Use Fortran-code in C - c

Question

I try to use a fortran-routine in C, but I doesn't work. I don't know where I made a mistake. Here my Fortran-code including the Integration-Module, which I want to use in C:
module integration
implicit none
contains
function Integrate(func, a,b, intsteps) result(integral)
interface
real function func(x)
real, intent(in) :: x
end function func
end interface
real :: integral, a, b
integer :: intsteps
intent(in) :: a, b, intsteps
optional :: intsteps
real :: x, dx
integer :: i,n
integer, parameter :: rk = kind(x)
n = 1000
if (present(intsteps)) n = intsteps
dx = (b-a)/n
integral = 0.0_rk
do i = 1,n
x = a + (1.0_rk * i - 0.5_rk) * dx
integral = integral + func(x)
end do
integral = integral * dx
end function
end module integration
real(c_float) function wrapper_integrate(func,a,b, intsteps) result(integral) bind(C, name='integrate')
use iso_c_binding
use integration
interface
real(c_float) function func(x) bind(C)
use, intrinsic :: iso_c_binding
real(c_float), intent(in) :: x
end function func
end interface
real(c_float) :: a,b
integer(c_int),intent(in) :: intsteps
optional :: intsteps
if (present(intsteps)) then
integral = Integrate(func,a,b,intsteps)
else
integral = Integrate(func,a,b)
endif
end function wrapper_integrate
and my C-Code:
#include <stdio.h>
#include <math.h>
float sin2(float x) {
return sin(x) * sin(x);
}
float integrate(float(*func)(float), float a, float b, int intsteps);
int main() {
float integral;
integral = integrate(sin2,0.,1.,10000);
printf("%f",integral);
return 0;
}
if I execute
g++ -c main.c
gfortran -c integration.f95
g++ main.o integration.o
I get
undefined reference to `integrate(float (*)(float), float, float, int)'
Does anyone know how to handle this?

Answer 1

If you are using the module ISO_C_Binding, you can directly passing a function from C to Fortran as a function pointer C_FUNPTR.
See here for details.
In your case, this would look like:
real(c_float) function wrapper_integrate(func, a, b, intsteps) result(integral) bind(C, name='integrate')
use iso_c_binding
use integration
abstract interface
function iFunc(x) bind(C)
use, intrinsic :: iso_c_binding
real(c_float) :: iFunc
real(c_float), intent(in) :: x
end function iFunc
end interface
type(C_FUNPTR), INTENT(IN), VALUE :: func
real(c_float) :: a,b
integer(c_int),intent(in) :: intsteps
optional :: intsteps
procedure(iFunc),pointer :: myfunc
call c_f_procpointer(func, myfunc)
if (present(intsteps)) then
integral = Integrate(myfunc,a,b,intsteps)
else
integral = Integrate(myfunc,a,b)
endif
end function wrapper_integrate
Obviously, your solution is more elegant ;-)
Also, please note that Fortran passes variables by reference (unless you specify the VALUE attribute, which you don't). Therefore, you need to change you C code accordingly:
#include <stdio.h>
#include <math.h>
float sin2(float *x) {
return sin(*x) * sin(*x);
}
float integrate(float(*func)(float*), float* a, float* b, int* intsteps);
int main() {
float integral;
float a=0.;
float b=1.;
int intsteps=10000;
integral = integrate(sin2, &a, &b, &intsteps);
printf("%f",integral);
return 0;
}

Answer 2

You are using the C++ compiler, not the C one. Linking conventions may be different.
And you forgot to link with the math library (because of sin)
gcc -c main.c
gfortran -c integration.f95
gcc main.o integration.o -lm

Answer 3

The OP used the gnu c++ compiler. Here's a solution, using information from several respondants, that worked, using the c++ compiler -- not the c compiler -- for me:
file 'integration.f95':
module integration
implicit none
contains
function Integrate(func, a, b, intsteps) result(integral)
interface
real function func(x)
real, intent(in) :: x
end function func
end interface
real :: integral, a, b
integer :: intsteps
intent(in) :: a, b, intsteps
optional :: intsteps
real :: x, dx
integer :: i,n
integer, parameter :: rk = kind(x)
n = 1000
if (present(intsteps)) n = intsteps
dx = (b-a)/n
integral = 0.0_rk
do i = 1,n
x = a + (1.0_rk * i - 0.5_rk) * dx
integral = integral + func(x)
end do
integral = integral * dx
end function
end module integration
real(c_float) function wrapper_integrate(func, a, b, intsteps) result(integral) bind(C, name='integrate')
use iso_c_binding
use integration
abstract interface
function iFunc(x) bind(C)
use, intrinsic :: iso_c_binding
real(c_float) :: iFunc
real(c_float), intent(in) :: x
end function iFunc
end interface
type(C_FUNPTR), INTENT(IN), VALUE :: func
real(c_float) :: a,b
integer(c_int),intent(in) :: intsteps
optional :: intsteps
procedure(iFunc),pointer :: myfunc
call c_f_procpointer(func, myfunc)
if (present(intsteps)) then
integral = Integrate(myfunc,a,b,intsteps)
else
integral = Integrate(myfunc,a,b)
endif
end function wrapper_integrate
file 'main.c':
#include <stdio.h>
#define _USE_MATH_DEFINES
#include <math.h>
float sin2(float *x) {
return sin(*x) * sin(*x);
}
float integrate(float(*func)(float*), float* a, float* b, int* intsteps);
int main() {
int intsteps=10000;
float integral;
float a=0.;
float b=3.1416;
integral = integrate(sin2, &a, &b, &intsteps);
printf("The numerical value of \\int_0^\\pi dx sin^2x = %f\n",integral);
printf("The exact value of \\int_0^\\pi dx sin^2x = %f\n",M_PI_2);
return 0;
}
file 'compile.txt':
gcc -c main.c
gfortran -c integration.f95
g++ -o intsinq main.o integration.o