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<a name="fseek_002c-fseeko_002d_002d_002dset-file-position"></a>
<h3 class="section">4.26 <code>fseek</code>, <code>fseeko</code>—set file position</h3>
<a name="index-fseek"></a>
<a name="index-fseeko"></a>
<a name="index-_005ffseek_005fr"></a>
<a name="index-_005ffseeko_005fr"></a>
<p><strong>Synopsis</strong>
</p><div class="example">
<pre class="example">#include <stdio.h>
int fseek(FILE *<var>fp</var>, long <var>offset</var>, int <var>whence</var>);
int fseeko(FILE *<var>fp</var>, off_t <var>offset</var>, int <var>whence</var>);
int _fseek_r(struct _reent *<var>ptr</var>, FILE *<var>fp</var>,
long <var>offset</var>, int <var>whence</var>);
int _fseeko_r(struct _reent *<var>ptr</var>, FILE *<var>fp</var>,
off_t <var>offset</var>, int <var>whence</var>);
</pre></div>
<p><strong>Description</strong><br>
Objects of type <code>FILE</code> can have a “position” that records how much
of the file your program has already read. Many of the <code>stdio</code> functions
depend on this position, and many change it as a side effect.
</p>
<p>You can use <code>fseek</code>/<code>fseeko</code> to set the position for the file identified by
<var>fp</var>. The value of <var>offset</var> determines the new position, in one
of three ways selected by the value of <var>whence</var> (defined as macros
in ‘<code>stdio.h</code>’):
</p>
<p><code>SEEK_SET</code>—<var>offset</var> is the absolute file position (an offset
from the beginning of the file) desired. <var>offset</var> must be positive.
</p>
<p><code>SEEK_CUR</code>—<var>offset</var> is relative to the current file position.
<var>offset</var> can meaningfully be either positive or negative.
</p>
<p><code>SEEK_END</code>—<var>offset</var> is relative to the current end of file.
<var>offset</var> can meaningfully be either positive (to increase the size
of the file) or negative.
</p>
<p>See <code>ftell</code>/<code>ftello</code> to determine the current file position.
</p>
<br>
<p><strong>Returns</strong><br>
<code>fseek</code>/<code>fseeko</code> return <code>0</code> when successful. On failure, the
result is <code>EOF</code>. The reason for failure is indicated in <code>errno</code>:
either <code>ESPIPE</code> (the stream identified by <var>fp</var> doesn’t support
repositioning) or <code>EINVAL</code> (invalid file position).
</p>
<br>
<p><strong>Portability</strong><br>
ANSI C requires <code>fseek</code>.
</p>
<p><code>fseeko</code> is defined by the Single Unix specification.
</p>
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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