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cfortraninteropfortran-iso-c-binding
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Reputation: 133

Fortran to pass C struct between two C functions

I have a Fortran application that is required to call two C routines. One to load a file and one to run a calculation on the file about 200 times. I understand that a C DLL cannot 'save' the loaded struct in a static variable to be reused in the calculate function so I am looking to parse back a void* to Fortran and send it to C calculate function.

The C functions:

 __declspec(dllexport) void loadfile(void * file); // Empty pointer should be filled with struct of loaded file

 __declspec(dllexport) void calculate(void * file, double * result); //filled void ptr is used (casted back to my struct first)

My Fortran code:

module globalFileHolder
  USE, INTRINSIC::ISO_C_BINDING
  type(C_PTR), save :: fileModule = C_NULL_PTR    
end module

Load file routine:

SUBROUTINE loadcfile()
  USE, INTRINSIC::ISO_C_BINDING
  use globalFileHolder
  IMPLICIT NONE
  INTERFACE
       SUBROUTINE loadfile(fm) BIND(C)
       USE, INTRINSIC::ISO_C_BINDING
          TYPE(C_PTR)   :: fm
       END SUBROUTINE loadfile
  END INTERFACE 

  TYPE(C_PTR) :: fms = c_null_ptr       
  call loadfile(fms)
  fileModule = fms
  return
  end

And finally my routine that is supposed to use the loaded file in a calculation:

SUBROUTINE calculatec()
 USE, INTRINSIC::ISO_C_BINDING 
  use globalFileHolder
  IMPLICIT NONE
  INTERFACE
       SUBROUTINE calculate(fm,res) BIND(C)
      USE, INTRINSIC::ISO_C_BINDING
      TYPE(C_PTR)    , VALUE :: fm  
      REAL(C_DOUBLE) , value :: res
  END SUBROUTINE calculate
  END INTERFACE 
  TYPE(C_PTR) :: fms 
  REAL(C_DOUBLE) result
  fms = C_LOC(fileModule)
  call calculate(fms,result)
  return
  end

Now the problem I currently have is that the module variable filemodule seems to be filled but when sending it to the calculate function the variable is null after casting it like: 

myStruct * ms = (myStruct*)file;

Where do I go wrong?

Upvotes: 2

Views: 568

Answers (1)

IanH
IanH

Reputation: 21431

The declaration of the fm dummy argument in the interface for loadfile is missing the VALUE attribute.

Upvotes: 1

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