IOMemoryDescriptor

IOMemoryDescriptor is an abstract base class in Apple's I/O Kit that describes a region of memory to be used in I/O operations. It encapsulates information about the memory's address, length, and applicable constraints, and can represent different kinds of memory, including kernel memory, user memory, or device-accessible memory. A descriptor may describe a single contiguous region or multiple segments, enabling flexible handling of various memory layouts.

The primary purpose of IOMemoryDescriptor is to allow drivers to specify the buffers involved in data transfer with devices, set up memory for DMA, or expose memory to user space in a controlled manner. Descriptors are used to describe memory that devices will read from or write to, as well as memory that may be mapped into a user task’s address space through IOMemoryMap. They provide a uniform interface for managing memory across different I/O sources and architectures.

Creation and usage of a descriptor typically involve factory methods that tailor the descriptor to the memory source, such as kernel-address-based buffers, user-space buffers associated with a task, or physically described memory. Some descriptors describe single regions, while others describe a list of segments to support scatter-gather I/O. The descriptor’s lifetime governs the validity of the underlying memory during I/O operations.

Related concepts include IOBufferMemoryDescriptor and IOMemoryMap, which provide concrete implementations and mapping capabilities, and IOMemoryDescriptor work with DMA-capable devices while ensuring appropriate synchronization and protection between kernel and user space.