Undefined reference to Fortran function in C++ - c++

Question

I can't seem to figure out why this isn't working.
/* main.cpp */
#include <stdio.h>
extern "C"
{
int __stdcall inhalf(int *);
}
int main()
{
int toHalf = 2;
int halved = inhalf(&toHalf);
printf("Half of 2 is %d", halved);
return 0;
}
Ok, that looks good.
$ g++ -c main.cpp
No errors.
! functions.f90
function inhalf(i) result(j)
integer, intent(in) :: i
integer :: j
j = i/2
end function inhalf
I'm pretty sure that's right.
$ gfortran -c functions.f90
So far so good...
$ gcc -o testo main.o functions.o
main.o:main.cpp:(.text+0x24): undefined reference to `inhalf#4'
collect2.exe: error: ld returned 1 exit status
I've been looking this up for over an hour, but I couldn't find anything that worked for this case. How should I solve this?

Answer 1

For full C compatibility you can use the bind feature of modern Fortran:
! functions.f90
function inhalf(i) result(j) bind(C,name='inhalf')
integer, intent(in) :: i
integer :: j
j = i/2
end function inhalf
This allows you to give a name to the function that you can use in C (and others) without relying on the naming scheme your compiler uses on its own.
The __stdcall is Win32 only (and the default behavior for linking, see here). You can safely remove it. [Actually it is required for compiling your code in Linux. ]