The LDI includes two categories of interfaces:
Kernel interfaces. User applications use system calls to open, read, and write to devices that are managed by a device driver within the kernel. Kernel modules can use the LDI kernel interfaces to open, read, and write to devices that are managed by another device driver within the kernel. For example, a user application might use read(2) and a kernel module might use ldi_read(9F) to read the same device. See Kernel Interfaces.
User interfaces. The LDI user interfaces can provide information to user processes regarding which devices are currently being used by other devices in the kernel. See User Interfaces.
The following terms are commonly used in discussing the LDI:
Target Device. A target device is a device within the kernel that is managed by a device driver and is being accessed by a device consumer.
Device Consumer. A device consumer is a user process or kernel module that opens and accesses a target device. A device consumer normally performs operations such as open, read, write, or ioctl on a target device.
Kernel Device Consumer. A kernel device consumer is a particular kind of device consumer. A kernel device consumer is a kernel module that accesses a target device. The kernel device consumer usually is not the device driver that manages the target device that is being accessed. Instead, the kernel device consumer accesses the target device indirectly through the device driver that manages the target device.
Layered Driver. A layered driver is a particular kind of kernel device consumer. A layered driver is a kernel driver that does not directly manage any piece of hardware. Instead, a layered driver accesses one of more target devices indirectly through the device drivers that manage those target devices. Volume managers and STREAMS multiplexers are good examples of layered drivers.