The spi_write() function writes data to a specific device on the SPI bus. The prototype for this function is:
int spi_write( int fd, uint32_t device, void *buf, int len );
The arguments are:
The function returns the number of bytes of data that it successfully wrote to the device. If an error occurred, the function returns -1 and sets errno:
An SPI driver typically considers it to be an error if the number of bytes returned by this function isn't the same as the number of bytes it asked the function to write.