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For user-level programs that read and write to terminals, the TTY subsystem in UNIX System V provides a flexible interface, known as the termio facility. The flexibility of the termio facility enables users to perform efficient TTY I/O in a wide range of applications. However, the improper use of this termio can result in inefficient user programs. This section discusses writing programs that use termio and focuses on the topics of buffer size, canonical mode, raw mode and flow control and provides several code examples.
User programs that read from terminal devices must read from TTYs in either canonical mode or raw mode.