SPI_connect — connect a procedure to the SPI manager
int SPI_connect(void)
   SPI_connect opens a connection from a
   procedure invocation to the SPI manager.  You must call this
   function if you want to execute commands through SPI.  Some utility
   SPI functions can be called from unconnected procedures.
  
SPI_OK_CONNECTon success
SPI_ERROR_CONNECTon error