SPI_connect, SPI_connect_ext — connect a C function to the SPI manager
int SPI_connect(void)
int SPI_connect_ext(int options)
   SPI_connect opens a connection from a
   C function 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 C functions.
  
   SPI_connect_ext does the same but has an argument that
   allows passing option flags.  Currently, the following option values are
   available:
   
SPI_OPT_NONATOMIC
       Sets the SPI connection to be nonatomic, which
       means that transaction control calls SPI_commit,
       SPI_rollback, and
       SPI_start_transaction are allowed.  Otherwise,
       calling these functions will result in an immediate error.
      
   SPI_connect() is equivalent to
   SPI_connect_ext(0).
  
SPI_OK_CONNECTon success
SPI_ERROR_CONNECTon error