Creating a FORTRAN interface to a C function that returns a char*

Question

I\'ve been held up on this for about a week, now, and have searched forum after forum for a clear explanation of how to send a char* from C to FORTRAN. To make the matter m

Answer 1

My thanks to heraldkl for giving me the solution to this very frustrating problem. I'm posting what I've eventually implemented, which roles the pointer conversion into the interface, meaning the final application can call the C function without having to know about the pointer conversion:

The C function:

// The C declaration header (using __cdecl in a def file):
extern "C" const char* GetLastErrorMessage();

The FORTRAN interface module:

MODULE mINTERFACES

USE ISO_C_BINDING

INTERFACE
    FUNCTION GetLastErrorMessagePtr [C, ALIAS: '_GetLastErrorMessage'] ()
        USE ISO_C_BINDING   
    TYPE(C_PTR) :: GetLastErrorMessagePtr                   
    END FUNCTION GetLastErrorMessagePtr
END INTERFACE

CONTAINS    ! this must be after all INTERFACE definitions

FUNCTION GetLastErrorMessage()
    USE ISO_C_BINDING   
    CHARACTER*255 :: GetLastErrorMessage
    CHARACTER, POINTER, DIMENSION(:) :: last_message_array
    CHARACTER*255 last_message
    INTEGER message_length

    CALL C_F_POINTER(GetLastErrorMessagePtr(), last_message_array, [ 255 ])

    DO 10 i=1, 255
        last_message(i:i+1) = last_message_array(i)
10  CONTINUE

    message_length = LEN_TRIM(last_message(1:INDEX(last_message, CHAR(0))))

    GetLastErrorMessage = last_message(1:message_length-1)

END FUNCTION GetLastErrorMessage

And to call this function from a FORTRAN program:

USE MINTERFACES

PRINT *, "--> GetLastErrorMessage:      '", TRIM(GetLastErrorMessage()), "'"

My thanks again to heraldkl for providing this solution - I wouldn't have had a clue how do do this without his input.