Fortran 2003 introduced language features which can guarantee interoperability between C and Fortran (and to more languages by using C as an intermediary). These features are mostly accessed through the intrinsic module
use, intrinsic :: iso_c_binding
intrinsic keyword here ensures the correct module is used, and not a user created module of the same name.
iso_c_binding gives access to interoperable kind type parameters:
integer(c_int) :: foo ! equivalent of 'int foo' in C real(c_float) :: bar ! equivalent of 'float bar' in C
Use of C kind type parameters guarantees that the data can be transferred between C and Fortran programs.
Interoperability of C char and Fortran characters is probably a topic for itself and so not discussed here
To actually call a C function from Fortran, first the interface must be declared. This is essentially equivalent to the C function prototype, and lets the compiler know about the number and type of the arguments, etc.
bind attribute is used to tell the compiler the name of the function in C, which may be different to the Fortran name.
// Count how many geese are in a given flock int howManyGeese(int flock);
! Interface to C routine interface integer(c_int) function how_many_geese(flock_num) bind(C, 'howManyGeese') ! Interface blocks don't know about their context, ! so we need to use iso_c_binding to get c_int definition use, intrinsic :: iso_c_binding, only : c_int integer(c_int) :: flock_num end function how_many_geese end interface
The Fortran program needs to be linked against the C library (compiler dependent, include here?) that includes the implementation of
howManyGeese(), and then
how_many_geese() can be called from Fortran.