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| <h3 class="section">17.3 Two Styles of Input: Canonical or Not</h3> |
| |
| <p>POSIX systems support two basic modes of input: canonical and |
| noncanonical. |
| |
| <p><a name="index-canonical-input-processing-1854"></a>In <dfn>canonical input processing</dfn> mode, terminal input is processed in |
| lines terminated by newline (<code>'\n'</code>), EOF, or EOL characters. No |
| input can be read until an entire line has been typed by the user, and |
| the <code>read</code> function (see <a href="I_002fO-Primitives.html#I_002fO-Primitives">I/O Primitives</a>) returns at most a |
| single line of input, no matter how many bytes are requested. |
| |
| <p>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 <a href="Editing-Characters.html#Editing-Characters">Editing Characters</a>. |
| |
| <p>The constants <code>_POSIX_MAX_CANON</code> and <code>MAX_CANON</code> parameterize |
| the maximum number of bytes which may appear in a single line of |
| canonical input. See <a href="Limits-for-Files.html#Limits-for-Files">Limits for Files</a>. You are guaranteed a maximum |
| line length of at least <code>MAX_CANON</code> bytes, but the maximum might be |
| larger, and might even dynamically change size. |
| |
| <p><a name="index-noncanonical-input-processing-1855"></a>In <dfn>noncanonical input processing</dfn> 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 <a href="Noncanonical-Input.html#Noncanonical-Input">Noncanonical Input</a>. |
| |
| <p>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. |
| |
| <p>The choice of canonical or noncanonical input is controlled by the |
| <code>ICANON</code> flag in the <code>c_lflag</code> member of <code>struct termios</code>. |
| See <a href="Local-Modes.html#Local-Modes">Local Modes</a>. |
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