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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"> |
| — 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"> |
| — 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"> |
| — 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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