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<h4 class="subsection">30.3.1 Mount Information</h4>

<p>For some programs it is desirable and necessary to access information
about whether a certain filesystem is mounted and, if it is, where, or
simply to get lists of all the available filesystems.  The GNU libc
provides some functions to retrieve this information portably.

   <p>Traditionally Unix systems have a file named <samp><span class="file">/etc/fstab</span></samp> which
describes all possibly mounted filesystems.  The <code>mount</code> program
uses this file to mount at startup time of the system all the
necessary filesystems.  The information about all the filesystems
actually mounted is normally kept in a file named either
<samp><span class="file">/var/run/mtab</span></samp> or <samp><span class="file">/etc/mtab</span></samp>.  Both files share the same
syntax and it is crucial that this syntax is followed all the time. 
Therefore it is best to never directly write the files.  The functions
described in this section can do this and they also provide the
functionality to convert the external textual representation to the
internal representation.

   <p>Note that the <samp><span class="file">fstab</span></samp> and <samp><span class="file">mtab</span></samp> files are maintained on a
system by <em>convention</em>.  It is possible for the files not to exist
or not to be consistent with what is really mounted or available to
mount, if the system's administration policy allows it.  But programs
that mount and unmount filesystems typically maintain and use these
files as described herein.

   <p><a name="index-g_t_005fPATH_005fFSTAB-3458"></a><a name="index-g_t_005fPATH_005fMNTTAB-3459"></a><a name="index-g_t_005fPATH_005fMOUNTED-3460"></a><a name="index-FSTAB-3461"></a><a name="index-MNTTAB-3462"></a><a name="index-MOUNTED-3463"></a>The filenames given above should never be used directly.  The portable
way to handle these file is to use the macro <code>_PATH_FSTAB</code>,
defined in <samp><span class="file">fstab.h</span></samp>, or <code>_PATH_MNTTAB</code>, defined in
<samp><span class="file">mntent.h</span></samp> and <samp><span class="file">paths.h</span></samp>, for <samp><span class="file">fstab</span></samp>; and the macro
<code>_PATH_MOUNTED</code>, also defined in <samp><span class="file">mntent.h</span></samp> and
<samp><span class="file">paths.h</span></samp>, for <samp><span class="file">mtab</span></samp>.  There are also two alternate macro
names <code>FSTAB</code>, <code>MNTTAB</code>, and <code>MOUNTED</code> defined but
these names are deprecated and kept only for backward compatibility. 
The names <code>_PATH_MNTTAB</code> and <code>_PATH_MOUNTED</code> should always be used.

<ul class="menu">
<li><a accesskey="1" href="fstab.html#fstab">fstab</a>:                        The <samp><span class="file">fstab</span></samp> file
<li><a accesskey="2" href="mtab.html#mtab">mtab</a>:                         The <samp><span class="file">mtab</span></samp> file
<li><a accesskey="3" href="Other-Mount-Information.html#Other-Mount-Information">Other Mount Information</a>:      Other (non-libc) sources of mount information
</ul>

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