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<h3 class="section">31.7 Optional Features in File Support</h3>

<p>POSIX defines certain system-specific options in the system calls for
operating on files.  Some systems support these options and others do
not.  Since these options are provided in the kernel, not in the
library, simply using the GNU C library does not guarantee that any of these
features is supported; it depends on the system you are using.  They can
also vary between file systems on a single machine.

   <p><a name="index-unistd_002eh-3660"></a>This section describes the macros you can test to determine whether a
particular option is supported on your machine.  If a given macro is
defined in <samp><span class="file">unistd.h</span></samp>, then its value says whether the
corresponding feature is supported.  (A value of <code>-1</code> indicates no;
any other value indicates yes.)  If the macro is undefined, it means
particular files may or may not support the feature.

   <p>Since all the machines that support the GNU C library also support NFS,
one can never make a general statement about whether all file systems
support the <code>_POSIX_CHOWN_RESTRICTED</code> and <code>_POSIX_NO_TRUNC</code>
features.  So these names are never defined as macros in the GNU C
library.

<!-- unistd.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>_POSIX_CHOWN_RESTRICTED</b><var><a name="index-g_t_005fPOSIX_005fCHOWN_005fRESTRICTED-3661"></a></var><br>
<blockquote><p>If this option is in effect, the <code>chown</code> function is restricted so
that the only changes permitted to nonprivileged processes is to change
the group owner of a file to either be the effective group ID of the
process, or one of its supplementary group IDs.  See <a href="File-Owner.html#File-Owner">File Owner</a>. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>_POSIX_NO_TRUNC</b><var><a name="index-g_t_005fPOSIX_005fNO_005fTRUNC-3662"></a></var><br>
<blockquote><p>If this option is in effect, file name components longer than
<code>NAME_MAX</code> generate an <code>ENAMETOOLONG</code> error.  Otherwise, file
name components that are too long are silently truncated. 
</p></blockquote></div>

<!-- unistd.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: unsigned char <b>_POSIX_VDISABLE</b><var><a name="index-g_t_005fPOSIX_005fVDISABLE-3663"></a></var><br>
<blockquote><p>This option is only meaningful for files that are terminal devices. 
If it is enabled, then handling for special control characters can
be disabled individually.  See <a href="Special-Characters.html#Special-Characters">Special Characters</a>. 
</p></blockquote></div>

   <p><a name="index-unistd_002eh-3664"></a>If one of these macros is undefined, that means that the option might be
in effect for some files and not for others.  To inquire about a
particular file, call <code>pathconf</code> or <code>fpathconf</code>. 
See <a href="Pathconf.html#Pathconf">Pathconf</a>.

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