here's a simple example I found on the Net, for Fortran 90
module numz
  integer, parameter:: b8 = selected_real_kind(14)
  integer gene_size,num_genes
  integer,allocatable :: a_gene(:),many_genes(:,:)
end module

program darwin
    use numz
    implicit none
    integer ierr
    call set_size()
    allocate(a_gene(gene_size),stat=ierr) !stat= allows for an error code return
    if(ierr /= 0)write(*,*)"allocation error"  ! /= is .ne.
    allocate(many_genes(gene_size,num_genes),stat=ierr)  !2d array
    if(ierr /= 0)write(*,*)"allocation error"
    write(*,*)lbound(a_gene),ubound(a_gene) ! get lower and upper bound
                                            ! for the array
    write(*,*)size(many_genes),size(many_genes,1) !get total size and size
                                                  !along 1st dimension
    deallocate(many_genes) ! free the space for the array and matrix
    deallocate(a_gene)
    allocate(a_gene(0:gene_size)) ! now allocate starting at 0 instead of 1
    write(*,*)allocated(many_genes),allocated(a_gene) ! shows if allocated
    write(*,*)lbound(a_gene),ubound(a_gene)
end program
 

subroutine set_size
    use numz
    write(*,*)'enter gene size:'
    read(*,*)gene_size
    write(*,*)'enter number of genes:'
    read(*,*)num_genes
end subroutine set_size

Here is another example for F95.  It won't work in F90.  This allocates within the routine but you can equally do it outside.  All you said you were interested in was the declaration.

You do it the same as you do any arrays.   From the perspective of the Subroutine,  the arrays it receives might as well be ones that were declared in the main routine without the allocatable attribute.  There's no need for you to use modules or anything else to handle this situation.  Keep in mind,  we are talking about simple arrays of standard data types here, ie. Integer, Real, Character, etc.  This doesn't apply to arrays of user defined Types.

Dan   
www.dtware.com