The example is to implement the handler for the task call that will use virtual_copy to copy a large struct from kernel memory to a user process's memory.

We have to specify a struct vir_addr to describe the virtual address of the kernel's source data and another struct vir_addr to describe the virtual address of the destination user's variable to receive the copy.

struct vir_addr {
  int proc_nr_e;
  int segment;
  vir_bytes offset;
};

  proc_nr_e:   endpoint value of the process
  segment:     This should be T, D, or S (0,1, or 2)
               T for Text, D for Data, S for Stack
  offset:      Byte offset from the beginning of the segment.

From the discussion above, for copying data, you can set the segment to be D or S and specify the byte offset as the address of the data item (using C expression to calculate this address). This should work if the data is either in the data segment or the stack segment. The umap_local function will change the segment if it is not the correct one before calculating the physical address.