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| <h3 class="section">13.17 Generic I/O Control operations</h3> |
| |
| <p><a name="index-generic-i_002fo-control-operations-1382"></a><a name="index-IOCTLs-1383"></a> |
| The GNU system can handle most input/output operations on many different |
| devices and objects in terms of a few file primitives - <code>read</code>, |
| <code>write</code> and <code>lseek</code>. However, most devices also have a few |
| peculiar operations which do not fit into this model. Such as: |
| |
| <ul> |
| <li>Changing the character font used on a terminal. |
| |
| <li>Telling a magnetic tape system to rewind or fast forward. (Since they |
| cannot move in byte increments, <code>lseek</code> is inapplicable). |
| |
| <li>Ejecting a disk from a drive. |
| |
| <li>Playing an audio track from a CD-ROM drive. |
| |
| <li>Maintaining routing tables for a network. |
| |
| </ul> |
| |
| <p>Although some such objects such as sockets and terminals |
| <a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> have special functions of their own, it would |
| not be practical to create functions for all these cases. |
| |
| <p>Instead these minor operations, known as <dfn>IOCTL</dfn>s, are assigned code |
| numbers and multiplexed through the <code>ioctl</code> function, defined in |
| <code>sys/ioctl.h</code>. The code numbers themselves are defined in many |
| different headers. |
| |
| <!-- sys/ioctl.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>ioctl</b> (<var>int filedes, int command, <small class="dots">...</small></var>)<var><a name="index-ioctl-1384"></a></var><br> |
| <blockquote> |
| <p>The <code>ioctl</code> function performs the generic I/O operation |
| <var>command</var> on <var>filedes</var>. |
| |
| <p>A third argument is usually present, either a single number or a pointer |
| to a structure. The meaning of this argument, the returned value, and |
| any error codes depends upon the command used. Often -1 is |
| returned for a failure. |
| |
| </blockquote></div> |
| |
| <p>On some systems, IOCTLs used by different devices share the same numbers. |
| Thus, although use of an inappropriate IOCTL <em>usually</em> only produces |
| an error, you should not attempt to use device-specific IOCTLs on an |
| unknown device. |
| |
| <p>Most IOCTLs are OS-specific and/or only used in special system utilities, |
| and are thus beyond the scope of this document. For an example of the use |
| of an IOCTL, see <a href="Out_002dof_002dBand-Data.html#Out_002dof_002dBand-Data">Out-of-Band Data</a>. |
| |
| <div class="footnote"> |
| <hr> |
| <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Actually, the terminal-specific functions are implemented with |
| IOCTLs on many platforms.</p> |
| |
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