Use lapack's dgemm (linker error)

Question

How can I multiply two matrices by Lapack package for Fortran? I use gfortran compiler in ubuntu. My code which doesn't work is:

program main

integer, parameter :: n = 10
double precision :: alpha = 1.0, beta = 0.0
real, dimension(10,10) :: a

do i1 = 1,n
  do j1 = 1,n
    a(i1,j1) = j1 + (i1-1)*n
   end do
end do

call cpu_time(start)

call DGEMM('N', 'N', n, n, n, alpha, a, n, a, n, beta, a, n)

call cpu_time(end)
print *, end - start

end program main

I used:

gfortran 0.f90 -llapack

It returned:

/tmp/ccPy78g5.o: In function `MAIN__':
0.f90:(.text+0x110): undefined reference to `dgemm_'
collect2: ld returned 1 exit status

Answer 1

The error message means, that your compiler (gfortran) cannot find lapack or rather dgemm. Please make sure, that lapack is in your path. Alternatively (I assume you are using Ubuntu Linux) you could try -lblas instead (after installing it, of course - afaik ubuntu follows a different naming convention):

gfortran 0.f90 -lblas -llapack

Edit

Alternatively, you may pass the path to the library directly as an argument. By default, gfortran will look in `/usr/local/lib/``for the specified libraries. If the library is located somewhere else, you may instead use something like

gfortran 0.f90 /path/to/my/library.a