[Bug fortran/84885] c_char bind length

[mdblack98 at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84885

--- Comment #2 from mdblack98 at yahoo dot com ---
I got what's below from Steve Lionel of the FORTRAN working group....With what
he said you flat should NOT be able to say anything other than LEN=1 for
c_char.  Whether it's in a type block or not.So the question becomes how to do
this...since this is likely to break a bit of FORTRAN code out in the
wild...like ours....It would appear you can't put a char array of any kind of
type block which we use for shared memory.

From Steve:
However, I don't see that Fortran 2018 has anything to do with this. The code
you show does not conform to Fortran 2003, which was the first revision to have
C interoperability. The issue is that in an interoperable type (a type declared
with BIND(C)), any entity of type CHARACTER must have length 1, because C
doesn't have the concept of character lengths. Instead you would make c here a
10-element array of single characters. It's ok outside of the type because
you're not declaring something interoperable then.

 Similarly, a dummy argument to an interoperable procedure can't have a
character length other than 1 up through Fortran 2008. In Fortran 2018, you're
allowed to use CHARACTER(*), but that requires that the corresponding C code
pass or accept a "C descriptor".

 That earlier versions of gcc allowed this to compile would be a bug in those
older versions.

----------------------------------- 
Michael D. Black 

    On Thursday, March 15, 2018, 11:42:40 AM CDT, kargl at gcc dot gnu.org
<gcc-bugzilla@gcc.gnu.org> wrote:  

 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84885

kargl at gcc dot gnu.org changed:

          What    |Removed                    |Added
----------------------------------------------------------------------------
            Status|UNCONFIRMED                |RESOLVED
                CC|                            |kargl at gcc dot gnu.org
        Resolution|---                        |INVALID

--- Comment #1 from kargl at gcc dot gnu.org ---
(In reply to mdblack98 from comment #0)
> subroutine foo(i,c)
>  use, intrinsic :: iso_c_binding, only: c_char
>        integer i
> 
>  type, bind(C) :: params_block
>        character(kind=c_char,len=10) :: c

I see length of 10 here.

>  end type params_block
>        write(*,*) 'X',c,'Z'
> end
> 
> This program fails to compile with gcc 8.0.1 20180304 -- but only if the
> character declaration is inside a type block
> Compiles fine with pre 8.0 compilers
> 
> gfortran -fPIC -g -c foo.f90
> foo.f90:6:42:
> 
>          character(kind=c_char,len=10) :: c
>                                          1
> Error: Component 'c' of BIND(C) type at (1) must have length one

From the F2018 standard,

18.3.2 Interoperability of intrinsic types

Table 18.2 shows the interoperability between Fortran intrinsic
types and C types.  A Fortran intrinsic type with particular type
parameter values is interoperable with a C type if the type and
kind type parameter value are listed in the table on the same row
as that C type.  If the type is character, the length type parameter
is interoperable if and only if its value is one.


C1806 (R726) Each component of a derived type with the BIND
attribute shall be a nonpointer, nonallocatable data component
with interoperable type and type parameters.

Your code is invalid, and the number constraint means
that gfortran must tell you about it.