The system abstractions described are:

A directory context is a position in the filesystem name space. Operations on files and other named objects in a filesystem are always specified relative to such a context.

Files are used to store uninterpreted sequences of bytes, which may be read and written randomly. Pages from files may also be mapped into the process address space. A directory may be read as a file if permitted by the underlying storage facility, though it is usually accessed using .Fn getdirentries (see section .Xr 2.2.3.1 ). (Local filesystems permit directories to be read, although most NFS implementations do not allow reading of directories.)

A communications domain represents an interprocess communications environment, such as the communications facilities of the 4.4BSD system, communications in the INTERNET, or the resource sharing protocols and access rights of a resource sharing system on a local network.

A socket is an endpoint of communication and the focal point for IPC in a communications domain. Sockets may be created in pairs, or given names and used to rendezvous with other sockets in a communications domain, accepting connections from these sockets or exchanging messages with them. These operations model a labeled or unlabeled communications graph, and can be used in a wide variety of communications domains. Sockets can have different types\| to provide different semantics of communication, increasing the flexibility of the model.

Devices include terminals (providing input editing, interrupt generation, output flow control, and editing), magnetic tapes, disks, and other peripherals. They normally support the generic read and write operations as well as a number of ioctl\|'s.

Process descriptors provide facilities for control and debugging of other processes.