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<h1 id="firstHeading" class="firstHeading">ftell</h1> <table class="t-dcl-begin"> <tr class="t-dsc-header"> <th> Defined in header <code><stdio.h></code> </th> <th> </th> <th> </th> </tr> <tr class="t-dcl"> <td class="t-dcl-nopad"> <pre data-language="c">long ftell( FILE *stream );</pre>
</td> <td class="t-dcl-nopad"> </td> <td class="t-dcl-nopad"> </td> </tr> </table> <p>Returns the file position indicator for the file stream <code>stream</code>.</p>
<p>If the stream is open in binary mode, the value obtained by this function is the number of bytes from the beginning of the file.</p>
<p>If the stream is open in text mode, the value returned by this function is unspecified and is only meaningful as the input to <code><a href="fseek" title="c/io/fseek">fseek()</a></code>.</p>
<h3 id="Parameters"> Parameters</h3> <table class="t-par-begin"> <tr class="t-par"> <td> stream </td> <td> - </td> <td> file stream to examine </td>
</tr>
</table> <h3 id="Return_value"> Return value</h3> <p>File position indicator on success or <code>-1L</code> if failure occurs.</p>
<p>On error, the <code>errno</code> variable is set to implementation-defined positive value.</p>
<h3 id="Example"> Example</h3> <div class="t-example">
<p>Demonstrates <code>ftell()</code> with error checking. Writes then reads a few floating-point (FP) values to/from a file.</p>
<div class="c source-c"><pre data-language="c">#include <stdio.h>
#include <stdlib.h>
/* If the condition is not met then exit the program with error message. */
void check(_Bool condition, const char* func, int line)
{
if (condition)
return;
perror(func);
fprintf(stderr, "%s failed in file %s at line # %d\n", func, __FILE__, line - 1);
exit(EXIT_FAILURE);
}
int main(void)
{
/* Prepare an array of FP values. */
#define SIZE 5
double A[SIZE] = {1.1,2.,3.,4.,5.};
/* Write array to a file. */
const char* fname = "/tmp/test.bin";
FILE* file = fopen(fname, "wb");
check(file != NULL, "fopen()", __LINE__);
const int write_count = fwrite(A, sizeof(double), SIZE, file);
check(write_count == SIZE, "fwrite()", __LINE__);
fclose(file);
/* Read the FP values into array B. */
double B[SIZE];
file = fopen(fname, "rb");
check(file != NULL, "fopen()", __LINE__);
long int pos = ftell(file); /* position indicator at start of file */
check(pos != -1L, "ftell()", __LINE__);
printf("pos: %ld\n", pos);
const int read_count = fread(B, sizeof(double), 1, file); /* read one FP value */
check(read_count == 1, "fread()", __LINE__);
pos = ftell(file); /* position indicator after reading one FP value */
check(pos != -1L, "ftell()", __LINE__);
printf("pos: %ld\n", pos);
printf("B[0]: %.1f\n", B[0]); /* print one FP value */
return EXIT_SUCCESS;
}</pre></div> <p>Possible output:</p>
<div class="text source-text"><pre data-language="c">pos: 0
pos: 8
B[0]: 1.1</pre></div> </div> <h3 id="References"> References</h3> <ul>
<li> C11 standard (ISO/IEC 9899:2011): </li>
<ul><li> 7.21.9.4 The ftell function (p: 337-338) </li></ul>
<li> C99 standard (ISO/IEC 9899:1999): </li>
<ul><li> 7.19.9.4 The ftell function (p: 303-304) </li></ul>
<li> C89/C90 standard (ISO/IEC 9899:1990): </li>
<ul><li> 4.9.9.4 The ftell function </li></ul>
</ul> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td> <div><a href="fgetpos" title="c/io/fgetpos"> <span class="t-lines"><span>fgetpos</span></span></a></div> </td> <td> gets the file position indicator <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="fseek" title="c/io/fseek"> <span class="t-lines"><span>fseek</span></span></a></div> </td> <td> moves the file position indicator to a specific location in a file <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="fsetpos" title="c/io/fsetpos"> <span class="t-lines"><span>fsetpos</span></span></a></div> </td> <td> moves the file position indicator to a specific location in a file <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/io/c/ftell" title="cpp/io/c/ftell">C++ documentation</a></span> for <code>ftell</code> </td>
</tr> </table> <div class="_attribution">
<p class="_attribution-p">
© cppreference.com<br>Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.<br>
<a href="https://en.cppreference.com/w/c/io/ftell" class="_attribution-link">https://en.cppreference.com/w/c/io/ftell</a>
</p>
</div>
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