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I'm having to write a subroutine that can be called both by C and by Fortran. This subroutine takes a file name as one of its arguments. I know that to interoperate nicely with C, the ISO C binding recommends using character arrays for interoperation.

The question I have is: is there such a thing as a character array literal that's easy to write? I have a subroutine like this:

subroutine my_sub(char_array)
  use iso_c_binding, only: c_char
  char(kind=c_char, len=1), dimension(:), intent(in) :: char_array
  ...
end subroutine my_sub

Is it possible to invoke this with something like:

call my_sub('Hello World!')

Or do I have to do something horrible like:

call my_sub((/ 'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!' /))

My main issue seems to be that it doesn't like the assumed-shape array and that giving it a set (large) size also outputs all the garbage memory that happens to get picked up afterwards.

Is there a better way of doing this?

Kyle_S-C
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2 Answers2

4

The rules of sequence association allow similar situation. You can use

call my_sub('Hello World!')

but only if you make your array assumed size dimension(*) or explicit shape.

Generally, in Fortran 2008 you cannot use assumed shape arrays for C-interoperable procedures. This will change i the future, but will be more complicated.

One other important remark. Just using iso_c_binding module doesn't make the procedure C-interoperable. You have to use bind(C), possibly with binding name. That's the important thing. You can create interoperable procedures even without the module (only limited ones). The feature of passing the string instead of 1-character array also depends on this.

This is why i don't like saying "use iso_c_binding to create interoperable procedures".

The interface to the procedure will have to explicit in Fortran.

Vladimir F Героям слава
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  • Sorry, I missed the bind(C) and the name (I use various different compilers, so I want to ensure that I'm getting consistent naming, as far as possible). – Kyle_S-C Mar 17 '14 at 15:26
  • `bind(C)` is the key! Even `iso_c_binding` is less important, and here almost not needed, it just contains couple of constants and procedures. – Vladimir F Героям слава Mar 17 '14 at 15:32
  • Oh, I meant in my code snippet. I have bind(C) in all of my subroutines. :) – Kyle_S-C Mar 17 '14 at 17:40
  • Ok, unfortunately, `gfortran` accepts the assumed shape array even though it is clearly a problem here. With `ifort` I get `error #8520: An array dummy argument of a BIND(C) procedure must be an explicit shape or assumed size array.` It may be relaxed now, but a warning would be in order anyway I believe. – Vladimir F Героям слава Mar 17 '14 at 17:47
3

You can use transfer function. Here is an example code.

program string0
  implicit none
  character, allocatable :: string(:)
  character(len = :), allocatable :: text

  text = 'this is a pen'
  string = transfer(text, ' ', size = len_trim(text))
  string = achar(iachar(string) - 32)
  text = transfer(string, text)
  print *, text

  stop
end program string0
yamajun
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