9 Synchronization variables
  
  
  9.1 Synchronization variables
  
  Synchronization  variables  (also  often  called  dataflow  variables in the
  literature)  are  variables  that can be written only once; attempts to read
  the variable block until it has been written to.
  
  Synchronization  variables  are  created with CreateSyncVar (9.1-1), written
  with SyncWrite (9.1-2) and read with SyncRead (9.1-3).
  
    Example  
    gap> sv := CreateSyncVar();;
    gap> RunAsyncTask(function()
    >      Sleep(10);
    >      SyncWrite(sv, MakeImmutable([1, 2, 3]));
    >    end);;
    gap> SyncRead(sv);
    [ 1, 2, 3 ]
  
  
  9.1-1 CreateSyncVar
  
  CreateSyncVar(  )  function
  
  The   function   CreateSyncVar   takes   no  arguments.  It  returns  a  new
  synchronization  variable.  There  is  no need to deallocate it; the garbage
  collector  will  free  the  memory  and  all related resources when it is no
  longer accessible.
  
  9.1-2 SyncWrite
  
  SyncWrite( syncvar, obj )  function
  
  SyncWrite  attempts  to assign the value obj to syncvar. If syncvar has been
  previously  assigned  a  value,  the  call  will  fail with a runtime error;
  otherwise, obj will be assigned to syncvar.
  
  In  order  to  make  sure  that  the  recipient can read the result, the obj
  argument  should  not  be  a  thread-local  object;  it  should  be  public,
  read-only, or shared.
  
  9.1-3 SyncRead
  
  SyncRead( syncvar )  function
  
  SyncRead  reads  the  value  previously  assigned  to syncvar with SyncWrite
  (9.1-2).  If  no  value  has  been  assigned  yet, it blocks. It returns the
  assigned value.