[Bug fortran/84885] c_char bind length

[sgk at troutmask dot apl.washington.edu]

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

--- Comment #6 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
On Fri, Mar 16, 2018 at 01:11:30PM +0000, mdblack98 at yahoo dot com wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84885
> 
> --- Comment #5 from mdblack98 at yahoo dot com ---
> I've been using Fortran since 1980 so caps is natural to me :-)
> My concern is that if gfortran has supported the len>1 convention for many
> years than perhaps it should continue supporting it to prevent breaking code
> like ours which will take some major effort to rewrite.Consider it an
> "extension" to gfortran which apparently it was.  Still fully 2018 compliant
> with this one little extension.  Enable it via a switch if you want to. 

First, gcc-bugzilla is probably not the appropriate forum
for learning nuances of BIND(C).  I suggest posting your
questions to the comp.lang.fortran newsgroup or Stack Overflow.

I've quoted two difference standards.  Both state

   If the type is character, the length type parameter is
   interoperable if and only if its value is one.

The above isn't a numbered constraint, so gfortran is not
obligated to tell a user that in the following

subroutine foo(c)
  use, intrinsic iso_c_binding
  character(len=1000, kind=c_char), bind(c) :: c
end subroutine foo

'c' is not interoperable.  Perhaps, gfortran should issue
at least a warning.  As long as a program does not try to
use 'c' in an interoperable manner, gfortran appears to
ignore the bind(c) attribute.  If a program, however, uses
'c' in an interoperable manner, the user may get a result
he expects even though it technically violates the Fortran
standard. 

Now, when you wrap the character statement in a derived type

subroutine foo(c)
  use, intrinsic :: iso_c_binding
  type, bind(c) :: bar
     character(len=1000, kind=c_char) :: c
  end type
  type(bar) bah
  bah%c = "123"
end subroutine foo

a numbered constraint comes into play.  gfortran must tell
the user that 'c' in type 'bar' is not interoperable.  I quoted
the relevant text already.  From F2018 (and similar appears in
other versions of the standard)

   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.