Undefined reference to main fortran program - c

Question

I am writing a fortran code for benchmarking three loop kernels:
program Kernel_benchmark
implicit none
double precision,dimension (:),save,allocatable:: a,b,c,d,x,y
double precision s
double precision,dimension (:,:),save,allocatable:: mat
double precision wcs,wce,ct,runtime, total
integer k,iter,r,i,j,N
do k = 3, 20
N = INT(2.5**k)
allocate (a(N),b(N),c(N),d(N))
do i=1,N
a(i) = 1.2
b(i) = 1.2
c(i) = 1.2
d(i) = 1.2
end do
iter = 1
runtime = 0.0
do while(runtime < 0.2)
call timing(wcs,ct)
do r =0, iter
do i=1,N
a(i) = b(i) + c(i) * d(i)
end do
if(a(ISHFT(N,-1)) < 0.0) then
call dummy(a)
end if
end do
call timing(wce,ct)
runtime = wce - wcs
iter = iter * 2
end do
iter = iter / 2
open(unit=1, file = 'vector_triad.dat',status = 'unknown')
write(1,*) N, (N * iter* 2) / (runtime * 1e-6)
close(1)
deallocate(a,b,c,d)
end do
do k = 3, 20
N = INT(2.5**k)
allocate(a(N))
do i = 1, N
a(i) = 1.2
end do
s = 2.2
iter = 1
runtime = 0.0
do while(runtime < 0.2)
call timing(wcs,ct)
do r = 0, iter
do i = 1, N
a(i) = s * a(i)
end do
if(a(ISHFT(N,-1)) < 0.0) then
call dummy(a)
end if
end do
call timing(wce,ct)
runtime = wce - wcs
iter = iter * 2
end do
iter = iter / 2
open (unit = 2, file = 'vector_update.txt', status = 'unknown' )
write(2,*) N, (N * iter) / (runtime * 1e-6)
close(2)
deallocate(a)
end do
do k = 10, 22
N = INT(1.5**k)
allocate (mat(N,N),x(N),y(N))
do i = 1, N
do j = 1, N
mat(i,j) = 1.2
end do
y(i) = 1.2
x(i) = 1.2
end do
iter = 1
runtime = 0.0
do while(runtime < 0.2)
call timing(wcs,ct)
do r = 0, iter
do i = 1, N
y(i) = 0.0
do j = 1, N
y(i) = y(i) + (mat(i,j) * x(i))
end do
end do
if(y(ISHFT(N,-1))< 0.0) then
call dummy(y)
end if
end do
call timing(wce,ct)
runtime = wce - wcs
iter = iter * 2
end do
iter = iter / 2
open (unit = 3, file = 'matrix_vector.txt', status ='unknown')
write(3,*) N, (2 * N * N * iter) / (runtime * 1e-6)
close(3)
deallocate(mat,x,y)
end do
end program Kernel_benchmark
The dummy function I have written inside a C source file as follows
#include "dummy.h"
void dummy(double *array){
printf ("Well if its printing this then you're pretty much screwed.");
}
and dummy.h simply contains the function prototype.
I made an dummy.o object file and I'm trying to link it with my fortran source code using an intel ifort compiler. Unfortunately, I'm getting an error
In function MAIN__':bench.f90:(.text+0x8ca): undefined reference todummy_'
every time the dummy function is called. Any suggestion? Thanks in advance.

Answer 1

The modern way to interface with Fortran is the interoperability with C and the iso_c_binding module, as discussed many times on this site.
Calling a FORTRAN subroutine from C
https://stackoverflow.com/search?tab=votes&q=iso_c_binding

Answer 2

Inside the Fortram program, the symbol dummy is taken to be a subroutine with an implicit interface. Naturally the Fortran compiler things that subroutine will be a Fortran subroutine and will arrange argument passing, linker name mangling, etc appropriately.
Because dummy procedure is a C function and not a Fortran subroutine, problems ensure.
If the Fortran compiler is explicitly told that the dummy symbol is a C function, then it will make the appropriate changes. In the specification part of your main program:
INTERFACE
SUBROUTINE dummy(array) BIND(C, NAME='dummy')
IMPLICIT NONE
DOUBLE PRECISION :: array(*)
END SUBROUTINE
END INTERFACE
Robust code would further set the kind of the array argument appropriately.

Answer 3

If you use GNU compilers, note that the name mangling is a little bit different for C and Fortran. If your fortran programm call subroutine xyz then the corresponding C subroutine should be named xyz_.
So in your case it is enough to rename dummy into dummy_ in the C sources. You may also need to link with -lg2c if I remember correctly.