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 <h4 class="subsection">14.9.1 The meaning of the File Attributes</h4>

 <p><a name="index-status-of-a-file-1491"></a><a name="index-attributes-of-a-file-1492"></a><a name="index-file-attributes-1493"></a>
 When you read the attributes of a file, they come back in a structure
 called <code>struct stat</code>.  This section describes the names of the
 attributes, their data types, and what they mean.  For the functions
 to read the attributes of a file, see <a href="Reading-Attributes.html#Reading-Attributes">Reading Attributes</a>.

    <p>The header file <samp><span class="file">sys/stat.h</span></samp> declares all the symbols defined
 in this section.
 <a name="index-sys_002fstat_002eh-1494"></a>
 <!-- sys/stat.h -->
 <!-- POSIX.1 -->

 <div class="defun">
 &mdash; Data Type: <b>struct stat</b><var><a name="index-struct-stat-1495"></a></var><br>
 <blockquote><p>The <code>stat</code> structure type is used to return information about the
 attributes of a file.  It contains at least the following members:

           <dl>
 <dt><code>mode_t st_mode</code><dd>Specifies the mode of the file.  This includes file type information
 (see <a href="Testing-File-Type.html#Testing-File-Type">Testing File Type</a>) and the file permission bits
 (see <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>).

           <br><dt><code>ino_t st_ino</code><dd>The file serial number, which distinguishes this file from all other
 files on the same device.

           <br><dt><code>dev_t st_dev</code><dd>Identifies the device containing the file.  The <code>st_ino</code> and
 <code>st_dev</code>, taken together, uniquely identify the file.  The
 <code>st_dev</code> value is not necessarily consistent across reboots or
 system crashes, however.

           <br><dt><code>nlink_t st_nlink</code><dd>The number of hard links to the file.  This count keeps track of how
 many directories have entries for this file.  If the count is ever
 decremented to zero, then the file itself is discarded as soon as no
 process still holds it open.  Symbolic links are not counted in the
 total.

           <br><dt><code>uid_t st_uid</code><dd>The user ID of the file's owner.  See <a href="File-Owner.html#File-Owner">File Owner</a>.

           <br><dt><code>gid_t st_gid</code><dd>The group ID of the file.  See <a href="File-Owner.html#File-Owner">File Owner</a>.

           <br><dt><code>off_t st_size</code><dd>This specifies the size of a regular file in bytes.  For files that are
 really devices this field isn't usually meaningful.  For symbolic links
 this specifies the length of the file name the link refers to.

           <br><dt><code>time_t st_atime</code><dd>This is the last access time for the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_atime_usec</code><dd>This is the fractional part of the last access time for the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>time_t st_mtime</code><dd>This is the time of the last modification to the contents of the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_mtime_usec</code><dd>This is the fractional part of the time of the last modification to the
 contents of the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>time_t st_ctime</code><dd>This is the time of the last modification to the attributes of the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_ctime_usec</code><dd>This is the fractional part of the time of the last modification to the
 attributes of the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <!-- !!! st_rdev -->
           <br><dt><code>blkcnt_t st_blocks</code><dd>This is the amount of disk space that the file occupies, measured in
 units of 512-byte blocks.

           <p>The number of disk blocks is not strictly proportional to the size of
 the file, for two reasons: the file system may use some blocks for
 internal record keeping; and the file may be sparse&mdash;it may have
 &ldquo;holes&rdquo; which contain zeros but do not actually take up space on the
 disk.

           <p>You can tell (approximately) whether a file is sparse by comparing this
 value with <code>st_size</code>, like this:

           <pre class="smallexample">               (st.st_blocks * 512 &lt; st.st_size)
 </pre>
           <p>This test is not perfect because a file that is just slightly sparse
 might not be detected as sparse at all.  For practical applications,
 this is not a problem.

           <br><dt><code>unsigned int st_blksize</code><dd>The optimal block size for reading of writing this file, in bytes.  You
 might use this size for allocating the buffer space for reading of
 writing the file.  (This is unrelated to <code>st_blocks</code>.)
 </dl>
         </p></blockquote></div>

    <p>The extensions for the Large File Support (LFS) require, even on 32-bit
 machines, types which can handle file sizes up to 2^63.
 Therefore a new definition of <code>struct stat</code> is necessary.

