More on mixing C and Fortran
Consider the following code in C and Fortran:

#include <stdio.h>

void extern ranvec_(double *, int *);

int main()
{
  int n = 5;
  double a[5];

  ranvec_(a, &n);

  printf("a = %f, %f, %f, %f, %f\n",
          a[0], a[1], a[2], a[3], a[4]);

  return 0;
}

subroutine ranvec(v, n)
   integer                        :: n
   double precision, dimension(n) :: v

   call random_seed()
   call random_number(v)

end


Giving the obvious commands give problems:

% g95 -c ranvec.f90
% gcc main.c ranvec.o
ranvec.o(.text+0x58): In function `ranvec_':
: undefined reference to `_g95_random_seed'
ranvec.o(.text+0x9d): In function `ranvec_':
: undefined reference to `_g95_random_8'
collect2: ld returned 1 exit status

% gfortran -c ranvec.f90
% gcc main.c ranvec.o
ranvec.o(.text+0x24): In function `ranvec_':
: undefined reference to `_gfortran_random_seed'
ranvec.o(.text+0x54): In function `ranvec_':
: undefined reference to `_gfortran_arandom_r8'
collect2: ld returned 1 exit status

The reason for this meassage is that a libraries containing  _g95_random_seed etc. are missing. These routines reside in Fortran libraries and we have to tell the C-compiler (the link-phase) to link with the missing libraries. Here are some examples:

% g95 -c ranvec.f90
% gcc main.c ranvec.o -L/chalmers/sw/unsup/g95/lib/gcc-lib/i686-pc-linux-gnu/4.0.3 -lf95
% a.out
a = 0.253645, 0.486569, 0.287456, 0.373990, 0.778394

% gfortran -c ranvec.f90
% gcc main.c ranvec.o -L. -lgfortran
% a.out
a = 0.126499, 0.082701, 0.330154, 0.401973, 0.248005

To make this work I had to make a soft link to the shared object under /usr/lib (there are problems with the name of the shared library):

% ln -s /usr/lib/libgfortran.so.1 libgfortran.so

How do I know what libraries to use? Well, I do not, but it is reasonable to look in standrad directories, like /usr/lib, and install directories, such as /chalmers/sw/unsup/g95/lib. Look for library names containing "fortran". Then try to list the contents using commands like ar, nm, objdump (depending on type of library and operating system) and see if you can find the missing routine.

It may work to compile with gcc and link with the Fortran-compiler. They may lead to problems with C-libraries, instead.

% gcc -c main.c
% gfortran main.o ranvec.f90

% gcc -c main.c
% g95 main.o ranvec.f90