Calling C function/subroutine in Fortran code

Question

I am attempting to compile and link a Fortran code calling c subroutine:

Fortran code:

program adder
integer a,b
a=1
b=2
call addnums(a,b)
stop    
e         


        

Answer 1

You need to provide an interface body for the C function inside the specification part of the Fortran main program that tells the Fortran compiler that the name addnums is a C function. Something like:

INTERFACE
  SUBROUTINE addnums(a, b) BIND(C)
    USE, INTRINSIC :: ISO_C_BINDING, ONLY: C_INT
    IMPLICIT NONE
    INTEGER(C_INT) :: a, b
  END SUBROUTINE addnums
END INTERFACE

(With those compilers on that platform without special options the default kind of integer is the same as C_INT - but being explicit about the integer KIND helps protect you if compiler/platform or compile options change.)