 <!-- sys/stat.h -->
 <!-- LFS -->
 <div class="defun">
 &mdash; Data Type: <b>struct stat64</b><var><a name="index-struct-stat64-1496"></a></var><br>
 <blockquote><p>The members of this type are the same and have the same names as those
 in <code>struct stat</code>.  The only difference is that the members
 <code>st_ino</code>, <code>st_size</code>, and <code>st_blocks</code> have a different
 type to support larger values.

           <dl>
 <dt><code>mode_t st_mode</code><dd>Specifies the mode of the file.  This includes file type information
 (see <a href="Testing-File-Type.html#Testing-File-Type">Testing File Type</a>) and the file permission bits
 (see <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>).

           <br><dt><code>ino64_t st_ino</code><dd>The file serial number, which distinguishes this file from all other
 files on the same device.

           <br><dt><code>dev_t st_dev</code><dd>Identifies the device containing the file.  The <code>st_ino</code> and
 <code>st_dev</code>, taken together, uniquely identify the file.  The
 <code>st_dev</code> value is not necessarily consistent across reboots or
 system crashes, however.

           <br><dt><code>nlink_t st_nlink</code><dd>The number of hard links to the file.  This count keeps track of how
 many directories have entries for this file.  If the count is ever
 decremented to zero, then the file itself is discarded as soon as no
 process still holds it open.  Symbolic links are not counted in the
 total.

           <br><dt><code>uid_t st_uid</code><dd>The user ID of the file's owner.  See <a href="File-Owner.html#File-Owner">File Owner</a>.

           <br><dt><code>gid_t st_gid</code><dd>The group ID of the file.  See <a href="File-Owner.html#File-Owner">File Owner</a>.

           <br><dt><code>off64_t st_size</code><dd>This specifies the size of a regular file in bytes.  For files that are
 really devices this field isn't usually meaningful.  For symbolic links
 this specifies the length of the file name the link refers to.

           <br><dt><code>time_t st_atime</code><dd>This is the last access time for the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_atime_usec</code><dd>This is the fractional part of the last access time for the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>time_t st_mtime</code><dd>This is the time of the last modification to the contents of the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_mtime_usec</code><dd>This is the fractional part of the time of the last modification to the
 contents of the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>time_t st_ctime</code><dd>This is the time of the last modification to the attributes of the file.
 See <a href="File-Times.html#File-Times">File Times</a>.

           <br><dt><code>unsigned long int st_ctime_usec</code><dd>This is the fractional part of the time of the last modification to the
 attributes of the file.  See <a href="File-Times.html#File-Times">File Times</a>.

           <!-- !!! st_rdev -->
           <br><dt><code>blkcnt64_t st_blocks</code><dd>This is the amount of disk space that the file occupies, measured in
 units of 512-byte blocks.

           <br><dt><code>unsigned int st_blksize</code><dd>The optimal block size for reading of writing this file, in bytes.  You
 might use this size for allocating the buffer space for reading of
 writing the file.  (This is unrelated to <code>st_blocks</code>.)
 </dl>
         </p></blockquote></div>

    <p>Some of the file attributes have special data type names which exist
 specifically for those attributes.  (They are all aliases for well-known
 integer types that you know and love.)  These typedef names are defined
 in the header file <samp><span class="file">sys/types.h</span></samp> as well as in <samp><span class="file">sys/stat.h</span></samp>.
 Here is a list of them.

 <!-- sys/types.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Data Type: <b>mode_t</b><var><a name="index-mode_005ft-1497"></a></var><br>
 <blockquote><p>This is an integer data type used to represent file modes.  In the
 GNU system, this is equivalent to <code>unsigned int</code>.
 </p></blockquote></div>

    <p><a name="index-inode-number-1498"></a><!-- sys/types.h -->
 <!-- POSIX.1 -->

 <div class="defun">
 &mdash; Data Type: <b>ino_t</b><var><a name="index-ino_005ft-1499"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent file serial numbers.
 (In Unix jargon, these are sometimes called <dfn>inode numbers</dfn>.)
 In the GNU system, this type is equivalent to <code>unsigned long int</code>.

