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| <h3 class="section">17.2 I/O Queues</h3> |
| |
| <p>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 <em>within the kernel</em> independent of the buffering |
| implemented by I/O streams (see <a href="I_002fO-on-Streams.html#I_002fO-on-Streams">I/O on Streams</a>). |
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| <p><a name="index-terminal-input-queue-1851"></a><a name="index-typeahead-buffer-1852"></a>The <dfn>terminal input queue</dfn> is also sometimes referred to as its |
| <dfn>typeahead buffer</dfn>. It holds the characters that have been received |
| from the terminal but not yet read by any process. |
| |
| <p>The size of the input queue is described by the <code>MAX_INPUT</code> and |
| <code>_POSIX_MAX_INPUT</code><!-- /@w --> parameters; see <a href="Limits-for-Files.html#Limits-for-Files">Limits for Files</a>. You |
| are guaranteed a queue size of at least <code>MAX_INPUT</code>, but the queue |
| might be larger, and might even dynamically change size. If input flow |
| control is enabled by setting the <code>IXOFF</code> input mode bit |
| (see <a href="Input-Modes.html#Input-Modes">Input Modes</a>), 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 <a href="Canonical-or-Not.html#Canonical-or-Not">Canonical or Not</a>. |
| |
| <p><a name="index-terminal-output-queue-1853"></a>The <dfn>terminal output queue</dfn> 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 <code>IXON</code> input mode bit (see <a href="Input-Modes.html#Input-Modes">Input Modes</a>), the |
| terminal driver obeys START and STOP characters sent by the terminal to |
| stop and restart transmission of output. |
| |
| <p><dfn>Clearing</dfn> 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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