Calling C function/subroutine in Fortran code

c, compilation, fortran, fortran-iso-c-binding

Solution

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.)

Problem

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 end program ``` C code: ``` void addnums( int* a, int* b ) { int c = (*a) + (*b); /* convert pointers to values, then add them */ printf("sum of %i and %i is %i\n", (*a), (*b), c ); } ``` I used the following commands to compile and link in windows environment. ``` ifort -c adder.f cl -c addnums.c ifort -o add adder.obj addnums.obj ``` I get the following error: ``` Microsoft (R) Incremental Linker Version 8.00.50727.762 Copyright (C) Microsoft Corporation. All rights reserved. -out:add.exe -subsystem:console adder.obj addnums.obj adder.obj : error LNK2019: unresolved external symbol ADDNUMS referenced in function MAIN__ add.exe : fatal error LNK1120: 1 unresolved externals ``` Please help me resolve this issue? Thanks.

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