         <p>If the source is compiled with <code>_FILE_OFFSET_BITS == 64</code> this type
 is transparently replaced by <code>ino64_t</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- Unix98 -->
 <div class="defun">
 &mdash; Data Type: <b>ino64_t</b><var><a name="index-ino64_005ft-1500"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent file serial numbers
 for the use in LFS.  In the GNU system, this type is equivalent to
 <code>unsigned long long int</code>.

         <p>When compiling with <code>_FILE_OFFSET_BITS == 64</code> this type is
 available under the name <code>ino_t</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Data Type: <b>dev_t</b><var><a name="index-dev_005ft-1501"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent file device numbers.
 In the GNU system, this is equivalent to <code>int</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- POSIX.1 -->
 <div class="defun">
 &mdash; Data Type: <b>nlink_t</b><var><a name="index-nlink_005ft-1502"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent file link counts.
 In the GNU system, this is equivalent to <code>unsigned short int</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- Unix98 -->
 <div class="defun">
 &mdash; Data Type: <b>blkcnt_t</b><var><a name="index-blkcnt_005ft-1503"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent block counts.
 In the GNU system, this is equivalent to <code>unsigned long int</code>.

         <p>If the source is compiled with <code>_FILE_OFFSET_BITS == 64</code> this type
 is transparently replaced by <code>blkcnt64_t</code>.
 </p></blockquote></div>

 <!-- sys/types.h -->
 <!-- Unix98 -->
 <div class="defun">
 &mdash; Data Type: <b>blkcnt64_t</b><var><a name="index-blkcnt64_005ft-1504"></a></var><br>
 <blockquote><p>This is an arithmetic data type used to represent block counts for the
 use in LFS.  In the GNU system, this is equivalent to <code>unsigned
 long long int</code>.

         <p>When compiling with <code>_FILE_OFFSET_BITS == 64</code> this type is
 available under the name <code>blkcnt_t</code>.
 </p></blockquote></div>

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	Next: <a rel="next" accesskey="n" href="Reading-Attributes.html#Reading-Attributes">Reading Attributes</a>,
	Up: <a rel="up" accesskey="u" href="File-Attributes.html#File-Attributes">File Attributes</a>
	<hr>
	</div>

	<h4 class="subsection">14.9.1 The meaning of the File Attributes</h4>

	<p><a name="index-status-of-a-file-1491"></a><a name="index-attributes-of-a-file-1492"></a><a name="index-file-attributes-1493"></a>
	When you read the attributes of a file, they come back in a structure
	called <code>struct stat</code>. This section describes the names of the
	attributes, their data types, and what they mean. For the functions
	to read the attributes of a file, see <a href="Reading-Attributes.html#Reading-Attributes">Reading Attributes</a>.

	<p>The header file <samp><span class="file">sys/stat.h</span></samp> declares all the symbols defined
	in this section.
	<a name="index-sys_002fstat_002eh-1494"></a>
	<!-- sys/stat.h -->
	<!-- POSIX.1 -->

	<div class="defun">
	— Data Type: <b>struct stat</b><var><a name="index-struct-stat-1495"></a></var><br>
	<blockquote><p>The <code>stat</code> structure type is used to return information about the
	attributes of a file. It contains at least the following members:

	<dl>
	<dt><code>mode_t st_mode</code><dd>Specifies the mode of the file. This includes file type information
	(see <a href="Testing-File-Type.html#Testing-File-Type">Testing File Type</a>) and the file permission bits
	(see <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>).

	<br><dt><code>ino_t st_ino</code><dd>The file serial number, which distinguishes this file from all other
	files on the same device.

	<br><dt><code>dev_t st_dev</code><dd>Identifies the device containing the file. The <code>st_ino</code> and
	<code>st_dev</code>, taken together, uniquely identify the file. The
	<code>st_dev</code> value is not necessarily consistent across reboots or
	system crashes, however.

	<br><dt><code>nlink_t st_nlink</code><dd>The number of hard links to the file. This count keeps track of how
	many directories have entries for this file. If the count is ever
	decremented to zero, then the file itself is discarded as soon as no
	process still holds it open. Symbolic links are not counted in the
	total.

