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.sh "System facilities This section discusses the system facilities that are not considered part of the kernel.
The system abstractions described are:
A directory context is a position in the UNIX file system name space. Operations on files and other named objects in a file system are always specified relative to such a context.
Files are used to store uninterpreted sequence of bytes on which random access reads and writes may occur. Pages from files may also be mapped into process address space.\(dg A directory may be read as a file. .FS \(dg Support for mapping files is not included in the 4.3 release. .FE
A communications domain represents an interprocess communications environment, such as the communications facilities of the UNIX 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 and interrupt generation and output flow control and editing, magnetic tapes, disks and other peripherals. They often 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.