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| <h3 class="section">4.28 <code>ftell</code>, <code>ftello</code>—return position in a stream or file</h3> |
| |
| <p><a name="index-ftell-204"></a><a name="index-ftello-205"></a><a name="index-g_t_005fftell_005fr-206"></a><a name="index-g_t_005fftello_005fr-207"></a><strong>Synopsis</strong> |
| <pre class="example"> #include <stdio.h> |
| long ftell(FILE *<var>fp</var>); |
| off_t ftello(FILE *<var>fp</var>); |
| long _ftell_r(struct _reent *<var>ptr</var>, FILE *<var>fp</var>); |
| off_t _ftello_r(struct _reent *<var>ptr</var>, FILE *<var>fp</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>The result of <code>ftell</code>/<code>ftello</code> is the current position for a file |
| identified by <var>fp</var>. If you record this result, you can later |
| use it with <code>fseek</code>/<code>fseeko</code> to return the file to this |
| position. The difference between <code>ftell</code> and <code>ftello</code> is that |
| <code>ftell</code> returns <code>long</code> and <code>ftello</code> returns <code>off_t</code>. |
| |
| <p>In the current implementation, <code>ftell</code>/<code>ftello</code> simply uses a character |
| count to represent the file position; this is the same number that |
| would be recorded by <code>fgetpos</code>. |
| |
| <p><br> |
| <strong>Returns</strong><br> |
| <code>ftell</code>/<code>ftello</code> return the file position, if possible. If they cannot do |
| this, they return <code>-1L</code>. Failure occurs on streams that do not support |
| positioning; the global <code>errno</code> indicates this condition with the |
| value <code>ESPIPE</code>. |
| |
| <p><br> |
| <strong>Portability</strong><br> |
| <code>ftell</code> is required by the ANSI C standard, but the meaning of its |
| result (when successful) is not specified beyond requiring that it be |
| acceptable as an argument to <code>fseek</code>. In particular, other |
| conforming C implementations may return a different result from |
| <code>ftell</code> than what <code>fgetpos</code> records. |
| |
| <p><code>ftello</code> is defined by the Single Unix specification. |
| |
| <p>No supporting OS subroutines are required. |
| |
| <p><br> |
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