	<br><dt><code>uid_t st_uid</code><dd>The user ID of the file's owner. See <a href="File-Owner.html#File-Owner">File Owner</a>.

	<br><dt><code>gid_t st_gid</code><dd>The group ID of the file. See <a href="File-Owner.html#File-Owner">File Owner</a>.

	<br><dt><code>off_t st_size</code><dd>This specifies the size of a regular file in bytes. For files that are
	really devices this field isn't usually meaningful. For symbolic links
	this specifies the length of the file name the link refers to.

	<br><dt><code>time_t st_atime</code><dd>This is the last access time for the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_atime_usec</code><dd>This is the fractional part of the last access time for the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>time_t st_mtime</code><dd>This is the time of the last modification to the contents of the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_mtime_usec</code><dd>This is the fractional part of the time of the last modification to the
	contents of the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>time_t st_ctime</code><dd>This is the time of the last modification to the attributes of the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_ctime_usec</code><dd>This is the fractional part of the time of the last modification to the
	attributes of the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<!-- !!! st_rdev -->
	<br><dt><code>blkcnt_t st_blocks</code><dd>This is the amount of disk space that the file occupies, measured in
	units of 512-byte blocks.

	<p>The number of disk blocks is not strictly proportional to the size of
	the file, for two reasons: the file system may use some blocks for
	internal record keeping; and the file may be sparse—it may have
	“holes” which contain zeros but do not actually take up space on the
	disk.

	<p>You can tell (approximately) whether a file is sparse by comparing this
	value with <code>st_size</code>, like this:

	<pre class="smallexample"> (st.st_blocks * 512 < st.st_size)
	</pre>
	<p>This test is not perfect because a file that is just slightly sparse
	might not be detected as sparse at all. For practical applications,
	this is not a problem.

	<br><dt><code>unsigned int st_blksize</code><dd>The optimal block size for reading of writing this file, in bytes. You
	might use this size for allocating the buffer space for reading of
	writing the file. (This is unrelated to <code>st_blocks</code>.)
	</dl>
	</p></blockquote></div>

	<p>The extensions for the Large File Support (LFS) require, even on 32-bit
	machines, types which can handle file sizes up to 2^63.
	Therefore a new definition of <code>struct stat</code> is necessary.

	<!-- sys/stat.h -->
	<!-- LFS -->
	<div class="defun">
	— Data Type: <b>struct stat64</b><var><a name="index-struct-stat64-1496"></a></var><br>
	<blockquote><p>The members of this type are the same and have the same names as those
	in <code>struct stat</code>. The only difference is that the members
	<code>st_ino</code>, <code>st_size</code>, and <code>st_blocks</code> have a different
	type to support larger values.

	<dl>
	<dt><code>mode_t st_mode</code><dd>Specifies the mode of the file. This includes file type information
	(see <a href="Testing-File-Type.html#Testing-File-Type">Testing File Type</a>) and the file permission bits
	(see <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>).

	<br><dt><code>ino64_t st_ino</code><dd>The file serial number, which distinguishes this file from all other
	files on the same device.

	<br><dt><code>dev_t st_dev</code><dd>Identifies the device containing the file. The <code>st_ino</code> and
	<code>st_dev</code>, taken together, uniquely identify the file. The
	<code>st_dev</code> value is not necessarily consistent across reboots or
	system crashes, however.

	<br><dt><code>nlink_t st_nlink</code><dd>The number of hard links to the file. This count keeps track of how
	many directories have entries for this file. If the count is ever
	decremented to zero, then the file itself is discarded as soon as no
	process still holds it open. Symbolic links are not counted in the
	total.

	<br><dt><code>uid_t st_uid</code><dd>The user ID of the file's owner. See <a href="File-Owner.html#File-Owner">File Owner</a>.

	<br><dt><code>gid_t st_gid</code><dd>The group ID of the file. See <a href="File-Owner.html#File-Owner">File Owner</a>.

	<br><dt><code>off64_t st_size</code><dd>This specifies the size of a regular file in bytes. For files that are
	really devices this field isn't usually meaningful. For symbolic links
	this specifies the length of the file name the link refers to.

