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| <h3 class="section">4.26 <code>fseek</code>, <code>fseeko</code>—set file position</h3> |
| |
| <p><a name="index-fseek-198"></a><a name="index-fseeko-199"></a><a name="index-g_t_005ffseek_005fr-200"></a><a name="index-g_t_005ffseeko_005fr-201"></a><strong>Synopsis</strong> |
| <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> |
| <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>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><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><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><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>See <code>ftell</code>/<code>ftello</code> to determine the current file position. |
| |
| <p><br> |
| <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> |
| <strong>Portability</strong><br> |
| ANSI C requires <code>fseek</code>. |
| |
| <p><code>fseeko</code> is defined by the Single Unix specification. |
| |
| <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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