For who wants to know i solved the question this way:

function setBBOX()
{
wait(1);
c_setminmax( me );
}


the paint_FUNCTION runs on the server of course, so i just used proc_local in the begining of that function this way:

if( connection == 3 )
{
proc_local( me, setBBOX );
}

This way all clients see their bounding box updated and server´s bbox too.