "present with value 0". If that's the one, mentioning it might certainly help
> an unaware reader.

No, that's a separate, still unfixed issue which only occurs if one 
mixes (pass by) VALUE with OPTIONAL arguments. (And a Fortran 2003 item 
as VALUE is allowed since 2003.)

(Side note: The upcoming technical report on further C interoperability 
will also allow C_NULL_PTR (C: "NULL") to denote an absent argument and, 
thus, allows "OPTIONAL" in BIND(C) procedure - but not together with VALUE.)

Tobias