Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see section Input/Output on Streams).
The terminal input queue is also sometimes referred to as its typeahead buffer. It holds the characters that have been received from the terminal but not yet read by any process.
The size of the terminal's input queue is described by the
MAX_INPUT and _POSIX_MAX_INPUT parameters; see section Limits on File System Capacity. You are guaranteed a queue size of at least
MAX_INPUT, but the queue might be larger, and might even
dynamically change size. If input flow control is enabled by setting
the IXOFF input mode bit (see section Input Modes), the terminal
driver transmits STOP and START characters to the terminal when
necessary to prevent the queue from overflowing. Otherwise, input may
be lost if it comes in too fast from the terminal. In canonical mode,
all input stays in the queue until a newline character is received, so
the terminal input queue can fill up when you type a very long line.
See section Two Styles of Input: Canonical or Not.
The terminal output queue is like the input queue, but for output;
it contains characters that have been written by processes, but not yet
transmitted to the terminal. If output flow control is enabled by
setting the IXON input mode bit (see section Input Modes), the
terminal driver obeys STOP and STOP characters sent by the terminal to
stop and restart transmission of output.
Clearing the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.
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