A STREAMS device driver is a character device driver that implements the STREAMS interface. A STREAMS device driver exists below the stream head and any modules. It can act on an external I/O device, or it can be an internal software driver, called a pseudo-device driver. The driver transfers data between the kernel and the device. The interfaces between the driver and kernel are known collectively as the Oracle Solaris operating environment Device Driver Interface/Driver Kernel Interface (Oracle Solaris operating environment DDI/DKI). The relationship between the driver and the rest of the UNIX kernel is explained in Writing Device Drivers for Oracle Solaris 11.2 . Details of device drivers are explained in Chapter 9, STREAMS Drivers.