POSIX systems support two basic modes of input: canonical and noncanonical.
In canonical input processing mode, terminal input is processed in
lines terminated by newline ('\n'), EOF, or EOL characters. No
input can be read until an entire line has been typed by the user, and
the read function (see Input and Output Primitives) returns at most a
single line of input, no matter how many bytes are requested.
In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. See Characters for Input Editing.
The constants _POSIX_MAX_CANON and MAX_CANON parameterize
the maximum number of bytes which may appear in a single line of
canonical input. See Limits on File System Capacity. You are guaranteed a maximum
line length of at least MAX_CANON bytes, but the maximum might be
larger, and might even dynamically change size.
In noncanonical input processing mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. See Noncanonical Input.
Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.
The choice of canonical or noncanonical input is controlled by the
ICANON flag in the c_lflag member of struct termios.
See Local Modes.