Using objdump to look at the symbols, we see
0000000000000000 g O .bss 000000000000000c __mymodule_MOD_aa
You need to add bind(C)
to your aa variable
module myModule
use iso_c_binding
implicit none
real(C_FLOAT), bind(C) :: aa(3)
contains
subroutine fortranFunction() bind(C)
print *,"hello world from Fortran 90"
aa(1)=1.0;
aa(2)=2.0;
aa(3)=3.0;
end subroutine
end module
now $ objdump -t fortrancode.o
says
000000000000000c O *COM* 0000000000000004 aa
and
#include "stdio.h"
extern void fortranfunction();
extern float aa[3];
int main()
{
printf("hello world from C\n");
fortranfunction();
printf("%f %f %f \n",aa[0],aa[1],aa[2]);
return 0;
}
and
$ ./myprogram
hello world from C
hello world from Fortran 90
1.000000 2.000000 3.000000