sc_start(0) warning: no activity or clock movement

Hi,

I'm trying to co-simulate verilog and systemc using vpi. The basic step is

1. At verilog side, whenever posedge of clock occurs, call sc_start(delta) to let systemc time catch up verilog time;
2. Execute clk.write(1)
3. Call sc_start(0) to let the write really occur at systemc side (clk posedge ocurrs at evaluation phase of systemc side )
4. Then call another sc_start(0) to let the updated systemc output to be propagated to verilog side.

Actually, I merge step3 and step4 to the following

      while(sc_pending_activity_at_current_time()) sc_start(0);

All works fine except the warning which fired at step 3:

      Warning: (W571) no activity or clock movement for sc_start() invocation
      In file: ../../../../src/sysc/kernel/sc_simcontext.cpp:1606

Here my question is that,

1. At what condition, sc_pending_activity_at_current_time() will return true? Will executing clk.write(1) let it to return true? I suppose yes.
2. What's the popurse for this warning? It seems this warning will not be fired at systemc-2.2.0, but only systemc-2.3.0

Thanks in advance,

Brian