	<br><dt><code>time_t st_atime</code><dd>This is the last access time for the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_atime_usec</code><dd>This is the fractional part of the last access time for the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>time_t st_mtime</code><dd>This is the time of the last modification to the contents of the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_mtime_usec</code><dd>This is the fractional part of the time of the last modification to the
	contents of the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>time_t st_ctime</code><dd>This is the time of the last modification to the attributes of the file.
	See <a href="File-Times.html#File-Times">File Times</a>.

	<br><dt><code>unsigned long int st_ctime_usec</code><dd>This is the fractional part of the time of the last modification to the
	attributes of the file. See <a href="File-Times.html#File-Times">File Times</a>.

	<!-- !!! st_rdev -->
	<br><dt><code>blkcnt64_t st_blocks</code><dd>This is the amount of disk space that the file occupies, measured in
	units of 512-byte blocks.

	<br><dt><code>unsigned int st_blksize</code><dd>The optimal block size for reading of writing this file, in bytes. You
	might use this size for allocating the buffer space for reading of
	writing the file. (This is unrelated to <code>st_blocks</code>.)
	</dl>
	</p></blockquote></div>

	<p>Some of the file attributes have special data type names which exist
	specifically for those attributes. (They are all aliases for well-known
	integer types that you know and love.) These typedef names are defined
	in the header file <samp><span class="file">sys/types.h</span></samp> as well as in <samp><span class="file">sys/stat.h</span></samp>.
	Here is a list of them.

	<!-- sys/types.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Data Type: <b>mode_t</b><var><a name="index-mode_005ft-1497"></a></var><br>
	<blockquote><p>This is an integer data type used to represent file modes. In the
	GNU system, this is equivalent to <code>unsigned int</code>.
	</p></blockquote></div>

	<p><a name="index-inode-number-1498"></a><!-- sys/types.h -->
	<!-- POSIX.1 -->

	<div class="defun">
	— Data Type: <b>ino_t</b><var><a name="index-ino_005ft-1499"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent file serial numbers.
	(In Unix jargon, these are sometimes called <dfn>inode numbers</dfn>.)
	In the GNU system, this type is equivalent to <code>unsigned long int</code>.

	<p>If the source is compiled with <code>_FILE_OFFSET_BITS == 64</code> this type
	is transparently replaced by <code>ino64_t</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- Unix98 -->
	<div class="defun">
	— Data Type: <b>ino64_t</b><var><a name="index-ino64_005ft-1500"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent file serial numbers
	for the use in LFS. In the GNU system, this type is equivalent to
	<code>unsigned long long int</code>.

	<p>When compiling with <code>_FILE_OFFSET_BITS == 64</code> this type is
	available under the name <code>ino_t</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Data Type: <b>dev_t</b><var><a name="index-dev_005ft-1501"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent file device numbers.
	In the GNU system, this is equivalent to <code>int</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- POSIX.1 -->
	<div class="defun">
	— Data Type: <b>nlink_t</b><var><a name="index-nlink_005ft-1502"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent file link counts.
	In the GNU system, this is equivalent to <code>unsigned short int</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- Unix98 -->
	<div class="defun">
	— Data Type: <b>blkcnt_t</b><var><a name="index-blkcnt_005ft-1503"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent block counts.
	In the GNU system, this is equivalent to <code>unsigned long int</code>.

	<p>If the source is compiled with <code>_FILE_OFFSET_BITS == 64</code> this type
	is transparently replaced by <code>blkcnt64_t</code>.
	</p></blockquote></div>

	<!-- sys/types.h -->
	<!-- Unix98 -->
	<div class="defun">
	— Data Type: <b>blkcnt64_t</b><var><a name="index-blkcnt64_005ft-1504"></a></var><br>
	<blockquote><p>This is an arithmetic data type used to represent block counts for the
	use in LFS. In the GNU system, this is equivalent to <code>unsigned
	long long int</code>.

	<p>When compiling with <code>_FILE_OFFSET_BITS == 64</code> this type is
	available under the name <code>blkcnt_t</code>.
	</p></blockquote></div>

	</body></html>